Dynamically Create a Python Subclass in a Function

Question

I want to write a function creating dynamic subclass. For example:

def get_class(num):
    ...
    ...
    return _class

This function accepts an integer num and then constructs such a class

class Test(BaseClass):
    attr_1 = 1
    attr_2 = 1
    ...
    attr_num = 1

and return it.

I wonder whether this is possible WITHOUT writing a metaclass.

What is a "dynamic subclass" exactly? Does get_class() return one of several predetermined classes, or an instance of one, or does it actually create entirely new types? — John Zwinck
– John Zwinck, Commented Jan 5, 2015 at 4:40
I hope this is just a simple example to explain your question, because creating a bunch of classes merely to accommodate a variable number of attributes named like that does not look like a good design: it would be better to make a single class that stores those attributes in a tuple or list. — PM 2Ring
– PM 2Ring, Commented Jan 5, 2015 at 5:22

miki725 · Accepted Answer · 2015-01-05 04:44:19Z

2

As mentioned in the other answer, you can either use type, in which case you dynamically construct attributes:

def foo(num, **kwargs):
    kwargs.update({'num': num})
    return type('Foo', (BaseClass,), kwargs)

or if you dont need dynamic keys you can then simply manually instantiate a class:

def foo(num):
    class Foo(BaseClass):
        num = num
    return Foo

answered Jan 5, 2015 at 4:44

miki725

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Comments

vicvicvic · Accepted Answer · 2015-01-05 04:41:38Z

1

You can use the type function to dynamically create a class. The documentation for type pretty much describes exactly what you're trying to accomplish. Something like this:

class BaseClass(object):
    pass

def get_class(num):
    attrs = [('attr_%d' % i, 1) for i in range(num)]
    return type('Test', (BaseClass,), dict(attrs))

answered Jan 5, 2015 at 4:41

vicvicvic

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