Plumber

Plumbing is an alternative to mixin-based extension of classes. In motivation an incomplete list of limitations and/or design choices of python's subclassing are given along with plumber's solutions for them. The plumbing system is described in detail with code examples. Some design choices and ongoing discussions are explained. Finally, in miscellanea you find nomenclature, coverage report, list of contributors, changes and some todos. All non-experimental features are fully test covered.

Contents

• Plumber
  • Motivation - Limitations of subclassing
  • The plumbing system
  • Miscellanea

Motivation - Limitations of subclassing

Plumbing is an alternative to mixin-based extension of classes, motivated by limitations and/or design choice of python's subclassing:

• Control of precedence only through order of mixins
• Impossible to provide default values to fill gaps on a base class
• super - chains are not verified during class creation
• No conditional super - chains
• Docstrings are not accumulated

Control of precedence only through order of mixins

Mixins are commonly used to extend classes with pre-defined behaviours: an attribute on the first mixin overwrites attributes with the same name on all following mixins and the base class being extended.

>>> class Mixin1(object):
...     a = 1

>>> class Mixin2(object):
...     a = 2
...     b = 2

>>> Base = dict
>>> class MixedClass(Mixin1, Mixin2, Base):
...     pass

>>> MixedClass.a
1
>>> MixedClass.b
2
>>> MixedClass.keys
<method 'keys' of 'dict' objects>

There is no way for a mixin later in the chain to take precedence over an earlier one.

Solution: plumber provides 3 decorators to enable finer control of precedence (default, override, finalize).

Impossible to provide default values to fill gaps on a base class

A dictionary-like storage at least needs to provide __getitem__, __setitem__, __delitem__ and __iter__, all other methods of a dictionary can be build upon these. A mixin that turns storages into full dictionaries needs to be able to provide default methods, taken if the base class does not provide a (more efficient) implementation.

Solution: plumber provides the default decorator to enable such defaults.

super - chains are not verified during class creation

It is possible to build a chain of methods using super: Mixin1 turns the key lowercase before passing it on, Mixin2 multiplies the result by 2 before returning it and both are chatty about start/stop.

>>> class Mixin1(object):
...     def __getitem__(self, key):
...         print('Mixin1 start')
...         key = key.lower()
...         ret = super(Mixin1, self).__getitem__(key)
...         print('Mixin1 stop')
...         return ret

>>> class Mixin2(object):
...     def __getitem__(self, key):
...         print('Mixin2 start')
...         ret = super(Mixin2, self).__getitem__(key)
...         ret = 2 * ret
...         print('Mixin2 stop')
...         return ret

>>> Base = dict
>>> class MixedClass(Mixin1, Mixin2, Base):
...     pass

>>> mc = MixedClass()
>>> mc['abc'] = 6
>>> mc['ABC']
Mixin1 start
Mixin2 start
Mixin2 stop
Mixin1 stop
12

dict.__getitem__ forms the endpoint of the chain as it returns a value without delegating to a method later in the chain (using super). If there is no endpoint an AttributeError is raised during runtime, not during class creation.

>>> class Mixin1(object):
...     def foo(self):
...         super(Mixin1, self).foo()

>>> class MixedClass(Mixin1, Base):
...     pass

>>> mc = MixedClass()
>>> mc.foo()
Traceback (most recent call last):
  ...
AttributeError: 'super' object has no attribute 'foo'

Solution: Plumber provides the plumb decorator to build similar chains using nested closures. These are create and verified during class creation.

No conditional super - chains

A mixin with subclassing needs to fit exactly the base class, there is no way to conditionally hook into method calls depending on whether the base class provides a method.

Solution: Plumber provides the plumbifexists decorator that behaves like plumb, if there is an endpoint available.

Docstrings are not accumulated

A class' docstring that uses mixins is not build from the docstrings of the mixins.

Solution: Plumber enables plumbing of docstrings using a special marker __plbnext__, which is replaced with the docstring of the next "mixin" Without the marker, docstrings are concatenated.

The plumbing system

The plumber metaclass creates plumbing classes according to instructions found on plumbing behaviors. First, all instructions are gathered, then they are applied in two stages: stage1: extension and stage2: pipelines, docstrings and optional zope.interfaces. There exists a class decorator plumbing which should be used in favor of setting metaclass directly as of plumber 1.3.

• Plumbing behaviors provide instructions
• The plumber gathers instructions
• Stage 1 - Extension
  • Interaction - finalize, plumbing declaration and base classes
  • Interaction - override, plumbing declaration and base classes
  • Interaction - default, plumbing declaration and base class
  • Interaction - finalize wins over override
  • Interaction - finalize wins over default
  • Subclassing Behaviors
• Stage 2 - Pipeline, docstrings and zope.interface instructions
  • Plumbing Pipelines in general
  • Method pipelines
  • Property pipelines
  • Subclassing Behaviors
  • Mixing methods and properties within the same pipeline is not possible
  • Docstrings of classes, methods and properties
  • Slots and plumbings
  • zope.interface (if available)
• Plumber metaclass hooks

Plumbing behaviors provide instructions

Plumbing behaviors correspond to mixins, but are more powerful and flexible. A plumbing behavior needs to inherit from plumber.Behavior and declares attributes with instructions on how to use them, here by example of the default instruction (more later).

>>> from plumber import Behavior
>>> from plumber import default

>>> class Behavior1(Behavior):
...     a = default(True)
...
...     @default
...     def foo(self):
...         return 42

>>> class Behavior2(Behavior):
...     @default
...     @property
...     def bar(self):
...         return 17

The instructions are given as behavior of assignments (a = default(None)) or as decorators (@default).

A plumbing declaration defines the plumber as metaclass and one or more plumbing behaviors to be processed from left to right. Further it may declare attributes like every normal class, they will be treated as implicit finalize instructions (see Stage 1: Extension).

>>> from plumber import plumbing

>>> Base = dict

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(Base):
...
...     def foobar(self):
...         return 5

The result is a plumbing class created according to the plumbing declaration.

>>> plb = Plumbing()
>>> plb.a
True
>>> plb.foo()
42
>>> plb.bar
17
>>> plb.foobar()
5
>>> plb['a'] = 1
>>> plb['a']
1

A plumbing class can be subclassed like normal classes.

>>> class Sub(Plumbing):
...     a = 'Sub'

>>> Sub.a
'Sub'
>>> Sub().foo()
42
>>> Sub().bar
17
>>> Sub().foobar()
5

The plumber gathers instructions

A plumbing declaration provides a list of behaviors via the plumbing decorator. Behaviors provide instructions to be applied in two stages:

Stage 1

• Extension via default, override and finalize. This happens by manipulating the class dict before the actual class is created.

Stage 2

• Creation of pipelines via plumb and plumbifexists.
• Plumbing of docstrings.
• Implemented interfaces from zope.interface, if available.
• Stage 2 instructions are applied on the class object.

The plumber iterates the Behavior list from left to right (behavior order) and gathers the instructions to apply. A history of all seen instructions is kept.

>>> pprint(Plumbing.__plumbing_stacks__.stage1)
{'a': <default 'a' of <class 'Behavior1'> payload=True>,
 'bar': <default 'bar' of <class 'Behavior2'> payload=<property object at ...>>,
 'foo': <default 'foo' of <class 'Behavior1'> payload=<function Behavior1.foo at ...>>}

>>> pprint(Plumbing.__plumbing_stacks__.stage2)
{'__interfaces__': <_implements '__interfaces__' of None payload=()>}

>>> pprint(Plumbing.__plumbing_stacks__.history)
[<_implements '__interfaces__' of None payload=()>,
 <default 'a' of <class 'Behavior1'> payload=True>,
 <default 'foo' of <class 'Behavior1'> payload=<function Behavior1.foo at ...>>,
 <_implements '__interfaces__' of None payload=()>,
 <default 'bar' of <class 'Behavior2'> payload=<property object at ...>>]

Each instruction belongs to a class member name and gets compared with the previous instruction for this name if exists. It is then either taken as is, discarded, merged or a PlumbingCollision is raised. This is detailed in the following sections.

After all instructions are collected, they are applied taking declarations on the plumbing class and base classes into account.

Note

The payload of an instruction is the attribute value passed to the instruction via function call or decoration. An instruction knows the behavior it is declared on.

Note

Behaviors are created by behaviormetaclass. If zope.interface is available, it will generate _implements instructions for each behavior. During behavior creation the interfaces are not yet implemented, they are checked at a later stage. Therefore the _implements instructions are generated even if the behaviors do not implement interfaces, which results in the empty tuple as payload (see also zope.interface support.

Warning

Do not rely on this structure within your programs it might change at any time. If you need information from the __plumbing_stacks__ or lack information in there, e.g. to create a plumbing inspector and earn yourself a box of your favorite beverage, please let us know.

Stage 1 - Extension

The extension stage creates endpoints for the pipelines created in stage 2. If no pipeline uses the endpoint, it will just live on as a normal attribute in the plumbing class' dictionary.

The extension decorators:

finalize

finalize is the strongest extension instruction. It will override declarations on base classes and all other extension instructions (override and default). Attributes declared as behavior of the plumbing declaration are implicit finalize declarations. Two finalize for one attribute name will collide and raise a PlumbingCollision during class creation.

override

override is weaker than finalize and overrides declarations on base classes and default declarations. Two override instructions for the same attribute name do not collide, instead the first one will be used.

default

default is the weakest extension instruction. It will not even override declarations of base classes. The first default takes precendence over later defaults.

• Interaction - finalize, plumbing declaration and base classes
• Interaction - override, plumbing declaration and base classes
• Interaction - default, plumbing declaration and base class
• Interaction - finalize wins over override
• Interaction - finalize wins over default
• Subclassing Behaviors

Interaction - finalize, plumbing declaration and base classes

In code.

>>> from plumber import finalize

>>> class Behavior1(Behavior):
...     N = finalize('Behavior1')
...

>>> class Behavior2(Behavior):
...     M = finalize('Behavior2')

>>> class Base(object):
...     K = 'Base'

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(Base):
...     L = 'Plumbing'

>>> for x in ['K', 'L', 'M', 'N']:
...     print('%s from %s' % (x, getattr(Plumbing, x)))
K from Base
L from Plumbing
M from Behavior2
N from Behavior1

summary:

• K-Q: attributes defined by behaviors, plumbing class and base classes
• f: finalize declaration
• x: declaration on plumbing class or base class
• ?: base class declaration is irrelevant
• Y: chosen end point
• collision: indicates an invalid combination, that raises a PlumbingCollision

Attr	Behavior1	Behavior2	Plumbing	Base	ok?
K				x
L			x	?
M		f		?
N	f			?
O	f		x	?	collision
P		f	x	?	collision
Q	f	f		?	collision

collisions.

>>> class Behavior1(Behavior):
...     O = finalize(False)

>>> @plumbing(Behavior1)
... class Plumbing(object):
...     O = True
Traceback (most recent call last):
  ...
plumber.exceptions.PlumbingCollision:
    Plumbing class
  with:
    <finalize 'O' of <class 'Behavior1'> payload=False>

>>> class Behavior2(Behavior):
...     P = finalize(False)

>>> @plumbing(Behavior2)
... class Plumbing(object):
...     P = True
Traceback (most recent call last):
  ...
plumber.exceptions.PlumbingCollision:
    Plumbing class
  with:
    <finalize 'P' of <class 'Behavior2'> payload=False>

>>> class Behavior1(Behavior):
...     Q = finalize(False)

>>> class Behavior2(Behavior):
...     Q = finalize(True)

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(object):
...     pass
Traceback (most recent call last):
  ...
plumber.exceptions.PlumbingCollision:
    <finalize 'Q' of <class 'Behavior1'> payload=False>
  with:
    <finalize 'Q' of <class 'Behavior2'> payload=True>

Interaction - override, plumbing declaration and base classes

in code.

>>> from plumber import override

>>> class Behavior1(Behavior):
...     K = override('Behavior1')
...     M = override('Behavior1')

>>> class Behavior2(Behavior):
...     K = override('Behavior2')
...     L = override('Behavior2')
...     M = override('Behavior2')

>>> class Base(object):
...     K = 'Base'
...     L = 'Base'
...     M = 'Base'

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(Base):
...     K = 'Plumbing'

>>> for x in ['K', 'L', 'M']:
...     print('%s from %s' % (x, getattr(Plumbing, x)))
K from Plumbing
L from Behavior2
M from Behavior1

summary:

• K-M: attributes defined by behaviors, plumbing class and base classes
• e: override declaration
• x: declaration on plumbing class or base class
• ?: base class declaration is irrelevant
• Y: chosen end point

Attr	Behavior1	Behavior2	Plumbing	Base
K	e	e	x	?
L		e		?
M	e	e		?

Interaction - default, plumbing declaration and base class

in code.

>>> class Behavior1(Behavior):
...     N = default('Behavior1')

>>> class Behavior2(Behavior):
...     K = default('Behavior2')
...     L = default('Behavior2')
...     M = default('Behavior2')
...     N = default('Behavior2')

>>> class Base(object):
...     K = 'Base'
...     L = 'Base'

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(Base):
...     L = 'Plumbing'

>>> for x in ['K', 'L', 'M', 'N']:
...     print('%s from %s' % (x, getattr(Plumbing, x)))
K from Base
L from Plumbing
M from Behavior2
N from Behavior1

summary:

• K-N: attributes defined by behaviors, plumbing class and base classes
• d = default declaration
• x = declaration on plumbing class or base class
• ? = base class declaration is irrelevant
• Y = chosen end point

Attr	Behavior1	Behavior2	Plumbing	Base
K		d		x
L		d	x	?
M		d
N	d	d

Interaction - finalize wins over override

in code.

>>> class Behavior1(Behavior):
...     K = override('Behavior1')
...     L = finalize('Behavior1')

>>> class Behavior2(Behavior):
...     K = finalize('Behavior2')
...     L = override('Behavior2')

>>> class Base(object):
...     K = 'Base'
...     L = 'Base'

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(Base):
...     pass

>>> for x in ['K', 'L']:
...     print('%s from %s' % (x, getattr(Plumbing, x)))
K from Behavior2
L from Behavior1

summary:

• K-L: attributes defined by behaviors, plumbing class and base classes
• e = override declaration
• f = finalize declaration
• ? = base class declaration is irrelevant
• Y = chosen end point

Attr	Behavior1	Behavior2	Plumbing	Base
K	e	f		?
L	f	e		?

Interaction - finalize wins over default

in code.

>>> class Behavior1(Behavior):
...     K = default('Behavior1')
...     L = finalize('Behavior1')

>>> class Behavior2(Behavior):
...     K = finalize('Behavior2')
...     L = default('Behavior2')

>>> class Base(object):
...     K = 'Base'
...     L = 'Base'

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(Base):
...     pass

>>> for x in ['K', 'L']:
...     print('%s from %s' % (x, getattr(Plumbing, x)))
K from Behavior2
L from Behavior1

summary:

• K-L: attributes defined by behaviors, plumbing class and base classes
• d = default declaration
• f = finalize declaration
• ? = base class declaration is irrelevant
• Y = chosen end point

Attr	Behavior1	Behavior2	Plumbing	Base
K	d	f		?
L	f	d		?

Interaction - override wins over default ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+~~~~~~~~~~~~~

in code.

>>> class Behavior1(Behavior):
...     K = default('Behavior1')
...     L = override('Behavior1')

>>> class Behavior2(Behavior):
...     K = override('Behavior2')
...     L = default('Behavior2')

>>> class Base(object):
...     K = 'Base'
...     L = 'Base'

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(Base):
...     pass

>>> for x in ['K', 'L']:
...     print('%s from %s' % (x, getattr(Plumbing, x)))
K from Behavior2
L from Behavior1

summary:

• K-L: attributes defined by behaviors, plumbing class and base classes
• d = default declaration
• e = override declaration
• ? = base class declaration is irrelevant
• Y = chosen end point

Attr	Behavior1	Behavior2	Plumbing	Base
K	d	e		?
L	e	d		?

Subclassing Behaviors

in code.

>>> class Behavior1(Behavior):
...     J = default('Behavior1')
...     K = default('Behavior1')
...     M = override('Behavior1')

>>> class Behavior2(Behavior1):
...     J = default('Behavior2') # overrides ``J`` of ``Behavior1``
...     L = default('Behavior2')
...     M = default('Behavior2') # this one wins, even if ``M`` on
...                              # superclass is ``override`` instruction.
...                              # due to ordinary inheritance behavior.

>>> @plumbing(Behavior2)
... class Plumbing(object):
...     pass

>>> plb = Plumbing()
>>> plb.J
'Behavior2'

>>> plb.K
'Behavior1'

>>> plb.L
'Behavior2'

>>> plb.M
'Behavior2'

Stage 2 - Pipeline, docstrings and zope.interface instructions

In stage1 plumbing class attributes were set, which can serve as endpoints for plumbing pipelines that are build in stage2. Plumbing pipelines correspond to super-chains. Docstrings of behaviors, methods in a pipeline and properties in a pipeline are accumulated. Plumber is zope.interface aware and takes implemeneted interfaces from behaviors, if it can be imported.

• Plumbing Pipelines in general
• Method pipelines
• Property pipelines
• Subclassing Behaviors
• Mixing methods and properties within the same pipeline is not possible
• Docstrings of classes, methods and properties
• Slots and plumbings
• zope.interface (if available)

Plumbing Pipelines in general

Elements for plumbing pipelines are declared with the plumb and plumbifexists decorators:

plumb

Marks a method to be used as behavior of a plumbing pipeline. The signature of such a plumbing method is def foo(next_, self, *args, **kw). Via next_ it is passed the next plumbing method to be called. self is an instance of the plumbing class, not the behavior.

plumbifexists

Like plumb, but only used if an endpoint exists.

The user of a plumbing class does not know which next_ to pass. Therefore, after the pipelines are built, an entrance method is generated for each pipe, that wraps the first plumbing method passing it the correct next_. Each next_ method is an entrance to the rest of the pipeline.

The pipelines are build in behavior order, skipping behaviors that do not define a pipeline element with the same attribute name:

+---+-----------+-----------+-----------+----------+
|   | Behavior1 | Behavior2 | Behavior3 | ENDPOINT |
+---+-----------+-----------+-----------+----------+
|   |      --------------------------------->      |
| E |     x     |           |           |    x     |
| N |      <---------------------------------      |
+ T +-----------+-----------+-----------+----------+
| R |      ----------> --------------------->      |
| A |     y     |     y     |           |    y     |
| N |      <---------- <---------------------      |
+ C +-----------+-----------+-----------+----------+
| E |           |           |      --------->      |
| S |           |           |     z     |    z     |
|   |           |           |      <---------      |
+---+-----------+-----------+-----------+----------+

Method pipelines

Two plumbing behaviors and a dict as base class. Behavior1 lowercases keys before passing them on, Behavior2 multiplies results before returning them.

>>> from plumber import plumb

>>> class Behavior1(Behavior):
...     @plumb
...     def __getitem__(next_, self, key):
...         print('Behavior1 start')
...         key = key.lower()
...         ret = next_(self, key)
...         print ('Behavior1 stop')
...         return ret

>>> class Behavior2(Behavior):
...     @plumb
...     def __getitem__(next_, self, key):
...         print('Behavior2 start')
...         ret = 2 * next_(self, key)
...         print('Behavior2 stop')
...         return ret

>>> Base = dict

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(Base):
...     pass

>>> plb = Plumbing()
>>> plb['abc'] = 6
>>> plb['AbC']
Behavior1 start
Behavior2 start
Behavior2 stop
Behavior1 stop
12

Plumbing pipelines need endpoints. If no endpoint is available an AttributeError is raised.

>>> class Behavior1(Behavior):
...     @plumb
...     def foo(next_, self):
...         pass

>>> @plumbing(Behavior1)
... class Plumbing(object):
...     pass
Traceback (most recent call last):
  ...
AttributeError: type object 'Plumbing' has no attribute 'foo'

If no endpoint is available and a behavior does not care about that, plumbifexists can be used to only plumb if an endpoint is available.

>>> from plumber import plumbifexists

>>> class Behavior1(Behavior):
...     @plumbifexists
...     def foo(next_, self):
...         pass
...
...     @plumbifexists
...     def bar(next_, self):
...         return 2 * next_(self)

>>> @plumbing(Behavior1)
... class Plumbing(object):
...
...     def bar(self):
...         return 6

>>> hasattr(Plumbing, 'foo')
False
>>> Plumbing().bar()
12

This enables one implementation of a certain behaviour, e.g. sending events for dictionaries, to be used for readwrite dictionaries that implement __getitem__ and __setitem__ and readonly dictionaries, that only implement __getitem__ but no __setitem__.

Property pipelines

Plumbing of read only properties.

>>> class Behavior1(Behavior):
...     @plumb
...     @property
...     def foo(next_, self):
...         return 2 * next_(self)

>>> @plumbing(Behavior1)
... class Plumbing(object):
...
...     @property
...     def foo(self):
...         return 3

>>> plb = Plumbing()
>>> plb.foo
6

It is possible to extend a property with so far unset getter/setter/deleter.

>>> class Behavior1(Behavior):
...     @plumb
...     @property
...     def foo(next_, self):
...         return 2 * next_(self)

>>> class Behavior2(Behavior):
...     def set_foo(self, value):
...         self._foo = value
...     foo = plumb(property(
...         None,
...         override(set_foo),
...         ))

>>> @plumbing(Behavior1, Behavior2)
... class Plumbing(object):
...
...     @property
...     def foo(self):
...         return self._foo

>>> plb = Plumbing()
>>> plb.foo = 4
>>> plb.foo
8

Subclassing Behaviors

Other than stage 1 instructions, which extend a class with properties and functions and thus override each other by the rules of ordinary subclassing, pipeline instructions are aggregated.

>>> class Behavior1(Behavior):
...
...     @plumb
...     def foo(next_, self):
...         return 'Behavior1 ' + next_(self)
...
...     @plumb
...     def bar(next_, self):
...         return 'Behavior1 ' + next_(self)

>>> class Behavior2(Behavior1):
...
...     @plumb
...     def foo(next_, self):
...         return 'Behavior2 ' + next_(self)

>>> @plumbing(Behavior2)
... class Plumbing(object):
...
...     def foo(self):
...         return 'foo'
...
...     def bar(self):
...         return 'bar'

>>> plb = Plumbing()
>>> plb.foo()
'Behavior2 Behavior1 foo'

>>> plb.bar()
'Behavior1 bar'

Mixing methods and properties within the same pipeline is not possible

Within a pipeline all elements need to be of the same type, it is not possible to mix properties with methods.

>>> class Behavior1(Behavior):
...     @plumb
...     def foo(next_, self):
...         return next_(self)

>>> @plumbing(Behavior1)
... class Plumbing(object):
...
...     @property
...     def foo(self):
...         return 5
Traceback (most recent call last):
  ...
plumber.exceptions.PlumbingCollision:
    <plumb 'foo' of <class 'Behavior1'> payload=<function Behavior1.foo at 0x...>>
  with:
    <class 'Plumbing'>

Docstrings of classes, methods and properties

Normal docstrings of the plumbing declaration and the behavior classes, plumbed methods and plumbed properties are joined by newlines starting with the plumbing declaration and followed by the behaviors in reverse order.

>>> class P1(Behavior):
...     """P1
...     """
...     @plumb
...     def foo(self):
...         """P1.foo
...         """
...     bar = plumb(property(None, None, None, 'P1.bar'))

>>> class P2(Behavior):
...     @override
...     def foo(self):
...         """P2.foo
...         """
...     bar = plumb(property(None, None, None, 'P2.bar'))

>>> @plumbing(P1, P2)
... class Plumbing(object):
...     """Plumbing
...     """
...     bar = property(None, None, None, 'Plumbing.bar')

>>> print(Plumbing.__doc__)
Plumbing
<BLANKLINE>
P1
<BLANKLINE>

>>> print(Plumbing.foo.__doc__)
P2.foo
<BLANKLINE>
P1.foo
<BLANKLINE>

>>> print(Plumbing.bar.__doc__)
Plumbing.bar
<BLANKLINE>
P2.bar
<BLANKLINE>
P1.bar

The accumulation of docstrings is an experimental feature and will probably change.

Slots and plumbings

A plumbing class can have __slots__ like normal classes.

>>> class P1(Behavior):
...     @default
...     def somewhing_which_writes_to_foo(self, foo_val):
...         self.foo = foo_val

>>> @plumbing(P1)
... class WithSlots(object):
...     __slots__ = 'foo'

>>> WithSlots.__dict__['foo']
<member 'foo' of 'WithSlots' objects>

>>> ob = WithSlots()
>>> ob.somewhing_which_writes_to_foo('foo')
>>> assert(ob.foo == 'foo')

zope.interface (if available)

The plumber does not depend on zope.interface but is aware of it. That means it will try to import it and if available will check plumbing behaviors for implemented interfaces and will make the plumbing implement them, too.

>>> from zope.interface import Interface
>>> from zope.interface import implementer

A class with an interface that will serve as base class of a plumbing.

>>> class IBase(Interface):
...     pass

>>> @implementer(IBase)
... class Base(object):
...     pass

>>> IBase.implementedBy(Base)
True

Two behaviors with corresponding interfaces, one with a base class that also implements an interface.

>>> class IBehavior1(Interface):
...     pass

>>> @implementer(IBehavior1)
... class Behavior1(Behavior):
...     blub = 1

>>> class IBehavior2Base(Interface):
...     pass

>>> @implementer(IBehavior2Base)
... class Behavior2Base(Behavior):
...     pass

>>> class IBehavior2(Interface):
...     pass

>>> @implementer(IBehavior2)
... class Behavior2(Behavior2Base):
...     pass

>>> IBehavior1.implementedBy(Behavior1)
True

>>> IBehavior2Base.implementedBy(Behavior2Base)
True

>>> IBehavior2Base.implementedBy(Behavior2)
True

>>> IBehavior2.implementedBy(Behavior2)
True

A plumbing based on Base using Behavior1 and Behavior2 and implementing IPlumbingClass.

>>> class IPlumbingClass(Interface):
...     pass

>>> @implementer(IPlumbingClass)
... @plumbing(Behavior1, Behavior2)
... class PlumbingClass(Base):
...     pass

The directly declared and inherited interfaces are implemented.

>>> IPlumbingClass.implementedBy(PlumbingClass)
True

>>> IBase.implementedBy(PlumbingClass)
True

The interfaces implemented by the Behaviors are also implemented.

>>> IBehavior1.implementedBy(PlumbingClass)
True

>>> IBehavior2.implementedBy(PlumbingClass)
True

>>> IBehavior2Base.implementedBy(PlumbingClass)
True

An instance of the class provides the interfaces.

>>> plb = PlumbingClass()

>>> IPlumbingClass.providedBy(plb)
True

>>> IBase.providedBy(plb)
True

>>> IBehavior1.providedBy(plb)
True

>>> IBehavior2.providedBy(plb)
True

>>> IBehavior2Base.providedBy(plb)
True

Plumber metaclass hooks

In case one writes a plumbing behavior requiring class manipulation at creation time, a decorator is provided for registering callbacks which are executed after stage 1 and stage 2 instructions have been applied.

>>> from plumber import plumber

>>> class IBehaviorInterface(Interface):
...     pass

>>> @plumber.metaclasshook
... def test_metclass_hook(cls, name, bases, dct):
...     if not IBehaviorInterface.implementedBy(cls):
...         return
...     cls.hooked = True

>>> @implementer(IBehaviorInterface)
... class MetaclassConsideredBehavior(Behavior):
...     pass

>>> @plumbing(MetaclassConsideredBehavior)
... class Plumbing(object):
...     pass

>>> Plumbing.hooked
True

Miscellanea

Nomenclature

plumber

Metaclass that creates a plumbing according to the instructions declared on plumbing behaviors. Instructions are given by decorators: default, override, finalize, plumb and plumbifexists.

plumbing

A plumbing is a class decorated by plumbing decorator which gets passed the behviors to apply, e.g. @plumbing(Behavior1, Behavior2). Apart from the behaviors, declarations on base classes and the class asking for the plumber are taken into account. Once created, a plumbing looks like any other class and can be subclassed as usual.

plumbing behavior

A plumbing behavior provides attributes (functions, properties and plain values) along with instructions for how to use them. Instructions are given via decorators: default, override, finalize, plumb and plumbifexists (see Stage 1:... and Stage 2:...).

plumbing pipeline

Plumbing methods/properties with the same name form a pipeline. The entrance and end-point have the signature of normal methods: def foo(self, *args, **kw). The plumbing pipelines is a series of nested closures (see next_).

entrance (method)

A method with a normal signature. i.e. expecting self as first argument, that is used to enter a pipeline. It is a next_ function. A method declared on the class with the same name, will be overwritten, but referenced in the pipelines as the innermost method, the endpoint.

``next_`` function

The next_ function is used to call the next method in a pipelines: in case of a plumbing method, it is a wrapper of it that passes the correct next next_ as first argument and in case of an end-point, just the end-point method itself.

end-point (method)

Method retrieved from the plumbing class with getattr(), before setting the entrance method on the class.

If you feel something is missing, please let us now or write a short corresponding text.

Python Versions

• Python 2.7, 3.7+
• May work with other versions (untested)

Contributors

• Florian Friesdorf
• Robert Niederreiter
• Jens W. Klein
• Marco Lempen
• Attila Oláh