Simplify and optimize iterable unstructuring (#516)

* Simplify and optimize iterable unstructuring

* Handle TypeVars after all

* Add test

src/cattrs/gen/__init__.py

@@ -675,29 +675,22 @@ def make_iterable_unstructure_fn(
     """Generate a specialized unstructure function for an iterable."""
     handler = converter.unstructure
 
-    fn_name = "unstructure_iterable"
-
     # Let's try fishing out the type args
     # Unspecified tuples have `__args__` as empty tuples, so guard
     # against IndexError.
     if getattr(cl, "__args__", None) not in (None, ()):
         type_arg = cl.__args__[0]
-        # We don't know how to handle the TypeVar on this level,
-        # so we skip doing the dispatch here.
-        if not isinstance(type_arg, TypeVar):
-            handler = converter.get_unstructure_hook(type_arg, cache_result=False)
-
-    globs = {"__cattr_seq_cl": unstructure_to or cl, "__cattr_u": handler}
-    lines = []
-
-    lines.append(f"def {fn_name}(iterable):")
-    lines.append("    res = __cattr_seq_cl(__cattr_u(i) for i in iterable)")
+        if isinstance(type_arg, TypeVar):
+            type_arg = getattr(type_arg, "__default__", Any)
+        handler = converter.get_unstructure_hook(type_arg, cache_result=False)
+        if handler == identity:
+            # Save ourselves the trouble of iterating over it all.
+            return unstructure_to or cl
 
-    total_lines = [*lines, "    return res"]
-
-    eval(compile("\n".join(total_lines), "", "exec"), globs)
+    def unstructure_iterable(iterable, _seq_cl=unstructure_to or cl, _hook=handler):
+        return _seq_cl(_hook(i) for i in iterable)
 
-    return globs[fn_name]
+    return unstructure_iterable
 
 
 #: A type alias for heterogeneous tuple unstructure hooks.

src/cattrs/preconf/msgspec.py

@@ -106,15 +106,14 @@ def configure_passthroughs(converter: Converter) -> None:
     )
 
 
-def seq_unstructure_factory(type, converter: BaseConverter) -> UnstructureHook:
+def seq_unstructure_factory(type, converter: Converter) -> UnstructureHook:
     """The msgspec unstructure hook factory for sequences."""
     if is_bare(type):
         type_arg = Any
-        handler = converter.get_unstructure_hook(type_arg, cache_result=False)
     else:
         args = get_args(type)
         type_arg = args[0]
-        handler = converter.get_unstructure_hook(type_arg, cache_result=False)
+    handler = converter.get_unstructure_hook(type_arg, cache_result=False)
 
     if handler in (identity, to_builtins):
         return handler

tests/test_generics_696.py

@@ -1,5 +1,5 @@
 """Tests for generics under PEP 696 (type defaults)."""
-from typing import Generic
+from typing import Generic, List
 
 import pytest
 from attrs import define, fields
@@ -48,3 +48,15 @@ class D(Generic[TD]):
     # But allows other types
     assert genconverter.structure({"a": "1"}, D[str]) == D("1")
     assert genconverter.structure({"a": 1}, D[int]) == D(1)
+
+
+def test_unstructure_iterable(genconverter):
+    """Unstructuring iterables with defaults works."""
+    genconverter.register_unstructure_hook(str, lambda v: v + "_str")
+
+    @define
+    class C(Generic[TD]):
+        a: List[TD]
+
+    assert genconverter.unstructure(C(["a"])) == {"a": ["a_str"]}
+    assert genconverter.unstructure(["a"], List[TD]) == ["a_str"]