Use the following attribute:
cls.__bases__
From the docs:
The tuple of base classes of a class
object.
Example:
>>> str.__bases__
(<type 'basestring'>,)
Another example:
>>> class A(object):
... pass
...
>>> class B(object):
... pass
...
>>> class C(A, B):
... pass
...
>>> C.__bases__
(<class '__main__.A'>, <class '__main__.B'>)
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