[Python-ideas] Composition over Inheritance

Wes Turner wes.turner at gmail.com
Mon Oct 30 02:29:55 EDT 2017

On Sunday, October 29, 2017, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 29 October 2017 at 12:25, Brendan Barnwell <brenbarn at brenbarn.net
> <javascript:_e(%7B%7D,'cvml','brenbarn at brenbarn.net');>> wrote:
>> On 2017-10-28 19:13, Soni L. wrote:
>>> And to have all cars have engines, you'd do:
>>> class Car:
>>>     def __init__(self, ???):
>>>       self[Engine] = GasEngine()
>>> car = Car()
>>> car[Engine].kickstart() # kickstart gets the car as second argument.
>>> And if you can't do that, then you can't yet do what I'm proposing, and
>>> thus the proposal makes sense, even if it still needs some refining...
>>         As near as I can tell you can indeed do that, although it's still
>> not clear to me why you'd want to.  You can give Car a __getitem__ that
>> on-the-fly generates an Engine object that knows which Car it is attached
>> to, and then you can make Engine.kickstart a descriptor that knows which
>> Engine it is attached to, and from that can figure out which Car it is
>> attached to.
> Right, I think a few different things are getting confused here related to
> how different folks use composition.
> For most data modeling use cases, the composition model you want is either
> a tree or an acyclic graph, where the subcomponents don't know anything
> about the whole that they're a part of. This gives you good component
> isolation, and avoids circular dependencies.
> However, for other cases, you *do* want the child object to be aware of
> the parent - XML etrees are a classic example of this, where we want to
> allow navigation back up the tree, so each node gains a reference to its
> parent node. This often takes the form of a combination of delegation
> (parent->child references) and dependency inversion (child->parent
> reference).

This is Java-y and maybe not opcode optimizable, but maybe there's a case
for defining __setattribute__ so that square brackets denote Rust-like


  def request_dev(self, req=None):

  > Observe that in the following example the child definitions must define
an optional ‘req’ argument; in runtime they will be passed its value basing
on what their parent object will return.

It's testable, but confusing to Java programmers who aren't familiar with
why Guice forces the patterns that it does:



> Like the factory, dependency injection is just a design pattern. The core
principle is to separate behaviour from dependency resolution. In our
example, the RealBillingService is not responsible for looking up the
TransactionLog and CreditCardProcessor. Instead, they're passed in as
constructor parameters:

When these are constructor parameters, we don't need to monkeypatch attrs
in order to write tests; which, IIUC, is also partly why you'd want
traits/mixins with the proposed special Rust-like syntax:


(this is too magic(), too)

But you want dynamic mixins that have an upward reference and Rust-like
syntax (and no factories).

> For the car/engine example, this relates to explicitly modeling the
> relationship whereby a car can have one or more engines

class MultiEngine():


But interfaces aren't yet justified because it's only a few lines and those
are just documentation or a too-complex adapter registry dict, anyway.

>  (but the engine may not currently be installed),

So it should default to a MockEngine which also implements(IEngine) and
raises NotImplementedError

>  while an engine can be installed in at most one car at any given point in
> time.

But the refcounts would be too difficult


> You don't even need the descriptor protocol for that though, you just need
> the subcomponent to accept the parent reference as a constructor parameter:
>     class Car:
>       def __init__(self, engine_type):
>         self.engine = engine_type(self)
> However, this form of explicit dependency inversion wouldn't work as well
> if you want to be able to explicitly create an "uninstalled engine"
> instance, and then pass the engine in as a parameter to the class
> constructor:
>     class Car:
>       def __init__(self, engine):
>         self.engine = engine # How would we ensure the engine is marked as
> installed here?
> As it turns out, Python doesn't need new syntax for this either, as it's
> all already baked into the regular attribute access syntax, whereby
> descriptor methods get passed a reference not only to the descriptor, but
> *also* to the object being accessed: https://docs.python.org/3/
> howto/descriptor.html#descriptor-protocol

> And then the property builtin lets you ignore the existence of the
> descriptor object entirely, and only care about the original object,
> allowing the above example to be written as:
>     class Car:
>       def __init__(self, engine):
>         self.engine = engine # This implicitly marks the engine as
> installed
>       @property
>       def engine(self):
>           return self._engine
>       @engine.setter
>       def engine(self, engine):
>          if engine is not None:
>              if self._engine is not None:
>                  raise RuntimeError("Car already has an engine installed")
>              if engine._car is not None:
>                  raise RuntimeError("Engine is already installed in
> another car")
>              engine._car = self
>          self._engine = engine
>      car = Car(GasEngine())

This could be less verbose. And less likely to raise a RuntimeError.

> ORMs use this kind of descriptor based composition management extensively
> in order to reliably model database foreign key relationships in a way
> that's mostly transparent to users of the ORM classes.

So there's a 'factory' which passes the ref as a constructor parameter for
such ORM instances; but they generally don't need to be dynamically
modified at runtime because traits.

> Cheers,
> Nick.
> --
> Nick Coghlan   |   ncoghlan at gmail.com
> <javascript:_e(%7B%7D,'cvml','ncoghlan at gmail.com');>   |   Brisbane,
> Australia
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