Monkey patch a function in a module for unit testing

Question

I have the following method in a module that calls another method imported from another module:

def imported_function():
    do_unnecessary_things_for_unittest()

The actual method that needs to be tested, imports and uses the above function:

from somewhere import imported_function

def function_to_be_tested():
    imported_function()
    do_something_more()
    return 42

The inner calls and related calculations inside imported_function are not important and they are not what I want to test so I just want to skip them while testing the actual function function_to_be_tested.

Thus, I tried to monkey patch the module named somewhere inside the test method but no luck.

def test_function_to_be_tested(self):
    import somewhere
    somewhere.__dict__['imported_function'] = lambda : True

The question is, how can I monkey patch a method of a module while testing so that it will not be called during the test phase?

Answer 1

I think better to use Mock Library

So you can do something like:

from somewhere import imported_function

@patch(imported_function)
def test_function_to_be_tested(self, imported_function):
    imported_function.return_value = True
    #Your test

I think for unit tests it's better than monkey patch.

Answer 2

Say you have the following files:

somewhere.py

def imported_function():
    return False

testme.py

from somewhere import imported_function

def function_to_be_tested():
    return imported_function()

A call to testme.function_to_be_tested() would return False.

---

Now, the trick is to import somewhere before testme:

import somewhere
somewhere.__dict__['imported_function'] = lambda : True

import testme
def test_function_to_be_tested():
    print testme.function_to_be_tested()

test_function_to_be_tested()

Output:

True

---

Or, reload the testme module

import testme

def test_function_to_be_tested():
    print testme.function_to_be_tested()
    import somewhere
    somewhere.__dict__['imported_function'] = lambda : True
    print testme.function_to_be_tested()
    reload(testme)
    print testme.function_to_be_tested()

test_function_to_be_tested()

Output:

False
False
True