Selecting a subclass at instantiation in Python

Question

I would like to choose a subclass at instantiation to select specific attributes from certain classes. I've worked out the following:

Code

class Foo:
    a = "foo"

    def __init__(self, dtype=Bar):
            self.__class__ = dtype


class Bar:
    b = "bar"


class Baz(Bar):
    c = "baz"

Demo

Foo(dtype=Bar).b
# 'bar'

Foo(dtype=Baz).b
# 'bar'

Foo(dtype=Baz).c
# 'baz'

This gives the desired result, selecting specific attributes from Bar while optionally extending features with Baz. Unlike subclassing however, we have no access to Foo's attributes.

Foo(dtype=Baz).a
# AttributeError: 'Baz' object has no attribute 'a'

There are occasions when not all attributes are desired, so subclassing Foo(Baz) is not preferred.

What's the idiomatic analog to accomplish this in Python?

Why can't you just use inheritance? You could dynamically pick the subclass like `var = Bar; var()`. — Kent Shikama, Dec 07 '19 at 00:14
This sounds like an [XY problem](https://en.wikipedia.org/wiki/XY_problem). I don't know that doing this would ever be idiomatic. — luther, Dec 07 '19 at 00:18
@KentShikama Choosing `var = Bar()` still excludes attributes in `Foo`. — pylang, Dec 07 '19 at 00:54
Per https://docs.python.org/3/reference/datamodel.html#object.__new__ , if `__new__` returns an instance (including subclasses), `__init__` will be called. But this is all performed by `type.__call__`; see also https://stackoverflow.com/questions/6966772/using-the-call-method-of-a-metaclass-instead-of-new — o11c, Dec 07 '19 at 02:03
That said, `__new__` returning a different type should be quite rare; I've only done it in FFI-related code to map `NULL` to `None`. — o11c, Dec 07 '19 at 02:05
@pylang Can you please elaborate? `var = Bar; var().a` outputs "foo" as expected. — Kent Shikama, Dec 07 '19 at 02:15
Note it isn't `Bar() but Bar`; you're dynamically choosing the class to instantiate. — Kent Shikama, Dec 07 '19 at 02:16

score 1 · Accepted Answer · answered Dec 07 '19 at 01:48

Why create an instance of Foo in the first place if you really want an instance of Bar or Baz? Instead, make Foo a factory for instance of Bar and Baz.

class Bar:
    b = "bar"

class Baz(Bar):
    c = "baz"

class Foo:
    a = "foo"

    def __new__(cls, dtype=Bar):
        return dtype()

score 0 · Answer 2 · answered Dec 07 '19 at 02:22

As I mentioned in the comments, just use plain inheritance.

class Foo:
    a = "foo"

class Bar(Foo):
    b = "bar"

class Baz(Foo):
    c = "baz"

# Example use
import random
class_to_instantiate = Bar if random.choice([True, False]) else Baz
print(class_to_instantiate().a)

Outputs

foo

Selecting a subclass at instantiation in Python

2 Answers2