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Is there a way to initialize all classes from a python module into a list of nameless objects?

Example I have a module rules which contains all child classes from a class Rule. Because of that, I'm certain they will all implement a method run() and will have a attribute name which will be generated during the call of __init__

I would like to have a list of objects dynamically initiated from those classes. By dynamically initialized i mean that they don't have to be named explicitly.

The questions are:

Is it possible to iterate through all classes in a module?

Can a nameless object be initiated?

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1 Answer

up vote 7 down vote accepted

There are at least two approaches you can take. You can get all of the subclasses of a class by calling a class's __subclasses__() method. So if your parent class is called Rule, you could call:

rule_list = [cls() for cls in Rule.__subclasses__()]

This will give you all subclasses of Rule, and will not limit them to the ones found in a particular module.

If you have a handle to your module, you can iterate over its content. Like so:

import rule
rule_list = []
for name in dir(rule):
    value = getattr(rule, name)
    if isinstance(value, type) and issubclass(value, Rule):

Unfortunately, issubclass throws TypeError if you give it an object that is not a class as its first argument. So you have to handle that somehow.

EDIT: dealing with the issubclass quirk per @Blckknght's suggestion.

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Better than my answer. You can use inspect.isclass() to avoid the TypeError. –  Antonis Christofides Aug 19 '13 at 15:10
@AntonisChristofides True, you can use inspect.isclass() as a filter. I try to avoid using anything from the inspect module in production code, however. –  Matt Anderson Aug 19 '13 at 15:15
You can also use isinstance(value, type) to make sure you have a class. (You'd also need to check for types.ClassType if you need to support old-style classes, but I'd just avoid those instead.) –  Blckknght Aug 20 '13 at 3:24
@Blckknght - good point. –  Matt Anderson Aug 20 '13 at 4:37
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