diff --git a/.pylint.ini b/.pylint.ini
index da7fdefe..b3cd7d41 100644
--- a/.pylint.ini
+++ b/.pylint.ini
@@ -26,11 +26,10 @@ disable=
     protected-access,
     # Disabling these helps to write more expressive tests:
     expression-not-assigned,
-    singleton-comparison,
-    bad-continuation,
     blacklisted-name,
     # Handled by automatic formatting:
     line-too-long,
+    bad-continuation,
     # False positives:
     no-member,
     unsupported-membership-test,
diff --git a/.travis.yml b/.travis.yml
index 08565a64..b612faba 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -21,6 +21,7 @@ install:
 script:
   - make test-repeat
   - make check
+  - make notebooks
   - make mkdocs
 
 after_success:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6dd37bc2..360441da 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+# 0.9 (2020-04-13)
+
+- Fixed serialization of optional nested dataclasses with a value of `None`.
+- Fixed preservation of comments on nested dataclass attributes.
+- Added support for using `enum.Enum` subclasses as type annotations.
+
 # 0.8.1 (2020-03-30)
 
 - Fixed loading of `Missing` nested dataclasses attributes.
diff --git a/Makefile b/Makefile
index 258f43a0..66c6e20b 100644
--- a/Makefile
+++ b/Makefile
@@ -89,7 +89,7 @@ test-profile: install
 MKDOCS_INDEX := site/index.html
 
 .PHONY: docs
-docs: mkdocs uml papermill  ## Generate documentation and UML
+docs: mkdocs uml notebooks  ## Generate documentation and UML
 
 .PHONY: mkdocs
 mkdocs: install $(MKDOCS_INDEX)
@@ -116,12 +116,12 @@ docs/*.png: $(MODULES)
 	- mv -f classes_$(PACKAGE).png docs/classes.png
 	- mv -f packages_$(PACKAGE).png docs/packages.png
 
-.PHONY: papermill
-papermill: install
+.PHONY: notebooks
+notebooks: install
 	@ cd notebooks; for filename in *.ipynb; do \
 	  poetry run papermill $$filename $$filename; \
 	done
-	git config filter.nbstripout.extrakeys 'metadata.papermill cell.metadata.papermill'
+	git config filter.nbstripout.extrakeys 'cell.metadata.execution cell.metadata.papermill metadata.papermill'
 	poetry run nbstripout --keep-output notebooks/*.ipynb
 
 # RELEASE #####################################################################
diff --git a/datafiles/converters/__init__.py b/datafiles/converters/__init__.py
index 95d61abc..ab4f7d00 100644
--- a/datafiles/converters/__init__.py
+++ b/datafiles/converters/__init__.py
@@ -1,4 +1,5 @@
 import dataclasses
+from enum import Enum
 from inspect import isclass
 from typing import Any, Dict, Optional, Union
 
@@ -9,6 +10,7 @@
 from ._bases import Converter
 from .builtins import Boolean, Float, Integer, String
 from .containers import Dataclass, Dictionary, List
+from .enumerations import Enumeration
 from .extensions import *  # pylint: disable=unused-wildcard-import
 
 
@@ -46,7 +48,7 @@ def map_type(cls, *, name: str = '', item_cls: Optional[type] = None):
         converters = {}
         for field in dataclasses.fields(cls):
             converters[field.name] = map_type(field.type, name=field.name)
-        converter = Dataclass.subclass(cls, converters)
+        converter = Dataclass.of_mappings(cls, converters)
         log.debug(f'Mapped {cls!r} to new converter: {converter}')
         return converter
 
@@ -57,11 +59,10 @@ def map_type(cls, *, name: str = '', item_cls: Optional[type] = None):
             try:
                 converter = map_type(item_cls or cls.__args__[0])
             except TypeError as e:
-                if '~T' in str(e):
-                    e = TypeError(f"Type is required with 'List' annotation")
-                raise e from None
+                assert '~T' in str(e), f'Unhandled error: ' + str(e)
+                raise TypeError("Type is required with 'List' annotation") from None
             else:
-                converter = List.subclass(converter)
+                converter = List.of_type(converter)
 
         elif cls.__origin__ == dict:
             if item_cls:
@@ -72,7 +73,7 @@ def map_type(cls, *, name: str = '', item_cls: Optional[type] = None):
                 key = map_type(cls.__args__[0])
                 value = map_type(cls.__args__[1])
 
-            converter = Dictionary.subclass(key, value)
+            converter = Dictionary.of_mapping(key, value)
 
         elif cls.__origin__ == Union:
             converter = map_type(cls.__args__[0])
@@ -101,4 +102,7 @@ def map_type(cls, *, name: str = '', item_cls: Optional[type] = None):
         log.debug(f'Mapped {cls!r} to existing converter (itself)')
         return cls
 
+    if issubclass(cls, Enum):
+        return Enumeration.of_type(cls)
+
     raise TypeError(f'Could not map type: {cls}')
diff --git a/datafiles/converters/_bases.py b/datafiles/converters/_bases.py
index ba80a2c4..5879f5be 100644
--- a/datafiles/converters/_bases.py
+++ b/datafiles/converters/_bases.py
@@ -7,7 +7,7 @@ class Converter:
     """Base class for immutable attribute conversion."""
 
     TYPE: type = object
-    DEFAULT: Any = None
+    DEFAULT: Any = NotImplemented
 
     @classmethod
     def as_optional(cls):
diff --git a/datafiles/converters/containers.py b/datafiles/converters/containers.py
index 1e6099e2..89e20520 100644
--- a/datafiles/converters/containers.py
+++ b/datafiles/converters/containers.py
@@ -11,10 +11,10 @@
 class List(Converter):
     """Base converter for homogeneous lists of another converter."""
 
-    CONVERTER: Converter = None  # type: ignore
+    CONVERTER: Converter = NotImplemented
 
     @classmethod
-    def subclass(cls, converter: type):
+    def of_type(cls, converter: type):
         name = f'{converter.__name__}List'
         bases = (cls,)
         attributes = {'CONVERTER': converter}
@@ -87,7 +87,7 @@ class Dictionary(Converter):
     """Base converter for raw dictionaries."""
 
     @classmethod
-    def subclass(cls, key: type, value: type):
+    def of_mapping(cls, key: type, value: type):
         name = f'{key.__name__}{value.__name__}Dict'
         bases = (cls,)
         return type(name, bases, {})
@@ -121,11 +121,11 @@ def to_preserialization_data(cls, python_value, *, default_to_skip=None):
 class Dataclass(Converter):
     """Base converter for dataclasses."""
 
-    DATACLASS: Callable = None  # type: ignore
-    CONVERTERS: Dict = None  # type: ignore
+    DATACLASS: Callable = NotImplemented
+    CONVERTERS: Dict = NotImplemented
 
     @classmethod
-    def subclass(cls, dataclass, converters: Dict[str, type]):
+    def of_mappings(cls, dataclass, converters: Dict[str, type]):
         name = f'{dataclass.__name__}Converter'
         bases = (cls,)
         attributes = {'DATACLASS': dataclass, 'CONVERTERS': converters}
@@ -138,6 +138,9 @@ def to_python_value(cls, deserialized_data, *, target_object):
         else:
             data = {}
 
+        if deserialized_data is None and cls.DEFAULT is None:
+            return None
+
         for name, value in list(data.items()):
             if name not in cls.CONVERTERS:
                 log.debug(f'Removed unmapped nested file attribute: {name}')
@@ -171,19 +174,22 @@ def to_python_value(cls, deserialized_data, *, target_object):
     def to_preserialization_data(cls, python_value, *, default_to_skip=None):
         data = {}
 
+        if python_value is None and cls.DEFAULT is None:
+            return None
+
         for name, converter in cls.CONVERTERS.items():
 
             if isinstance(python_value, dict):
                 try:
                     value = python_value[name]
-                except KeyError as e:
-                    log.debug(e)
+                except KeyError:
+                    log.debug(f'Added missing nested attribute: {name}')
                     value = None
             else:
                 try:
                     value = getattr(python_value, name)
-                except AttributeError as e:
-                    log.debug(e)
+                except AttributeError:
+                    log.debug(f'Added missing nested attribute: {name}')
                     value = None
 
             with suppress(AttributeError):
diff --git a/datafiles/converters/enumerations.py b/datafiles/converters/enumerations.py
new file mode 100644
index 00000000..c4a67461
--- /dev/null
+++ b/datafiles/converters/enumerations.py
@@ -0,0 +1,23 @@
+# pylint: disable=unused-argument,not-callable
+
+from ._bases import Converter
+
+
+class Enumeration(Converter):
+
+    ENUM: type = NotImplemented
+
+    @classmethod
+    def of_type(cls, enum: type):
+        name = f'{enum.__name__}Converter'
+        bases = (cls,)
+        attributes = {'ENUM': enum}
+        return type(name, bases, attributes)
+
+    @classmethod
+    def to_python_value(cls, deserialized_data, *, target_object=None):
+        return cls.ENUM(deserialized_data)
+
+    @classmethod
+    def to_preserialization_data(cls, python_value, *, default_to_skip=None):
+        return python_value.value
diff --git a/datafiles/formats.py b/datafiles/formats.py
index 704ec364..20c87e91 100644
--- a/datafiles/formats.py
+++ b/datafiles/formats.py
@@ -9,7 +9,7 @@
 
 import log
 
-from . import settings
+from . import settings, types
 
 
 _REGISTRY: Dict[str, type] = {}
@@ -87,15 +87,24 @@ def deserialize(cls, file_object):
         from ruamel import yaml
 
         try:
-            return yaml.round_trip_load(file_object, preserve_quotes=True) or {}
+            data = yaml.round_trip_load(file_object, preserve_quotes=True)
         except NotImplementedError as e:
             log.error(str(e))
             return {}
+        else:
+            return data or {}
 
     @classmethod
     def serialize(cls, data):
         from ruamel import yaml
 
+        yaml.representer.RoundTripRepresenter.add_representer(
+            types.List, yaml.representer.RoundTripRepresenter.represent_list
+        )
+        yaml.representer.RoundTripRepresenter.add_representer(
+            types.Dict, yaml.representer.RoundTripRepresenter.represent_dict
+        )
+
         if settings.INDENT_YAML_BLOCKS:
             f = StringIO()
             y = yaml.YAML()
@@ -104,8 +113,11 @@ def serialize(cls, data):
             text = f.getvalue().strip() + '\n'
         else:
             text = yaml.round_trip_dump(data) or ""
-            text = text.replace('- \n', '-\n')
-        return "" if text == "{}\n" else text
+
+        if text == "{}\n":
+            return ""
+
+        return text.replace('- \n', '-\n')
 
 
 class PyYAML(Formatter):
@@ -144,19 +156,21 @@ def increase_indent(self, flow=False, indentless=False):
         return text
 
 
-def deserialize(path: Path, extension: str) -> Dict:
-    formatter = _get_formatter(extension)
+def deserialize(path: Path, extension: str, *, formatter=None) -> Dict:
+    if formatter is None:
+        formatter = _get_formatter(extension)
     with path.open('r') as file_object:
         return formatter.deserialize(file_object)
 
 
-def serialize(data: Dict, extension: str = '.yml') -> str:
-    formatter = _get_formatter(extension)
+def serialize(data: Dict, extension: str = '.yml', *, formatter=None) -> str:
+    if formatter is None:
+        formatter = _get_formatter(extension)
     return formatter.serialize(data)
 
 
 def _get_formatter(extension: str):
-    if settings.YAML_LIBRARY == 'PyYAML':
+    if settings.YAML_LIBRARY == 'PyYAML':  # pragma: no cover
         register('.yml', PyYAML)
 
     with suppress(KeyError):
diff --git a/datafiles/hooks.py b/datafiles/hooks.py
index dcf44454..61c7f0da 100644
--- a/datafiles/hooks.py
+++ b/datafiles/hooks.py
@@ -4,7 +4,7 @@
 
 import log
 
-from . import settings
+from . import settings, types
 from .mapper import create_mapper
 
 
@@ -27,14 +27,6 @@
 FLAG = '_patched'
 
 
-class List(list):
-    """Patchable `list` type."""
-
-
-class Dict(dict):
-    """Patchable `dict` type."""
-
-
 def apply(instance, mapper):
     """Path methods that get or set attributes."""
     cls = instance.__class__
@@ -56,10 +48,10 @@ def apply(instance, mapper):
         for attr_name in instance.datafile.attrs:
             attr = getattr(instance, attr_name)
             if isinstance(attr, list):
-                attr = List(attr)
+                attr = types.List(attr)
                 setattr(instance, attr_name, attr)
             elif isinstance(attr, dict):
-                attr = Dict(attr)
+                attr = types.Dict(attr)
                 setattr(instance, attr_name, attr)
             elif not is_dataclass(attr):
                 continue
diff --git a/datafiles/mapper.py b/datafiles/mapper.py
index e8d19bd4..3bd41c9f 100644
--- a/datafiles/mapper.py
+++ b/datafiles/mapper.py
@@ -13,14 +13,8 @@
 
 from . import config, formats, hooks
 from .converters import Converter, List, map_type
-from .utils import (
-    Missing,
-    Trilean,
-    display,
-    get_default_field_value,
-    recursive_update,
-    write,
-)
+from .types import Missing, Trilean
+from .utils import display, get_default_field_value, recursive_update, write
 
 
 class Mapper:
@@ -138,20 +132,13 @@ def _get_data(self, include_default_values: Trilean = None) -> Dict:
 
             if getattr(converter, 'DATACLASS', None):
                 log.debug(f"Converting '{name}' dataclass with {converter}")
-                if value is None:
-                    value = {}
-
-                for field in dataclasses.fields(converter.DATACLASS):
-                    if field.name not in value:
-                        log.debug(f'Added missing nested attribute: {field.name}')
-                        value[field.name] = None
-
-                data[name] = converter.to_preserialization_data(
+                new_value = converter.to_preserialization_data(
                     value,
                     default_to_skip=Missing
                     if include_default_values
                     else get_default_field_value(self._instance, name),
                 )
+                data[name] = recursive_update(value, new_value)
 
             elif (
                 value == get_default_field_value(self._instance, name)
@@ -171,16 +158,16 @@ def _get_data(self, include_default_values: Trilean = None) -> Dict:
     def text(self) -> str:
         return self._get_text()
 
+    @text.setter
+    def text(self, value: str):
+        write(self.path, value.strip() + '\n', display=True)
+
     def _get_text(self, **kwargs) -> str:
         data = self._get_data(**kwargs)
         if self.path and self.path.suffix:
             return formats.serialize(data, self.path.suffix)
         return formats.serialize(data)
 
-    @text.setter  # type: ignore
-    def text(self, value: str):
-        write(self.path, value.strip() + '\n')
-
     def load(self, *, _log=True, _first_load=False) -> None:
         if self._root:
             self._root.load(_log=_log, _first_load=_first_load)
@@ -282,7 +269,7 @@ def save(self, *, include_default_values: Trilean = None, _log=True) -> None:
         with hooks.disabled():
             text = self._get_text(include_default_values=include_default_values)
 
-        write(self.path, text)
+        write(self.path, text, display=True)
 
         self.modified = False
 
diff --git a/datafiles/tests/test_converters.py b/datafiles/tests/test_converters.py
index 2901ceb0..d2a045b1 100644
--- a/datafiles/tests/test_converters.py
+++ b/datafiles/tests/test_converters.py
@@ -1,6 +1,7 @@
 # pylint: disable=unused-variable
 
 from dataclasses import dataclass
+from enum import Enum
 from typing import ByteString, Dict, List, Optional
 
 import pytest
@@ -29,12 +30,17 @@ class MyCustomString:
     pass
 
 
-IntegerList = converters.List.subclass(converters.Integer)
-StringList = converters.List.subclass(converters.String)
-MyDict = converters.Dictionary.subclass(converters.String, converters.Integer)
+class Color(Enum):
+    RED = 1
+    GREEN = 2
+    BLUE = 3
+
+
+IntegerList = converters.List.of_type(converters.Integer)
+StringList = converters.List.of_type(converters.String)
+MyDict = converters.Dictionary.of_mapping(converters.String, converters.Integer)
 MyDataclassConverter = converters.map_type(MyDataclass)
 MyDataclassConverterList = converters.map_type(List[MyDataclass])
-MyNestedDataclassConverter = converters.map_type(MyNestedDataclass)
 
 
 def describe_map_type():
@@ -68,11 +74,15 @@ def it_handles_dataclasses(expect):
             'flag': converters.Boolean,
         }
 
+    def it_handles_enums(expect):
+        converter = converters.map_type(Color)
+        expect(converter.__name__) == 'ColorConverter'
+
     def it_handles_optionals(expect):
         converter = converters.map_type(Optional[str])
         expect(converter.__name__) == 'OptionalString'
         expect(converter.TYPE) == str
-        expect(converter.DEFAULT) == None
+        expect(converter.DEFAULT).is_(None)
 
     def it_handles_string_type_annotations(expect):
         converter = converters.map_type('float')
@@ -146,11 +156,6 @@ def when_immutable(expect, converter, data, value):
                 (MyDataclassConverter, None, MyDataclass(foobar=0)),
                 (MyDataclassConverterList, None, []),
                 (MyDataclassConverterList, 42, [MyDataclass(foobar=0)]),
-                (
-                    MyNestedDataclassConverter,
-                    None,
-                    MyNestedDataclass(name='', dc=MyDataclass(foobar=0, flag=False)),
-                ),
             ],
         )
         def when_mutable(expect, converter, data, value):
@@ -167,6 +172,12 @@ def when_text(expect):
             expect(convert("Hello, world!")) == "Hello, world!"
             expect(convert("Line 1\nLine 2\n")) == "Line 1\nLine 2\n"
 
+        def when_enum(expect):
+            convert = converters.map_type(Color).to_python_value
+            expect(convert(1)).is_(Color.RED)
+            with expect.raises(ValueError):
+                convert(42)
+
         def when_invalid(expect):
             message = "invalid literal for int() with base 10: 'a'"
             with expect.raises(ValueError, message):
@@ -245,6 +256,10 @@ def when_text(expect):
             expect(convert("Line 1\nLine 2")) == "Line 1\nLine 2\n"
             expect(convert("Line 1\nLine 2")).isinstance(LiteralScalarString)
 
+        def when_enum(expect):
+            convert = converters.map_type(Color).to_preserialization_data
+            expect(convert(Color.RED)) == 1
+
         def when_invalid(expect):
             message = "invalid literal for int() with base 10: 'a'"
             with expect.raises(ValueError, message):
diff --git a/datafiles/tests/test_decorators.py b/datafiles/tests/test_decorators.py
index 2f4707aa..8cefeafe 100644
--- a/datafiles/tests/test_decorators.py
+++ b/datafiles/tests/test_decorators.py
@@ -12,7 +12,7 @@ class Normal:
 
         cls = decorators.datafile("<pattern>")(Normal)
 
-        expect(is_dataclass(cls)) == True
+        expect(is_dataclass(cls)).is_(True)
 
     def it_can_reuse_existing_dataclass(expect):
         @dataclass
@@ -29,7 +29,7 @@ class Sample:
 
         cls = decorators.datafile(Sample)
 
-        expect(is_dataclass(cls)) == True
+        expect(is_dataclass(cls)).is_(True)
 
     def it_forwards_arguments_dataclass_decorator(expect):
         class Sample:
@@ -37,4 +37,4 @@ class Sample:
 
         cls = decorators.datafile(order=True)(Sample)
 
-        expect(is_dataclass(cls)) == True
+        expect(is_dataclass(cls)).is_(True)
diff --git a/datafiles/tests/test_formats.py b/datafiles/tests/test_formats.py
index 6cac2472..94228db5 100644
--- a/datafiles/tests/test_formats.py
+++ b/datafiles/tests/test_formats.py
@@ -13,7 +13,7 @@ def data():
 
     def describe_ruamel_yaml():
         def it_indents_blocks_by_default(expect, data):
-            text = formats.RuamelYAML.serialize(data)
+            text = formats.serialize(data, '.yaml')
             expect(text) == dedent(
                 """
             key: value
@@ -26,7 +26,7 @@ def it_indents_blocks_by_default(expect, data):
 
         def it_can_render_lists_inline(expect, data, monkeypatch):
             monkeypatch.setattr(settings, 'INDENT_YAML_BLOCKS', False)
-            text = formats.RuamelYAML.serialize(data)
+            text = formats.serialize(data, '.yaml')
             expect(text) == dedent(
                 """
             key: value
@@ -39,7 +39,7 @@ def it_can_render_lists_inline(expect, data, monkeypatch):
 
     def describe_pyyaml():
         def it_indents_blocks_by_default(expect, data):
-            text = formats.PyYAML.serialize(data)
+            text = formats.serialize(data, '.yaml', formatter=formats.PyYAML)
             expect(text) == dedent(
                 """
             key: value
@@ -52,7 +52,7 @@ def it_indents_blocks_by_default(expect, data):
 
         def it_can_render_lists_inline(expect, data, monkeypatch):
             monkeypatch.setattr(settings, 'INDENT_YAML_BLOCKS', False)
-            text = formats.PyYAML.serialize(data)
+            text = formats.serialize(data, '.yaml', formatter=formats.PyYAML)
             expect(text) == dedent(
                 """
             key: value
@@ -71,18 +71,26 @@ def path(tmp_path):
         path.write_text("")
         return path
 
-    def with_empty_yaml_file(expect, path):
-        data = formats.deserialize(path, '.yaml')
-        expect(data) == {}
+    def describe_ruamel_yaml():
+        def with_empty_file(expect, path):
+            data = formats.deserialize(path, '.yaml')
+            expect(data) == {}
+
+    def describe_pyyaml():
+        def with_empty_file(expect, path):
+            data = formats.deserialize(path, '.yaml', formatter=formats.PyYAML)
+            expect(data) == {}
 
-    def with_empty_json_file(expect, path):
-        path.write_text("{}")
-        data = formats.deserialize(path, '.json')
-        expect(data) == {}
+    def describe_json():
+        def with_empty_file(expect, path):
+            path.write_text("{}")
+            data = formats.deserialize(path, '.json')
+            expect(data) == {}
 
-    def with_empty_toml_file(expect, path):
-        data = formats.deserialize(path, '.toml')
-        expect(data) == {}
+    def describe_toml():
+        def with_empty_file(expect, path):
+            data = formats.deserialize(path, '.toml')
+            expect(data) == {}
 
     def with_unknown_extension(expect, path):
         with expect.raises(ValueError):
diff --git a/datafiles/tests/test_hooks.py b/datafiles/tests/test_hooks.py
index 903046a4..9fca5b19 100644
--- a/datafiles/tests/test_hooks.py
+++ b/datafiles/tests/test_hooks.py
@@ -22,29 +22,29 @@ def it_can_be_called_twice(expect, mocker):
         setattr(instance, 'datafile', mocker.MagicMock(attrs=['key', 'items']))
 
         hooks.apply(instance, None)
-        expect(hasattr(instance.__setattr__, '_patched')) == True
+        expect(hasattr(instance.__setattr__, '_patched')).is_(True)
 
         hooks.apply(instance, None)
-        expect(hasattr(instance.__setattr__, '_patched')) == True
+        expect(hasattr(instance.__setattr__, '_patched')).is_(True)
 
     def it_patches_list_elements(expect, mocker):
         instance = Sample(items=[Item('a'), Item('b')])
         setattr(instance, 'datafile', mocker.MagicMock(attrs=['key', 'items']))
 
         hooks.apply(instance, None)
-        expect(hasattr(instance.items[0].__setattr__, '_patched')) == True
+        expect(hasattr(instance.items[0].__setattr__, '_patched')).is_(True)
 
 
 def describe_disabled():
     def when_nested(expect):
-        expect(settings.HOOKS_ENABLED) == True
+        expect(settings.HOOKS_ENABLED).is_(True)
 
         with hooks.disabled():
-            expect(settings.HOOKS_ENABLED) == False
+            expect(settings.HOOKS_ENABLED).is_(False)
 
             with hooks.disabled():
-                expect(settings.HOOKS_ENABLED) == False
+                expect(settings.HOOKS_ENABLED).is_(False)
 
-            expect(settings.HOOKS_ENABLED) == False
+            expect(settings.HOOKS_ENABLED).is_(False)
 
-        expect(settings.HOOKS_ENABLED) == True
+        expect(settings.HOOKS_ENABLED).is_(True)
diff --git a/datafiles/tests/test_manager.py b/datafiles/tests/test_manager.py
index 22a4465e..c19a836b 100644
--- a/datafiles/tests/test_manager.py
+++ b/datafiles/tests/test_manager.py
@@ -27,11 +27,11 @@ def describe_get_or_none():
         @patch('datafiles.mapper.Mapper.modified', False)
         def when_file_exists(mock_load, expect, manager):
             expect(manager.get_or_none(foo=1, bar=2)) == MyClass(foo=1, bar=2)
-            expect(mock_load.called) == True
+            expect(mock_load.called).is_(True)
 
         @patch('datafiles.mapper.Mapper.exists', False)
         def when_file_missing(expect, manager):
-            expect(manager.get_or_none(foo=3, bar=4)) == None
+            expect(manager.get_or_none(foo=3, bar=4)).is_(None)
 
     def describe_get_or_create():
         @patch('datafiles.mapper.Mapper.load')
@@ -40,14 +40,14 @@ def describe_get_or_create():
         @patch('datafiles.mapper.Mapper.modified', False)
         def when_file_exists(mock_load, mock_save, expect, manager):
             expect(manager.get_or_create(foo=1, bar=2)) == MyClass(foo=1, bar=2)
-            expect(mock_load.called) == False
-            expect(mock_save.called) == True
+            expect(mock_load.called).is_(False)
+            expect(mock_save.called).is_(True)
 
         @patch('datafiles.mapper.Mapper.save')
         @patch('datafiles.mapper.Mapper.exists', False)
         def when_file_missing(mock_save, expect, manager):
             expect(manager.get_or_create(foo=1, bar=2)) == MyClass(foo=1, bar=2)
-            expect(mock_save.called) == True
+            expect(mock_save.called).is_(True)
 
     def describe_all():
         @patch('datafiles.mapper.Mapper.exists', False)
diff --git a/datafiles/tests/test_mapper.py b/datafiles/tests/test_mapper.py
index 236a6059..2f5cfb83 100644
--- a/datafiles/tests/test_mapper.py
+++ b/datafiles/tests/test_mapper.py
@@ -37,7 +37,7 @@ def mapper():
 
     def describe_path():
         def is_none_when_no_pattern(expect, mapper):
-            expect(mapper.path) == None
+            expect(mapper.path).is_(None)
 
         def is_relative_to_file_by_default(expect, mapper):
             mapper._pattern = '../../tmp/sample.yml'
diff --git a/datafiles/tests/test_utils.py b/datafiles/tests/test_utils.py
index 868f63d8..dd4e5a49 100644
--- a/datafiles/tests/test_utils.py
+++ b/datafiles/tests/test_utils.py
@@ -6,48 +6,96 @@
 
 
 def describe_recursive_update():
-    def it_preserves_root_id(expect):
-        old: Dict = {}
-        new = {'a': 1}
-        id_ = id(old)
+    def describe_id_preservation():
+        def with_dict(expect):
+            old = {'my_dict': {'a': 1}}
+            new = {'my_dict': {'a': 2}}
+            previous_id = id(old['my_dict'])
 
-        old = recursive_update(old, new)
+            recursive_update(old, new)
 
-        expect(old) == new
-        expect(id(old)) == id_
+            expect(old) == new
+            expect(id(old['my_dict'])) == previous_id
 
-    def it_preserves_nested_dict_id(expect):
-        old = {'a': {'b': 1}}
-        new = {'a': {'b': 2}}
-        id_ = id(old['a'])
+        def with_dict_value(expect):
+            old = {'my_dict': {'my_nested_dict': {'a': 1}}}
+            new = {'my_dict': {'my_nested_dict': {'a': 2}}}
+            previous_id = id(old['my_dict']['my_nested_dict'])
 
-        old = recursive_update(old, new)
+            recursive_update(old, new)
 
-        expect(old) == new
-        expect(id(old['a'])) == id_
+            expect(old) == new
+            expect(id(old['my_dict']['my_nested_dict'])) == previous_id
 
-    def it_preserves_nested_list_id(expect):
-        old = {'a': [1]}
-        new = {'a': [2]}
-        id_ = id(old['a'])
+        def with_list(expect):
+            old = {'my_list': [1]}
+            new = {'my_list': [2]}
+            previous_id = id(old['my_list'])
 
-        old = recursive_update(old, new)
+            recursive_update(old, new)
 
-        expect(old) == new
-        expect(id(old['a'])) == id_
+            expect(old) == new
+            expect(id(old['my_list'])) == previous_id
 
-    def it_adds_missing_dict(expect):
-        old: Dict = {}
-        new = {'a': {'b': 2}}
+        def with_list_item(expect):
+            old = {'my_list': [{'name': "John"}]}
+            new = {'my_list': [{'name': "Jane"}]}
+            previous_id = id(old['my_list'][0])
 
-        old = recursive_update(old, new)
+            recursive_update(old, new)
 
-        expect(old) == new
+            expect(old) == new
+            expect(id(old['my_list'][0])) == previous_id
 
-    def it_adds_missing_list(expect):
-        old: Dict = {}
-        new = {'a': [1]}
+        def with_nested_list(expect):
+            old = {'my_dict': {'my_list': [{'name': "John"}]}}
+            new = {'my_dict': {'my_list': [{'name': "Jane"}]}}
+            previous_id = id(old['my_dict']['my_list'][0])
 
-        old = recursive_update(old, new)
+            recursive_update(old, new)
 
-        expect(old) == new
+            expect(old) == new
+            expect(id(old['my_dict']['my_list'][0])) == previous_id
+
+    def describe_merge():
+        def with_shoreter_list_into_longer(expect):
+            old = {'my_list': [1, 2, 3]}
+            new = {'my_list': [5, 6]}
+
+            recursive_update(old, new)
+
+            expect(old) == new
+
+        def with_longer_list_into_shorter(expect):
+            old = {'my_list': [1, 2]}
+            new = {'my_list': [3, 4, 5]}
+
+            recursive_update(old, new)
+
+            expect(old) == new
+
+    def describe_missing():
+        def with_dict(expect):
+            old: Dict = {}
+            new = {'my_dict': {'a': 1}}
+
+            recursive_update(old, new)
+
+            expect(old) == new
+
+        def with_list(expect):
+            old: Dict = {}
+            new = {'my_list': [1]}
+
+            recursive_update(old, new)
+
+            expect(old) == new
+
+    def describe_extra():
+        def with_dict(expect):
+            old = {'my_dict': {'a': 1, 'b': 2}}
+            new = {'my_dict': {'a': 1}}
+
+            recursive_update(old, new)
+
+            expect(old) == new
diff --git a/datafiles/types.py b/datafiles/types.py
new file mode 100644
index 00000000..dcd1b100
--- /dev/null
+++ b/datafiles/types.py
@@ -0,0 +1,14 @@
+import dataclasses
+from typing import Optional
+
+
+Trilean = Optional[bool]
+Missing = dataclasses._MISSING_TYPE
+
+
+class List(list):
+    """Patchable `list` type."""
+
+
+class Dict(dict):
+    """Patchable `dict` type."""
diff --git a/datafiles/utils.py b/datafiles/utils.py
index 7b4ed1d4..94e55c9b 100644
--- a/datafiles/utils.py
+++ b/datafiles/utils.py
@@ -6,13 +6,11 @@
 from pathlib import Path
 from pprint import pformat
 from shutil import get_terminal_size
-from typing import Dict, Optional, Union
+from typing import Any, Dict, Union
 
 import log
 
-
-Trilean = Optional[bool]
-Missing = dataclasses._MISSING_TYPE
+from .types import Missing
 
 
 cached = lru_cache()
@@ -52,24 +50,40 @@ def dictify(value):
     return value
 
 
-def recursive_update(old: Dict, new: Dict):
-    """Recursively update a dictionary."""
+def recursive_update(old: Dict, new: Dict) -> Dict:
+    """Recursively update a dictionary, keeping equivalent objects."""
+    return _merge(old, new)
+
+
+def _merge(old: Any, new: Any) -> Any:
+    if old is None:
+        return new
+
+    if isinstance(new, dict):
+        for key, value in new.items():
+            old[key] = _merge(old.get(key), value)
+
+        for key in list(old.keys()):
+            if key not in new:
+                old.pop(key)
 
-    for key, value in new.items():
-        if isinstance(value, dict):
-            if key in old:
-                recursive_update(old[key], value)
-            else:
-                old[key] = value
-        elif isinstance(value, list):
-            if key in old:
-                old[key][:] = value
-            else:
-                old[key] = value
-        else:
-            old[key] = value
+        return old
 
-    return old
+    if isinstance(new, list):
+        for index, new_item in enumerate(new):
+            try:
+                old_item = old[index]
+            except IndexError:
+                old_item = None
+                old.append(old_item)
+            old[index] = _merge(old_item, new_item)
+
+        while len(old) > len(new):
+            old.pop()
+
+        return old
+
+    return new
 
 
 def dedent(text: str) -> str:
@@ -79,8 +93,8 @@ def dedent(text: str) -> str:
     return text.replace('    ' * indent, '')
 
 
-def write(filename_or_path: Union[str, Path], text: str) -> None:
-    """Write text to a given file with logging."""
+def write(filename_or_path: Union[str, Path], text: str, *, display=False) -> None:
+    """Write text to a given file and optionally log it."""
     if isinstance(filename_or_path, Path):
         path = filename_or_path
     else:
@@ -93,14 +107,17 @@ def write(filename_or_path: Union[str, Path], text: str) -> None:
         content = text.replace(' \n', '␠\n')
     else:
         content = '∅\n'
-    log.debug(message + '\n' + line + '\n' + content + line)
+    if display:
+        log.debug(message + '\n' + line + '\n' + content + line)
+    else:
+        log.critical(message)
 
     path.parent.mkdir(parents=True, exist_ok=True)
     path.write_text(text)
 
 
-def read(filename: str) -> str:
-    """Read text from a file with logging."""
+def read(filename: str, *, display=False) -> str:
+    """Read text from a file and optionally log it."""
     path = Path(filename).resolve()
     message = f'Reading file: {path}'
     line = '=' * (31 + len(message))
@@ -109,7 +126,10 @@ def read(filename: str) -> str:
         content = text.replace(' \n', '␠\n')
     else:
         content = '∅\n'
-    log.debug(message + '\n' + line + '\n' + content + line)
+    if display:
+        log.debug(message + '\n' + line + '\n' + content + line)
+    else:
+        log.critical(message)
     return text
 
 
diff --git a/docs/types/builtins.md b/docs/types/builtins.md
index 41b468d0..6382457c 100644
--- a/docs/types/builtins.md
+++ b/docs/types/builtins.md
@@ -8,36 +8,53 @@ from typing import Optional
 
 ## Booleans
 
-| Type Annotation | Python Value | YAML Content |
-| --- | --- | --- |
-| `foobar: bool` | `foobar = True` | `foobar: true` |
-| `foobar: bool` | `foobar = False` | `foobar: false` |
-| `foobar: bool` | `foobar = None` | `foobar: false` |
-| `foobar: Optional[bool]` | `foobar = False` | `foobar: ` |
+| Type Annotation          | Python Value     | YAML Content    |
+| ------------------------ | ---------------- | --------------- |
+| `foobar: bool`           | `foobar = True`  | `foobar: true`  |
+| `foobar: bool`           | `foobar = False` | `foobar: false` |
+| `foobar: bool`           | `foobar = None`  | `foobar: false` |
+| `foobar: Optional[bool]` | `foobar = False` | `foobar:`       |
 
 ## Integers
 
-| Type Annotation | Python Value | YAML Content |
-| --- | --- | --- |
-| `foobar: int` | `foobar = 42` | `foobar: 42` |
-| `foobar: int` | `foobar = 1.23` | `foobar: 1` |
-| `foobar: int` | `foobar = None` | `foobar: 0` |
-| `foobar: Optional[int]` | `foobar = None` | `foobar: ` |
+| Type Annotation         | Python Value    | YAML Content |
+| ----------------------- | --------------- | ------------ |
+| `foobar: int`           | `foobar = 42`   | `foobar: 42` |
+| `foobar: int`           | `foobar = 1.23` | `foobar: 1`  |
+| `foobar: int`           | `foobar = None` | `foobar: 0`  |
+| `foobar: Optional[int]` | `foobar = None` | `foobar:`    |
 
 ## Floats
 
-| Type Annotation | Python Value | YAML Content |
-| --- | --- | --- |
-| `foobar: float` | `foobar = 1.23` | `foobar: 1.23` |
-| `foobar: float` | `foobar = 42` | `foobar: 42.0` |
-| `foobar: float` | `foobar = None` | `foobar: 0.0` |
-| `foobar: Optional[float]` | `foobar = None` | `foobar: ` |
+| Type Annotation           | Python Value    | YAML Content   |
+| ------------------------- | --------------- | -------------- |
+| `foobar: float`           | `foobar = 1.23` | `foobar: 1.23` |
+| `foobar: float`           | `foobar = 42`   | `foobar: 42.0` |
+| `foobar: float`           | `foobar = None` | `foobar: 0.0`  |
+| `foobar: Optional[float]` | `foobar = None` | `foobar:`      |
 
 ## Strings
 
-| Type Annotation | Python Value | YAML Content |
-| --- | --- | --- |
-| `foobar: str` | `foobar = "Hello, world!"` | `foobar: Hello, world!` |
-| `foobar: str` | `foobar = 42` | `foobar: '42'` |
-| `foobar: str` | `foobar = None` | `foobar: ''` |
-| `foobar: Optional[str]` | `foobar = None` | `foobar: ` |
+| Type Annotation         | Python Value               | YAML Content            |
+| ----------------------- | -------------------------- | ----------------------- |
+| `foobar: str`           | `foobar = "Hello, world!"` | `foobar: Hello, world!` |
+| `foobar: str`           | `foobar = 42`              | `foobar: '42'`          |
+| `foobar: str`           | `foobar = None`            | `foobar: ''`            |
+| `foobar: Optional[str]` | `foobar = None`            | `foobar:`               |
+
+## Enumerations
+
+Subclasses of `enum.Enum` can also be used as type annotations:
+
+```python
+from enum import Enum
+
+class Color:
+    RED = 1
+    GREEN = 2
+    BLUE = 3
+```
+
+| Type Annotation | Python Value         | YAML Content |
+| --------------- | -------------------- | ------------ |
+| `color: Color`  | `color = Color.BLUE` | `color: 3`   |
diff --git a/docs/types/extensions.md b/docs/types/extensions.md
index 72680f31..f96340e1 100644
--- a/docs/types/extensions.md
+++ b/docs/types/extensions.md
@@ -12,12 +12,12 @@ from typing import Optional
 from datafiles.converters import Number
 ```
 
-| Type Annotation | Python Value | YAML Content |
-| --- | --- | --- |
-| `foobar: Number` | `foobar = 42` | `foobar: 42` |
-| `foobar: Number` | `foobar = 1.23` | `foobar: 1.23` |
-| `foobar: Number` | `foobar = None` | `foobar: 0.0` |
-| `foobar: Optional[Number]` | `foobar = None` | `foobar: ` |
+| Type Annotation            | Python Value    | YAML Content   |
+| -------------------------- | --------------- | -------------- |
+| `foobar: Number`           | `foobar = 42`   | `foobar: 42`   |
+| `foobar: Number`           | `foobar = 1.23` | `foobar: 1.23` |
+| `foobar: Number`           | `foobar = None` | `foobar: 0.0`  |
+| `foobar: Optional[Number]` | `foobar = None` | `foobar:`      |
 
 ## Text
 
@@ -29,9 +29,9 @@ from typing import Optional
 from datafiles.converters import Text
 ```
 
-| Type Annotation | Python Value | YAML Content |
-| --- | --- | --- |
-| `foobar: Text` | `foobar = "Hello, world!"` | `foobar: Hello, world!` |
-| `foobar: Text` | `foobar = "First\nSecond\n"` | `foobar: | `<br>&nbsp;&nbsp;&nbsp;&nbsp;`First`<br>&nbsp;&nbsp;&nbsp;&nbsp;`Second` |
-| `foobar: Text` | `foobar = None` | `foobar: ''` |
-| `foobar: Optional[Text]` | `foobar = None` | `foobar: ` |
+| Type Annotation          | Python Value                 | YAML Content                                                                       |
+| ------------------------ | ---------------------------- | ---------------------------------------------------------------------------------- |
+| `foobar: Text`           | `foobar = "Hello, world!"`   | `foobar: Hello, world!`                                                            |
+| `foobar: Text`           | `foobar = "First\nSecond\n"` | `foobar: |`<br>&nbsp;&nbsp;&nbsp;&nbsp;`First`<br>&nbsp;&nbsp;&nbsp;&nbsp;`Second` |
+| `foobar: Text`           | `foobar = None`              | `foobar: ''`                                                                       |
+| `foobar: Optional[Text]` | `foobar = None`              | `foobar:`                                                                          |
diff --git a/mkdocs.yml b/mkdocs.yml
index 6699505b..7e40a349 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -18,8 +18,8 @@ nav:
       - Mapper: api/mapper.md
   - Supported Types:
       - Builtins: types/builtins.md
-      - Extensions: types/extensions.md
       - Containers: types/containers.md
+      - Extended Types: types/extensions.md
       - Custom Types: types/custom.md
   - File Formats: formats.md
   - Utilities: utilities.md
diff --git a/notebooks/file_inference.ipynb b/notebooks/file_inference.ipynb
index 58cedf7f..fca59460 100644
--- a/notebooks/file_inference.ipynb
+++ b/notebooks/file_inference.ipynb
@@ -157,7 +157,7 @@
       "script:\n",
       "  - make test-repeat\n",
       "  - make check\n",
-      "  - make mkdocs\n",
+      "  - make docs\n",
       "\n",
       "after_success:\n",
       "  - pip install coveralls\n",
@@ -237,7 +237,9 @@
       "  - 3.8\n",
       "cache:\n",
       "  pip: true\n",
-      "  directories: []\n",
+      "  directories:\n",
+      "    - ${VIRTUAL_ENV}\n",
+      "\n",
       "before_install:\n",
       "  - curl -sSL https://raw.githubusercontent.com/sdispater/poetry/master/get-poetry.py\n",
       "    | python\n",
@@ -249,7 +251,7 @@
       "script:\n",
       "  - make test-repeat\n",
       "  - make check\n",
-      "  - make mkdocs\n",
+      "  - make docs\n",
       "after_success:\n",
       "  - pip install coveralls\n",
       "  - coveralls\n",
@@ -283,7 +285,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.2"
+   "version": "3.8.2"
   }
  },
  "nbformat": 4,
diff --git a/notebooks/format_options.ipynb b/notebooks/format_options.ipynb
index 2a820d46..59af66c2 100644
--- a/notebooks/format_options.ipynb
+++ b/notebooks/format_options.ipynb
@@ -256,7 +256,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.2"
+   "version": "3.8.2"
   }
  },
  "nbformat": 4,
diff --git a/notebooks/manager_api.ipynb b/notebooks/manager_api.ipynb
index d6496bbc..0648d764 100644
--- a/notebooks/manager_api.ipynb
+++ b/notebooks/manager_api.ipynb
@@ -331,7 +331,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.2"
+   "version": "3.8.2"
   }
  },
  "nbformat": 4,
diff --git a/notebooks/mapper_api.ipynb b/notebooks/mapper_api.ipynb
index db04dfea..3ec4df3d 100644
--- a/notebooks/mapper_api.ipynb
+++ b/notebooks/mapper_api.ipynb
@@ -325,7 +325,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.2"
+   "version": "3.8.2"
   }
  },
  "nbformat": 4,
diff --git a/notebooks/nested_dataclass.ipynb b/notebooks/nested_dataclass.ipynb
index b069e54b..b4db7fb2 100644
--- a/notebooks/nested_dataclass.ipynb
+++ b/notebooks/nested_dataclass.ipynb
@@ -19,7 +19,7 @@
    "source": [
     "%%sh\n",
     "\n",
-    "rm -f files/nested_values.yml"
+    "rm -f files/nested_dataclass.yml"
    ]
   },
   {
@@ -65,7 +65,7 @@
      "name": "stderr",
      "output_type": "stream",
      "text": [
-      "INFO: datafiles.mapper: Loading 'Sample' object from 'files/nested_dataclass.yml'\n"
+      "INFO: datafiles.mapper: Saving 'Sample' object to 'files/nested_dataclass.yml'\n"
      ]
     }
    ],
@@ -87,7 +87,7 @@
       "foo: 42\n",
       "bar:\n",
       "  alpha: true\n",
-      "  beta: true\n"
+      "  beta: false\n"
      ]
     }
    ],
@@ -157,6 +157,15 @@
     "\n",
     "cat files/nested_dataclass.yml"
    ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "tags": []
+   },
+   "outputs": [],
+   "source": []
   }
  ],
  "metadata": {
@@ -175,7 +184,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.2"
+   "version": "3.8.2"
   }
  },
  "nbformat": 4,
diff --git a/notebooks/patched_containers.ipynb b/notebooks/patched_containers.ipynb
index b2af098c..f4be6de8 100644
--- a/notebooks/patched_containers.ipynb
+++ b/notebooks/patched_containers.ipynb
@@ -10,7 +10,7 @@
    "source": [
     "%%sh\n",
     "\n",
-    "rm -f files/patched_containers.*"
+    "rm -f files/patched_containers.yml"
    ]
   },
   {
@@ -337,7 +337,6 @@
       "  - 2.3\n",
       "data:\n",
       "  a: 1\n",
-      "  b: 2\n",
       "  c: 3\n"
      ]
     }
@@ -365,7 +364,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.2"
+   "version": "3.8.2"
   }
  },
  "nbformat": 4,
diff --git a/notebooks/readme.ipynb b/notebooks/readme.ipynb
index db613cff..191500b0 100644
--- a/notebooks/readme.ipynb
+++ b/notebooks/readme.ipynb
@@ -244,7 +244,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.2"
+   "version": "3.8.2"
   }
  },
  "nbformat": 4,
diff --git a/notebooks/roundtrip_comments.ipynb b/notebooks/roundtrip_comments.ipynb
index bacb777d..8b925cc3 100644
--- a/notebooks/roundtrip_comments.ipynb
+++ b/notebooks/roundtrip_comments.ipynb
@@ -239,7 +239,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.2"
+   "version": "3.8.2"
   }
  },
  "nbformat": 4,
diff --git a/poetry.lock b/poetry.lock
index ec8668ad..b1b0e45d 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -43,6 +43,14 @@ wrapt = ">=1.11.0,<1.12.0"
 python = "<3.8"
 version = ">=1.4.0,<1.5"
 
+[[package]]
+category = "dev"
+description = "Async generators and context managers for Python 3.5+"
+name = "async-generator"
+optional = false
+python-versions = ">=3.5"
+version = "1.10"
+
 [[package]]
 category = "dev"
 description = "Atomic file writes."
@@ -117,7 +125,7 @@ description = "Python package for providing Mozilla's CA Bundle."
 name = "certifi"
 optional = false
 python-versions = "*"
-version = "2019.11.28"
+version = "2020.4.5.1"
 
 [[package]]
 category = "dev"
@@ -157,7 +165,7 @@ description = "Code coverage measurement for Python"
 name = "coverage"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <4"
-version = "5.0.4"
+version = "5.1"
 
 [package.extras]
 toml = ["toml"]
@@ -539,7 +547,7 @@ description = "Optional static typing for Python"
 name = "mypy"
 optional = false
 python-versions = ">=3.5"
-version = "0.761"
+version = "0.770"
 
 [package.dependencies]
 mypy-extensions = ">=0.4.3,<0.5.0"
@@ -562,12 +570,14 @@ category = "dev"
 description = "A client library for executing notebooks. Formally nbconvert's ExecutePreprocessor."
 name = "nbclient"
 optional = false
-python-versions = ">=3.5"
-version = "0.1.0"
+python-versions = ">=3.6"
+version = "0.2.0"
 
 [package.dependencies]
-jupyter-client = ">=5.3.4"
+async-generator = "*"
+jupyter-client = ">=6.1.0"
 nbformat = ">=5.0"
+nest-asyncio = "*"
 traitlets = ">=4.2"
 
 [package.extras]
@@ -608,7 +618,7 @@ description = "The Jupyter Notebook format"
 name = "nbformat"
 optional = false
 python-versions = ">=3.5"
-version = "5.0.4"
+version = "5.0.5"
 
 [package.dependencies]
 ipython-genutils = "*"
@@ -630,6 +640,14 @@ version = "0.3.7"
 [package.dependencies]
 nbformat = "*"
 
+[[package]]
+category = "dev"
+description = "Patch asyncio to allow nested event loops"
+name = "nest-asyncio"
+optional = false
+python-versions = ">=3.5"
+version = "1.3.2"
+
 [[package]]
 category = "dev"
 description = "A web-based notebook environment for interactive computing"
@@ -681,25 +699,22 @@ category = "dev"
 description = "Parametrize and run Jupyter and nteract Notebooks"
 name = "papermill"
 optional = false
-python-versions = ">=3.5"
-version = "2.0.0"
+python-versions = ">=3.6"
+version = "2.1.0"
 
 [package.dependencies]
 ansiwrap = "*"
+black = "*"
 click = "*"
 entrypoints = "*"
 jupyter-client = "*"
-nbclient = "*"
+nbclient = ">=0.2.0"
 nbformat = "*"
 pyyaml = "*"
 requests = "*"
 tenacity = "*"
 tqdm = ">=4.32.2"
 
-[package.dependencies.black]
-python = ">=3.6"
-version = "*"
-
 [package.extras]
 all = ["boto3", "azure-datalake-store (>=0.0.30)", "azure-storage-blob (>=12.1.0)", "requests (>=2.21.0)", "gcsfs (>=0.2.0)", "pyarrow"]
 azure = ["azure-datalake-store (>=0.0.30)", "azure-storage-blob (>=12.1.0)", "requests (>=2.21.0)"]
@@ -854,7 +869,7 @@ description = "Python parsing module"
 name = "pyparsing"
 optional = false
 python-versions = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"
-version = "2.4.6"
+version = "2.4.7"
 
 [[package]]
 category = "dev"
@@ -919,6 +934,10 @@ version = "0.12.0"
 [package.dependencies]
 pytest = ">=2.6.0"
 
+[package.source]
+reference = "43245ecfc3c432c4bfb5b12f8868790c7f2ebe5d"
+type = "git"
+url = "https://github.com/jacebrowning/pytest-describe"
 [[package]]
 category = "dev"
 description = "Better testing with expecter and pytest."
@@ -929,17 +948,17 @@ version = "2.1"
 
 [[package]]
 category = "dev"
-description = "Thin-wrapper around the mock package for easier use with py.test"
+description = "Thin-wrapper around the mock package for easier use with pytest"
 name = "pytest-mock"
 optional = false
-python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
-version = "2.0.0"
+python-versions = ">=3.5"
+version = "3.0.0"
 
 [package.dependencies]
 pytest = ">=2.7"
 
 [package.extras]
-dev = ["pre-commit", "tox"]
+dev = ["pre-commit", "tox", "pytest-asyncio"]
 
 [[package]]
 category = "dev"
@@ -1212,7 +1231,7 @@ description = "Fast, Extensible Progress Meter"
 name = "tqdm"
 optional = false
 python-versions = ">=2.6, !=3.0.*, !=3.1.*"
-version = "4.44.1"
+version = "4.45.0"
 
 [package.extras]
 dev = ["py-make (>=0.1.0)", "twine", "argopt", "pydoc-markdown"]
@@ -1247,7 +1266,7 @@ description = "Backported and Experimental Type Hints for Python 3.5+"
 name = "typing-extensions"
 optional = false
 python-versions = "*"
-version = "3.7.4.1"
+version = "3.7.4.2"
 
 [[package]]
 category = "dev"
@@ -1325,7 +1344,7 @@ docs = ["sphinx", "jaraco.packaging (>=3.2)", "rst.linker (>=1.9)"]
 testing = ["jaraco.itertools", "func-timeout"]
 
 [metadata]
-content-hash = "98f1afc1fb2314c5d660a2d8fa7f0e25c7f6af2bde1459dafe39283d5a692d1c"
+content-hash = "3aa5a06bdedb79daee99cea522a995dd9a9c2d17f2d549194198adfb663f1d41"
 python-versions = "^3.7"
 
 [metadata.files]
@@ -1345,6 +1364,10 @@ astroid = [
     {file = "astroid-2.3.3-py3-none-any.whl", hash = "sha256:840947ebfa8b58f318d42301cf8c0a20fd794a33b61cc4638e28e9e61ba32f42"},
     {file = "astroid-2.3.3.tar.gz", hash = "sha256:71ea07f44df9568a75d0f354c49143a4575d90645e9fead6dfb52c26a85ed13a"},
 ]
+async-generator = [
+    {file = "async_generator-1.10-py3-none-any.whl", hash = "sha256:01c7bf666359b4967d2cda0000cc2e4af16a0ae098cbffcb8472fb9e8ad6585b"},
+    {file = "async_generator-1.10.tar.gz", hash = "sha256:6ebb3d106c12920aaae42ccb6f787ef5eefdcdd166ea3d628fa8476abe712144"},
+]
 atomicwrites = [
     {file = "atomicwrites-1.3.0-py2.py3-none-any.whl", hash = "sha256:03472c30eb2c5d1ba9227e4c2ca66ab8287fbfbbda3888aa93dc2e28fc6811b4"},
     {file = "atomicwrites-1.3.0.tar.gz", hash = "sha256:75a9445bac02d8d058d5e1fe689654ba5a6556a1dfd8ce6ec55a0ed79866cfa6"},
@@ -1370,8 +1393,8 @@ cached-property = [
     {file = "cached_property-1.5.1-py2.py3-none-any.whl", hash = "sha256:3a026f1a54135677e7da5ce819b0c690f156f37976f3e30c5430740725203d7f"},
 ]
 certifi = [
-    {file = "certifi-2019.11.28-py2.py3-none-any.whl", hash = "sha256:017c25db2a153ce562900032d5bc68e9f191e44e9a0f762f373977de9df1fbb3"},
-    {file = "certifi-2019.11.28.tar.gz", hash = "sha256:25b64c7da4cd7479594d035c08c2d809eb4aab3a26e5a990ea98cc450c320f1f"},
+    {file = "certifi-2020.4.5.1-py2.py3-none-any.whl", hash = "sha256:1d987a998c75633c40847cc966fcf5904906c920a7f17ef374f5aa4282abd304"},
+    {file = "certifi-2020.4.5.1.tar.gz", hash = "sha256:51fcb31174be6e6664c5f69e3e1691a2d72a1a12e90f872cbdb1567eb47b6519"},
 ]
 chardet = [
     {file = "chardet-3.0.4-py2.py3-none-any.whl", hash = "sha256:fc323ffcaeaed0e0a02bf4d117757b98aed530d9ed4531e3e15460124c106691"},
@@ -1389,37 +1412,37 @@ colorama = [
     {file = "colorama-0.3.9.tar.gz", hash = "sha256:48eb22f4f8461b1df5734a074b57042430fb06e1d61bd1e11b078c0fe6d7a1f1"},
 ]
 coverage = [
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+    {file = "coverage-5.1-cp39-cp39-win_amd64.whl", hash = "sha256:bb28a7245de68bf29f6fb199545d072d1036a1917dca17a1e75bbb919e14ee8e"},
+    {file = "coverage-5.1.tar.gz", hash = "sha256:f90bfc4ad18450c80b024036eaf91e4a246ae287701aaa88eaebebf150868052"},
 ]
 coveragespace = [
     {file = "coveragespace-3.1.1-py3-none-any.whl", hash = "sha256:cc62bf4f2feb419032920270a0c16f1a379b80ac9fc6bd3cefce918bfc90ba27"},
@@ -1588,41 +1611,45 @@ more-itertools = [
     {file = "more_itertools-8.2.0-py3-none-any.whl", hash = "sha256:5dd8bcf33e5f9513ffa06d5ad33d78f31e1931ac9a18f33d37e77a180d393a7c"},
 ]
 mypy = [
-    {file = "mypy-0.761-cp35-cp35m-macosx_10_6_x86_64.whl", hash = "sha256:7f672d02fffcbace4db2b05369142e0506cdcde20cea0e07c7c2171c4fd11dd6"},
-    {file = "mypy-0.761-cp35-cp35m-manylinux1_x86_64.whl", hash = "sha256:87c556fb85d709dacd4b4cb6167eecc5bbb4f0a9864b69136a0d4640fdc76a36"},
-    {file = "mypy-0.761-cp35-cp35m-win_amd64.whl", hash = "sha256:c6d27bd20c3ba60d5b02f20bd28e20091d6286a699174dfad515636cb09b5a72"},
-    {file = "mypy-0.761-cp36-cp36m-macosx_10_6_x86_64.whl", hash = "sha256:4b9365ade157794cef9685791032521233729cb00ce76b0ddc78749abea463d2"},
-    {file = "mypy-0.761-cp36-cp36m-manylinux1_x86_64.whl", hash = "sha256:634aef60b4ff0f650d3e59d4374626ca6153fcaff96ec075b215b568e6ee3cb0"},
-    {file = "mypy-0.761-cp36-cp36m-win_amd64.whl", hash = "sha256:53ea810ae3f83f9c9b452582261ea859828a9ed666f2e1ca840300b69322c474"},
-    {file = "mypy-0.761-cp37-cp37m-macosx_10_6_x86_64.whl", hash = "sha256:0a9a45157e532da06fe56adcfef8a74629566b607fa2c1ac0122d1ff995c748a"},
-    {file = "mypy-0.761-cp37-cp37m-manylinux1_x86_64.whl", hash = "sha256:7eadc91af8270455e0d73565b8964da1642fe226665dd5c9560067cd64d56749"},
-    {file = "mypy-0.761-cp37-cp37m-win_amd64.whl", hash = "sha256:e2bb577d10d09a2d8822a042a23b8d62bc3b269667c9eb8e60a6edfa000211b1"},
-    {file = "mypy-0.761-cp38-cp38-macosx_10_9_x86_64.whl", hash = "sha256:2c35cae79ceb20d47facfad51f952df16c2ae9f45db6cb38405a3da1cf8fc0a7"},
-    {file = "mypy-0.761-cp38-cp38-manylinux1_x86_64.whl", hash = "sha256:f97a605d7c8bc2c6d1172c2f0d5a65b24142e11a58de689046e62c2d632ca8c1"},
-    {file = "mypy-0.761-cp38-cp38-win_amd64.whl", hash = "sha256:a6bd44efee4dc8c3324c13785a9dc3519b3ee3a92cada42d2b57762b7053b49b"},
-    {file = "mypy-0.761-py3-none-any.whl", hash = "sha256:7e396ce53cacd5596ff6d191b47ab0ea18f8e0ec04e15d69728d530e86d4c217"},
-    {file = "mypy-0.761.tar.gz", hash = "sha256:85baab8d74ec601e86134afe2bcccd87820f79d2f8d5798c889507d1088287bf"},
+    {file = "mypy-0.770-cp35-cp35m-macosx_10_6_x86_64.whl", hash = "sha256:a34b577cdf6313bf24755f7a0e3f3c326d5c1f4fe7422d1d06498eb25ad0c600"},
+    {file = "mypy-0.770-cp35-cp35m-manylinux1_x86_64.whl", hash = "sha256:86c857510a9b7c3104cf4cde1568f4921762c8f9842e987bc03ed4f160925754"},
+    {file = "mypy-0.770-cp35-cp35m-win_amd64.whl", hash = "sha256:a8ffcd53cb5dfc131850851cc09f1c44689c2812d0beb954d8138d4f5fc17f65"},
+    {file = "mypy-0.770-cp36-cp36m-macosx_10_6_x86_64.whl", hash = "sha256:7687f6455ec3ed7649d1ae574136835a4272b65b3ddcf01ab8704ac65616c5ce"},
+    {file = "mypy-0.770-cp36-cp36m-manylinux1_x86_64.whl", hash = "sha256:3beff56b453b6ef94ecb2996bea101a08f1f8a9771d3cbf4988a61e4d9973761"},
+    {file = "mypy-0.770-cp36-cp36m-win_amd64.whl", hash = "sha256:15b948e1302682e3682f11f50208b726a246ab4e6c1b39f9264a8796bb416aa2"},
+    {file = "mypy-0.770-cp37-cp37m-macosx_10_6_x86_64.whl", hash = "sha256:b90928f2d9eb2f33162405f32dde9f6dcead63a0971ca8a1b50eb4ca3e35ceb8"},
+    {file = "mypy-0.770-cp37-cp37m-manylinux1_x86_64.whl", hash = "sha256:c56ffe22faa2e51054c5f7a3bc70a370939c2ed4de308c690e7949230c995913"},
+    {file = "mypy-0.770-cp37-cp37m-win_amd64.whl", hash = "sha256:8dfb69fbf9f3aeed18afffb15e319ca7f8da9642336348ddd6cab2713ddcf8f9"},
+    {file = "mypy-0.770-cp38-cp38-macosx_10_9_x86_64.whl", hash = "sha256:219a3116ecd015f8dca7b5d2c366c973509dfb9a8fc97ef044a36e3da66144a1"},
+    {file = "mypy-0.770-cp38-cp38-manylinux1_x86_64.whl", hash = "sha256:7ec45a70d40ede1ec7ad7f95b3c94c9cf4c186a32f6bacb1795b60abd2f9ef27"},
+    {file = "mypy-0.770-cp38-cp38-win_amd64.whl", hash = "sha256:f91c7ae919bbc3f96cd5e5b2e786b2b108343d1d7972ea130f7de27fdd547cf3"},
+    {file = "mypy-0.770-py3-none-any.whl", hash = "sha256:3b1fc683fb204c6b4403a1ef23f0b1fac8e4477091585e0c8c54cbdf7d7bb164"},
+    {file = "mypy-0.770.tar.gz", hash = "sha256:8a627507ef9b307b46a1fea9513d5c98680ba09591253082b4c48697ba05a4ae"},
 ]
 mypy-extensions = [
     {file = "mypy_extensions-0.4.3-py2.py3-none-any.whl", hash = "sha256:090fedd75945a69ae91ce1303b5824f428daf5a028d2f6ab8a299250a846f15d"},
     {file = "mypy_extensions-0.4.3.tar.gz", hash = "sha256:2d82818f5bb3e369420cb3c4060a7970edba416647068eb4c5343488a6c604a8"},
 ]
 nbclient = [
-    {file = "nbclient-0.1.0-py3-none-any.whl", hash = "sha256:02af062077ae0ec00201f52e207942a03a49e7dbb2dd7589c1713a3f40bea0df"},
-    {file = "nbclient-0.1.0.tar.gz", hash = "sha256:39fa0b3b24cb597827f6b104718dc952c679a8f5ba2d6f1a4113a3bd7f1528c6"},
+    {file = "nbclient-0.2.0-py3-none-any.whl", hash = "sha256:193731fd5039061dbd7d6d3f765a2fa59d23012594d68b1798deea9c3eae4a01"},
+    {file = "nbclient-0.2.0.tar.gz", hash = "sha256:44dde0356def1d9345908c8f58dc604a434f2fe61c49ac13fac6e2da2ae429de"},
 ]
 nbconvert = [
     {file = "nbconvert-5.6.1-py2.py3-none-any.whl", hash = "sha256:f0d6ec03875f96df45aa13e21fd9b8450c42d7e1830418cccc008c0df725fcee"},
     {file = "nbconvert-5.6.1.tar.gz", hash = "sha256:21fb48e700b43e82ba0e3142421a659d7739b65568cc832a13976a77be16b523"},
 ]
 nbformat = [
-    {file = "nbformat-5.0.4-py3-none-any.whl", hash = "sha256:f4bbbd8089bd346488f00af4ce2efb7f8310a74b2058040d075895429924678c"},
-    {file = "nbformat-5.0.4.tar.gz", hash = "sha256:562de41fc7f4f481b79ab5d683279bf3a168858268d4387b489b7b02be0b324a"},
+    {file = "nbformat-5.0.5-py3-none-any.whl", hash = "sha256:65a79936a128fd85aef392b7fea520166364037118b6fe3ed52de742d06c4558"},
+    {file = "nbformat-5.0.5.tar.gz", hash = "sha256:f0c47cf93c505cb943e2f131ef32b8ae869292b5f9f279db2bafb35867923f69"},
 ]
 nbstripout = [
     {file = "nbstripout-0.3.7-py2.py3-none-any.whl", hash = "sha256:cf745ae8c49fccdb3068b73fc3b783898d5d62ee929429e9af37a6dfefba34b7"},
     {file = "nbstripout-0.3.7.tar.gz", hash = "sha256:62f1b1fe9c7c298061089fd9bd5d297eb6209f7fbef0758631dbe58d38fc828f"},
 ]
+nest-asyncio = [
+    {file = "nest_asyncio-1.3.2-py3-none-any.whl", hash = "sha256:b4cdd08655e2848098d204a26590cbfa39fcbc4ad1811c568678ffc8a0c8e279"},
+    {file = "nest_asyncio-1.3.2.tar.gz", hash = "sha256:14e194b72144052a82173ca9109bd07c57813a320f42c7acfad1e4d329988350"},
+]
 notebook = [
     {file = "notebook-6.0.3-py3-none-any.whl", hash = "sha256:3edc616c684214292994a3af05eaea4cc043f6b4247d830f3a2f209fa7639a80"},
     {file = "notebook-6.0.3.tar.gz", hash = "sha256:47a9092975c9e7965ada00b9a20f0cf637d001db60d241d479f53c0be117ad48"},
@@ -1635,8 +1662,8 @@ pandocfilters = [
     {file = "pandocfilters-1.4.2.tar.gz", hash = "sha256:b3dd70e169bb5449e6bc6ff96aea89c5eea8c5f6ab5e207fc2f521a2cf4a0da9"},
 ]
 papermill = [
-    {file = "papermill-2.0.0-py3-none-any.whl", hash = "sha256:7ad0dd5bce86df5e973a88e29457be99662099d365b99d640acb27119b1b7812"},
-    {file = "papermill-2.0.0.tar.gz", hash = "sha256:83459eeb378b2a2f885fc2b36890306a99fda160d483d28847aa112454311bab"},
+    {file = "papermill-2.1.0-py3-none-any.whl", hash = "sha256:10f86551cd28d09acea1f63b04a61d061ed4c3d3e82aa675b6a593c4451860c6"},
+    {file = "papermill-2.1.0.tar.gz", hash = "sha256:1ff390a2bea5ea1538c1fcb5e2abb08c2a261d4427286905bc803416c72c26b0"},
 ]
 parse = [
     {file = "parse-1.15.0.tar.gz", hash = "sha256:a6d4e2c2f1fbde6717d28084a191a052950f758c0cbd83805357e6575c2b95c0"},
@@ -1685,8 +1712,8 @@ pygments = [
 ]
 pylint = []
 pyparsing = [
-    {file = "pyparsing-2.4.6-py2.py3-none-any.whl", hash = "sha256:c342dccb5250c08d45fd6f8b4a559613ca603b57498511740e65cd11a2e7dcec"},
-    {file = "pyparsing-2.4.6.tar.gz", hash = "sha256:4c830582a84fb022400b85429791bc551f1f4871c33f23e44f353119e92f969f"},
+    {file = "pyparsing-2.4.7-py2.py3-none-any.whl", hash = "sha256:ef9d7589ef3c200abe66653d3f1ab1033c3c419ae9b9bdb1240a85b024efc88b"},
+    {file = "pyparsing-2.4.7.tar.gz", hash = "sha256:c203ec8783bf771a155b207279b9bccb8dea02d8f0c9e5f8ead507bc3246ecc1"},
 ]
 pyrsistent = [
     {file = "pyrsistent-0.16.0.tar.gz", hash = "sha256:28669905fe725965daa16184933676547c5bb40a5153055a8dee2a4bd7933ad3"},
@@ -1699,16 +1726,14 @@ pytest-cov = [
     {file = "pytest-cov-2.8.1.tar.gz", hash = "sha256:cc6742d8bac45070217169f5f72ceee1e0e55b0221f54bcf24845972d3a47f2b"},
     {file = "pytest_cov-2.8.1-py2.py3-none-any.whl", hash = "sha256:cdbdef4f870408ebdbfeb44e63e07eb18bb4619fae852f6e760645fa36172626"},
 ]
-pytest-describe = [
-    {file = "pytest-describe-0.12.0.tar.gz", hash = "sha256:569bda96401fe512f4f345f33fd23fa4d718639d42afac62bc03254b5f2b3fdf"},
-]
+pytest-describe = []
 pytest-expecter = [
     {file = "pytest-expecter-2.1.tar.gz", hash = "sha256:70b05fec2a9ae5ec6f15d5c5467c1e2cb9aa2fd85ec9c4783b26e2cf68e66f96"},
     {file = "pytest_expecter-2.1-py3-none-any.whl", hash = "sha256:ab66120671a22be41f7df4c29d76c02d62e3420b8277b1455d2e683a042a0608"},
 ]
 pytest-mock = [
-    {file = "pytest-mock-2.0.0.tar.gz", hash = "sha256:b35eb281e93aafed138db25c8772b95d3756108b601947f89af503f8c629413f"},
-    {file = "pytest_mock-2.0.0-py2.py3-none-any.whl", hash = "sha256:cb67402d87d5f53c579263d37971a164743dc33c159dfb4fb4a86f37c5552307"},
+    {file = "pytest-mock-3.0.0.tar.gz", hash = "sha256:a4494016753a30231f8519bfd160242a0f3c8fb82ca36e7b6f82a7fb602ac6b8"},
+    {file = "pytest_mock-3.0.0-py2.py3-none-any.whl", hash = "sha256:98e02534f170e4f37d7e1abdfc5973fd4207aa609582291717f643764e71c925"},
 ]
 pytest-profiling = [
     {file = "pytest-profiling-1.7.0.tar.gz", hash = "sha256:93938f147662225d2b8bd5af89587b979652426a8a6ffd7e73ec4a23e24b7f29"},
@@ -1885,8 +1910,8 @@ tornado = [
     {file = "tornado-6.0.4.tar.gz", hash = "sha256:0fe2d45ba43b00a41cd73f8be321a44936dc1aba233dee979f17a042b83eb6dc"},
 ]
 tqdm = [
-    {file = "tqdm-4.44.1-py2.py3-none-any.whl", hash = "sha256:be5ddeec77d78ba781ea41eacb2358a77f74cc2407f54b82222d7ee7dc8c8ccf"},
-    {file = "tqdm-4.44.1.tar.gz", hash = "sha256:03d2366c64d44c7f61e74c700d9b202d57e9efe355ea5c28814c52bfe7a50b8c"},
+    {file = "tqdm-4.45.0-py2.py3-none-any.whl", hash = "sha256:ea9e3fd6bd9a37e8783d75bfc4c1faf3c6813da6bd1c3e776488b41ec683af94"},
+    {file = "tqdm-4.45.0.tar.gz", hash = "sha256:00339634a22c10a7a22476ee946bbde2dbe48d042ded784e4d88e0236eca5d81"},
 ]
 traitlets = [
     {file = "traitlets-4.3.3-py2.py3-none-any.whl", hash = "sha256:70b4c6a1d9019d7b4f6846832288f86998aa3b9207c6821f3578a6a6a467fe44"},
@@ -1916,9 +1941,9 @@ typed-ast = [
     {file = "typed_ast-1.4.1.tar.gz", hash = "sha256:8c8aaad94455178e3187ab22c8b01a3837f8ee50e09cf31f1ba129eb293ec30b"},
 ]
 typing-extensions = [
-    {file = "typing_extensions-3.7.4.1-py2-none-any.whl", hash = "sha256:910f4656f54de5993ad9304959ce9bb903f90aadc7c67a0bef07e678014e892d"},
-    {file = "typing_extensions-3.7.4.1-py3-none-any.whl", hash = "sha256:cf8b63fedea4d89bab840ecbb93e75578af28f76f66c35889bd7065f5af88575"},
-    {file = "typing_extensions-3.7.4.1.tar.gz", hash = "sha256:091ecc894d5e908ac75209f10d5b4f118fbdb2eb1ede6a63544054bb1edb41f2"},
+    {file = "typing_extensions-3.7.4.2-py2-none-any.whl", hash = "sha256:f8d2bd89d25bc39dabe7d23df520442fa1d8969b82544370e03d88b5a591c392"},
+    {file = "typing_extensions-3.7.4.2-py3-none-any.whl", hash = "sha256:6e95524d8a547a91e08f404ae485bbb71962de46967e1b71a0cb89af24e761c5"},
+    {file = "typing_extensions-3.7.4.2.tar.gz", hash = "sha256:79ee589a3caca649a9bfd2a8de4709837400dfa00b6cc81962a1e6a1815969ae"},
 ]
 urllib3 = [
     {file = "urllib3-1.25.8-py2.py3-none-any.whl", hash = "sha256:2f3db8b19923a873b3e5256dc9c2dedfa883e33d87c690d9c7913e1f40673cdc"},
diff --git a/pyproject.toml b/pyproject.toml
index 9cd72c71..5f9b6e5f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,7 +1,7 @@
 [tool.poetry]
 
 name = "datafiles"
-version = "0.8.1"
+version = "0.9"
 description = "File-based ORM for dataclasses."
 
 license = "MIT"
@@ -43,8 +43,8 @@ python = "^3.7"
 
 # Formats
 PyYAML = "^5.2"
-"ruamel.yaml" = "^0.16.7"
-tomlkit = "^0.5.3"
+"ruamel.yaml" = "~0.16.10"
+tomlkit = "~0.5.3"
 
 # ORM
 parse = "^1.12"
@@ -64,14 +64,14 @@ black = "=19.3b0"
 isort = "=4.3.4"
 
 # Linters
-mypy = "^0.761"
+mypy = "~0.770"
 pylint = { git = "https://github.com/PyCQA/pylint", rev = "e169e83e52fedc6624235d45e8e8737294a0fedf" } # 2.4.5 has not yet been released
 pydocstyle = "*"
 
 # Testing
 pytest = "^5.3.2"
-pytest-describe = "*"
-pytest-expecter = "^2.0"
+pytest-describe = { git = "https://github.com/jacebrowning/pytest-describe", branch = "pytest-5.4-support" }
+pytest-expecter = "^2.1"
 pytest-mock = "*"
 pytest-random = "*"
 pytest-repeat = "*"
diff --git a/tests/test_file_inference.py b/tests/test_file_inference.py
index 92cc7fbd..3d2caf29 100644
--- a/tests/test_file_inference.py
+++ b/tests/test_file_inference.py
@@ -6,17 +6,13 @@ def test_auto_with_sample_file(expect):
     write(
         'tmp/sample.yml',
         """
-
         homogeneous_list:
           - 1
           - 2
-
         heterogeneous_list:
           - 1
           - 'abc'
-
         empty_list: []
-
         """,
     )
 
@@ -36,7 +32,6 @@ def test_auto_with_sample_file(expect):
     logbreak("Reading file")
     expect(read('tmp/sample.yml')) == dedent(
         """
-
         homogeneous_list:
           - 1
           - 2
diff --git a/tests/test_instantiation.py b/tests/test_instantiation.py
index 58ebb54b..79ef3514 100644
--- a/tests/test_instantiation.py
+++ b/tests/test_instantiation.py
@@ -115,12 +115,12 @@ class SampleAutomatic:
     def it_is_created_automatically_by_default(expect):
         sample = SampleAutomatic()
 
-        expect(sample.datafile.exists) == True
+        expect(sample.datafile.exists).is_(True)
 
     def it_is_not_created_automatically_when_manual(expect):
         sample = SampleManual()
 
-        expect(sample.datafile.exists) == False
+        expect(sample.datafile.exists).is_(False)
 
 
 def describe_factory_defaults():
diff --git a/tests/test_loading.py b/tests/test_loading.py
index 22bc85b7..e6bf7f6a 100644
--- a/tests/test_loading.py
+++ b/tests/test_loading.py
@@ -39,7 +39,7 @@ def with_matching_types(sample, expect):
 
         sample.datafile.load()
 
-        expect(sample.bool_) == True
+        expect(sample.bool_).is_(True)
         expect(sample.int_) == 1
         expect(sample.float_) == 2.3
         expect(sample.str_) == 'foobar'
@@ -57,7 +57,7 @@ def with_convertable_types(sample, expect):
 
         sample.datafile.load()
 
-        expect(sample.bool_) == True
+        expect(sample.bool_).is_(True)
         expect(sample.int_) == 2
         expect(sample.float_) == 3.0
         expect(sample.str_) == '4'
@@ -77,7 +77,7 @@ def with_extra_fields(sample, expect):
 
         sample.datafile.load()
 
-        expect(hasattr(sample, 'extra')) == False
+        expect(hasattr(sample, 'extra')).is_(False)
 
 
 def describe_alternate_formats():
@@ -100,7 +100,7 @@ def with_json(sample, expect):
 
         sample.datafile.load()
 
-        expect(sample.bool_) == True
+        expect(sample.bool_).is_(True)
         expect(sample.int_) == 1
         expect(sample.float_) == 2.3
         expect(sample.str_) == 'foobar'
@@ -230,7 +230,7 @@ def with_extra_attributes(sample, expect):
         expect(sample.score) == 1.2
         expect(sample.nested.name) == 'b'
         expect(sample.nested.score) == 3.4
-        expect(hasattr(sample.nested, 'extra')) == False
+        expect(hasattr(sample.nested, 'extra')).is_(False)
 
     @pytest.mark.flaky
     def with_multiple_levels(expect):
diff --git a/tests/test_orm_usage.py b/tests/test_orm_usage.py
index aaba1842..adf9ff1c 100644
--- a/tests/test_orm_usage.py
+++ b/tests/test_orm_usage.py
@@ -1,11 +1,11 @@
 """Tests that represent usage as an ORM."""
 
-from dataclasses import dataclass
+from typing import List, Optional
 
 import pytest
 
 from datafiles import datafile
-from datafiles.utils import logbreak
+from datafiles.utils import logbreak, write
 
 
 # This model is based on the example dataclass from:
@@ -48,7 +48,7 @@ def test_multiple_instances_are_distinct(expect):
 
 
 def test_classes_can_share_a_nested_dataclass(expect):
-    @dataclass
+    @datafile
     class Nested:
         value: int
 
@@ -78,3 +78,58 @@ def test_values_are_filled_from_disk(expect):
     items = list(InventoryItem.objects.all())
 
     expect(items[0]) == InventoryItem(42, "Things", 0.99)
+
+
+def test_missing_optional_fields_are_loaded(expect):
+    @datafile
+    class Name:
+        value: str
+
+    @datafile("../tmp/samples/{self.key}.json")
+    class Sample:
+
+        key: int
+        name: Optional[Name]
+        value: float = 0.0
+
+    sample = Sample(42, None)
+
+    logbreak("get key=42")
+    sample2 = Sample.objects.get(42)
+    expect(sample2.name) == sample.name
+
+
+def test_comments_in_matched_files(expect):
+    @datafile("../tmp/templates/{self.key}/config.yml")
+    class LegacyTemplate:
+        key: str
+        name: str
+        link: str
+        default: List[str]
+        aliases: List[str]
+
+    write(
+        'tmp/templates/foo/config.yml',
+        """
+        link: # placeholder
+        default:
+          - # placeholder
+          - # placeholder
+        aliases:
+          - # placeholder
+        """,
+    )
+    write(
+        'tmp/templates/bar/config.yml',
+        """
+        link: http://example.com
+        default:
+          - abc
+          - def
+        aliases:
+          - qux
+        """,
+    )
+
+    items = list(LegacyTemplate.objects.all())
+    expect(len(items)) == 2
diff --git a/tests/test_saving.py b/tests/test_saving.py
index bb4585d4..e127b77f 100644
--- a/tests/test_saving.py
+++ b/tests/test_saving.py
@@ -1,9 +1,12 @@
 """Tests for saving to a file."""
 
-# pylint: disable=unused-variable,assigning-non-slot
+# pylint: disable=unused-variable,assigning-non-slot,unsubscriptable-object
+
+from typing import Optional
 
 import pytest
 
+from datafiles import datafile
 from datafiles.utils import dedent, logbreak, read, write
 
 from .samples import (
@@ -224,6 +227,26 @@ def with_nones(expect):
             """
         )
 
+    def when_nested_dataclass_is_none(expect):
+        @datafile
+        class Name:
+            value: str
+
+        @datafile("../tmp/samples/{self.key}.yml")
+        class Sample:
+
+            key: int
+            name: Optional[Name]
+            value: float = 0.0
+
+        sample = Sample(42, None)
+
+        expect(read('tmp/samples/42.yml')) == dedent(
+            """
+            name:
+            """
+        )
+
 
 def describe_defaults():
     def with_custom_values(expect):
@@ -293,8 +316,8 @@ def with_comments(expect):
         write(
             'tmp/sample.yml',
             """
-            # Heading
-            required: 1.0  # Line
+            # Header
+            required: 1.0       # Line
             optional: 2.0
             """,
         )
@@ -305,8 +328,8 @@ def with_comments(expect):
 
         expect(read('tmp/sample.yml')) == dedent(
             """
-            # Heading
-            required: 3.0  # Line
+            # Header
+            required: 3.0       # Line
             optional: 2.0
             """
         )
@@ -317,76 +340,112 @@ def with_comments_in_nested_objects(expect):
         write(
             'tmp/sample.yml',
             """
-            # Heading
+            # Header
             name: a
-            score: 1.0  # Line
+            score: 1.0      # Line
 
             nested:
-              # Nested heading
+              # Nested header
               name: n
               score: 2
             """,
         )
 
-        logbreak("Loading")
         sample.datafile.load()
-
         sample.score = 3
         sample.nested.score = 4
-
-        logbreak("Saving")
         sample.datafile.save()
 
         expect(read('tmp/sample.yml')) == dedent(
             """
-            # Heading
+            # Header
             name: a
-            score: 3.0  # Line
+            score: 3.0      # Line
 
             nested:
-              # Nested heading
+              # Nested header
               name: n
               score: 4.0
             """
         )
 
-    @pytest.mark.xfail(reason="Unknown ruamel.yaml bug")
     def with_comments_on_nested_lines(expect):
         sample = SampleWithNestingAndDefaults(None)
 
         write(
             'tmp/sample.yml',
             """
-            # Heading
+            # Header
             name: a
-            score: 1  # Line
+            score: 1        # Line
 
             nested:
-              # Nested heading
+              # Nested header
               name: n
-              score: 2  # Nested line
+              score: 2      # Nested line
             """,
         )
 
-        logbreak("Loading")
         sample.datafile.load()
-
         sample.score = 3
         sample.nested.score = 4
-
-        logbreak("Saving")
         sample.datafile.save()
 
         expect(read('tmp/sample.yml')) == dedent(
             """
-            # Heading
+            # Header
             name: a
-            score: 3.0  # Line
+            score: 3.0      # Line
 
             nested:
-              # Nested heading
+              # Nested header
               name: n
-              score: 4.0  # Nested line
+              score: 4.0    # Nested line
+            """
+        )
+
+    @pytest.mark.xfail(reason="https://github.com/jacebrowning/datafiles/issues/171")
+    def with_comments_on_list_items(expect):
+        sample = SampleWithListOfDataclasses()
+
+        write(
+            'tmp/sample.yml',
+            """
+            # Header
+
+            items:  # Subheader
+
+              # Section
+
+              - name: a     # Item
+                score: 1
+
+              # Section
+
+              - name: b     # Item
+                score: 2
+            """,
+        )
+
+        sample.datafile.load()
+        sample.items[1].name = 'c'
+        sample.datafile.save()
+
+        expect(read('tmp/sample.yml')) == dedent(
+            """
+            # Header
+
+            items:  # Subheader
+
+              # Section
+
+              - name: a     # Item
+                score: 1
+
+              # Section
+
+              - name: c     # Item
+                score: 2
             """
         )
 
@@ -401,6 +460,7 @@ def with_quotations(expect):
         )
 
         sample.datafile.load()
+        sample.float_ = 1
         sample.datafile.save()
 
         expect(read('tmp/sample.yml')) == dedent(
@@ -408,6 +468,6 @@ def with_quotations(expect):
             str_: "42"
             bool_: false
             int_: 0
-            float_: 0.0
+            float_: 1.0
             """
         )
diff --git a/tests/test_setup.py b/tests/test_setup.py
index 879d338e..17f85a06 100644
--- a/tests/test_setup.py
+++ b/tests/test_setup.py
@@ -133,7 +133,7 @@ def it_inferrs_attrs(expect, sample):
         }
 
     def it_has_no_path(expect, sample):
-        expect(sample.datafile.path) == None
+        expect(sample.datafile.path).is_(None)
 
     def it_converts_attributes(expect, sample):
         expect(sample.key) == 2
@@ -163,7 +163,7 @@ def it_uses_attrs_from_meta(expect, sample):
         expect(sample.datafile.attrs) == {'name': datafiles.converters.String}
 
     def it_has_no_path(expect, sample):
-        expect(sample.datafile.path) == None
+        expect(sample.datafile.path).is_(None)
 
     def it_converts_attributes(expect, sample):
         expect(sample.datafile.data) == {'name': "c"}