How can I replace a module without importing it?

Question

I would like to test a module named "x" which imports a module named "y". The problem is that it takes several seconds to import y. How can I write a test for x that replaces y with my own module while preventing y from being imported?

All of the Python libraries that I'm aware of that allow one to replace a module require one to first import the module.

Answer 1

So, let's say we have these modules x and y:

# x.py
import y
y.foo()

# y.py
def foo():
    raise RuntimeError("Nope.")

Given those modules, the following will obviously fail:

# test_real.py
import x

$ python test_real.py 
Traceback (most recent call last):
File "test_real.py", line 2, in <module>
    import x
File "x.py", line 3, in <module>
    y.foo()
File "y.py", line 3, in foo
    raise RuntimeError("Nope.")
RuntimeError: Nope.

If we have a fake y (called z):

# z.py
def foo():
    print "Yessir!"

... we need to convince Python, before we import x, that y has already been imported (substituting z for it). That's actually quite simple:

# test_fake.py
import sys
sys.modules['y'] = __import__('z')

import x

... and it works:

$ python test_fake.py 
Yessir!