python - Overwrite import, show names of modules importing a module

I want to find out which modules are importing my sample module "foo":

foo.py

# pseudocode, this should be triggered when "foo" is imported
on_import():
    print "foo is imported by module X"

bar.py

# this should print "foo is imported by module bar"

import foo 

How can I implement this behavior?

Answer

@Wooble you are right

Here's the solution from know filename:line_no where an import to my_module was made

import inspect

last_frame = inspect.stack()[1]
print 'Module imported from file:line_no = %s:%i' % last_frame[1:3]