Python Instantiate All Classes Within a Module

Question

I am working on a text-based adventure game and have a module that has all of the action classes, e.g. Move(Action), Look(Action). I need a method of instantiating all of the classes in the module that are a subclass of the Action class into a list like this:

actions = [Move(), Look()]

Is there a way of doing this without having to individually instantiate the classes by typing their names?

I'm sure you can work out how to instantiate them all from there! — jonrsharpe, Jan 01 '16 at 09:45

Mike Müller · Accepted Answer · 2016-01-01T10:07:38.630

Solution

This works:

class Action:
    pass

class Move(Action):
    pass

class Look(Action):
    pass


actions = []
global_objs = list(globals().items())

for name, obj in global_objs:
    if obj is not Action and isinstance(obj, type) and issubclass(obj, Action):
        actions.append(obj())
print(actions)

prints:

[<__main__.Move object at 0x101916ba8>, <__main__.Look object at 0x101916be0>]

Steps

First, we get all names an an object in the current module:

global_objs = list(globals().items())

We need to convert into a list because in Python 3 items() return a dictview object that reflects changes in the underlaying dictionary. Since we work in the same module, defining new objects will change this dictionary. Converting into a list solves this problem.

Next, we go through all objects. There are three conditions they need to fulfill to be subclass of Action:

obj is not Action- Since Action is a subclass of itself, we need filter it out.
isinstance(obj, type) - The object has to be a class. Otherwise, the test under 3. would not be possible.
issubclass(obj, Action) - Finally, we can do our subclass test.

Now we can make instances of all filtered classes and append them to our list:

actions.append(obj())

Shorter version

If you are sure all classes are defined in one module or you even want all subclasses that are distributed over several modules, this would be much shorter:

>>> actions = [obj() for obj in Action.__subclasses__()]
>>> actions
[<__main__.Move at 0x10fc14fd0>, <__main__.Look at 0x10fc14668>]

`for obj in Action.__subclasses__()` would save you a lot of faff... — jonrsharpe, Jan 01 '16 at 09:50
@jonrsharpe Good point. But it would find all subclasses; not only these in the module. Still would need to filter for these. — Mike Müller, Jan 01 '16 at 09:56
Good point, but the OP does say they're all in the one module — jonrsharpe, Jan 01 '16 at 09:58
If the subclasses are spread across multiple modules but you only want to include them from the one module, you could use `actions = [obj() for obj in Action.__subclasses__() if obj.__module__ == 'mymodule']` — David Gard, Jan 24 '22 at 13:43

score 1 · Answer 2 · answered Jan 01 '16 at 09:15

from other_module import Action

def is_action_class(var):
    return var != Action and type(var) == type and issubclass(var, Action)

classes = []    

for attr in dir(my_module):
    var = getattr(my_module, attr)
    if is_action_class(var):
        classes.append(var)
instances = [c() for c in classes]

Python Instantiate All Classes Within a Module

2 Answers2

Solution

Steps

Shorter version

Linked