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.

• John Zwinck: 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?
• PM 2Ring: 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.

Answer 1

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

Answer 2

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))