What is the fastest way to check if a class has a function defined?

It works in both Python 2 and Python 3

hasattr(connection, 'invert_opt')

hasattr returns True if connection object has a function invert_opt defined. Here is the documentation for you to graze

https://docs.python.org/2/library/functions.html#hasattr https://docs.python.org/3/library/functions.html#hasattr

Yes, use getattr() to get the attribute, and callable() to verify it is a method:

invert_op = getattr(self, "invert_op", None)
if callable(invert_op):
    invert_op(self.path.parent_op)

Note that getattr() normally throws exception when the attribute doesn't exist. However, if you specify a default value (None, in this case), it will return that instead.