Python: Mock a module without importing it or needing it to exist

Question

I am starting to use a python mock library for my testing. I want to mock a module that is imported within the namespace of the module under test without actually importing it o

Answer 1

You're kind of missing the point of what a Mock is. You're supposed to build them when you want an object with a particular interface, regardless of how it's implemented.

What you're doing is trying to re-implement python's module system, plus it's a pretty horrible abuse of global variables to boot.

Instead of making foo a module, make a Foo class, and pass in the helpers in the constructor.

class Foo(object):
    def __init__(self, helpers):
        self.helpers = helpers

# then, instead of import foo:
foo = Foo(mock_helpers)

Even if the real "helpers" is actually going to be a module, there is no reason you need to be messing with sys.modules and setting it up via import in your tests.

And if foo has to be a module, that's fine too, but you do it like this:

# foo.py
class Foo(object):
    pass # same code as before, plus foo_func

try:
   import whatever
   _singleton = Foo(whatever)
except ImportError:
   _singleton = Foo(something_else)

def foo_func():
   return _singleton.foo_func()

Large chunks of the standard library work this way. It's pretty much the standard for defining singleton-like modules.