How to determine file where particular function is defined

...at execution time, given that all you have is an instance of function object:

>>> type(func)
<class 'function'>

Knowing where it is defined may save you some time during debugging session: say, when you are not sure that function you are dealing with is imported from “correct” location. Despite being easy, such sanity check may help.

Most obvious option is to use co_filename attribute of function’s code:

>>> func.__code__.co_filename
'/home/user/.../something.py'

This also may work with methods:

>>> class SomeClass:
...     def some_method(self):
...         pass
... 
>>> obj = SomeClass()
>>> obj.some_method.__code__.co_filename
'<stdin>'
>>> obj.some_method.__func__.__code__.co_filename
'<stdin>'

Usually it’s better to use inspect.getsourcefile (that uses inspect.getfile, which uses something similar to above, but in more consistent way—for many different types):

>>> import inspect
>>> inspect.getsourcefile(func)
'/home/user/.../something.py'

But if func is defined in CPython extension (that is shared library which uses C API), inspect.getsourcefile will fail with TypeError:

>>> inspect.getsourcefile(func)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/.../lib/python3.4/inspect.py", line 571, in getsourcefile
    filename = getfile(object)
  File "/home/user/.../lib/python3.4/inspect.py", line 536, in getfile
    'function, traceback, frame, or code object'.format(object))
TypeError: <built-in function func> is not a module, class, method, function, traceback, frame, or code object

In such case it is still possible to get file path of .so:

>>> inspect.getfile(inspect.getmodule(func))
'/home/user/.../lib/python3.4/site-packages/something.cpython-34m.so'