Alex Alex - 1 month ago 6
Python Question

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

I'm writing an AI state space search algorithm, and I have a generic class which can be used to quickly implement a search algorithm. A subclass would define the necessary operations, and the algorithm does the rest.

Here is where I get stuck: I want to avoid regenerating the parent state over and over again, so I have the following function, which returns the operations that can be legally applied to any state:

def get_operations(self, include_parent=True):
ops = self._get_operations()
if not include_parent and self.path.parent_op:
try:
parent_inverse = self.invert_op(self.path.parent_op)
ops.remove(parent_inverse)
except NotImplementedError:
pass
return ops


And the invert_op function throws by default.

Is there a faster way to check to see if the function is not defined than catching an exception?

I was thinking something on the lines of checking for present in dir, but that doesn't seem right. hasattr is implemented by calling getattr and checking if it raises, which is not what I want.

Answer

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.

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