How does one monkey patch a function in python?

Question

I'm having trouble replacing a function from a different module with another function and it's driving me crazy.

Let's say I have a module bar.py that looks like this:

from a_package.baz import do_something_expensive  def a_function():     print do_something_expensive() 

And I have another module that looks like this:

from bar import a_function a_function()  from a_package.baz import do_something_expensive do_something_expensive = lambda: 'Something really cheap.' a_function()  import a_package.baz a_package.baz.do_something_expensive = lambda: 'Something really cheap.' a_function() 

I would expect to get the results:

Something expensive! Something really cheap. Something really cheap. 

But instead I get this:

Something expensive! Something expensive! Something expensive! 

What am I doing wrong?

Answer 1

It may help to think of how Python namespaces work: they're essentially dictionaries. So when you do this:

from a_package.baz import do_something_expensive do_something_expensive = lambda: 'Something really cheap.' 

think of it like this:

do_something_expensive = a_package.baz['do_something_expensive'] do_something_expensive = lambda: 'Something really cheap.' 

Hopefully you can realize why this doesn't work then :-) Once you import a name into a namespace, the value of the name in the namespace you imported from is irrelevant. You're only modifying the value of do_something_expensive in the local module's namespace, or in a_package.baz's namespace, above. But because bar imports do_something_expensive directly, rather than referencing it from the module namespace, you need to write to its namespace:

import bar bar.do_something_expensive = lambda: 'Something really cheap.' 

Answer 2

There's a really elegant decorator for this: Guido van Rossum: Python-Dev list: Monkeypatching Idioms.

There's also the dectools package, which I saw an PyCon 2010, which may be able to be used in this context too, but that might actually go the other way (monkeypatching at the method declarative level... where you're not)