How to mock the return value of class method imported from another class in Python?

Question

I have these files:

/foo.py 
/foo2.py 
/test_food.py

in foo.py, I have this:

from foo2 import Foo2

class Foo(object):

    def GetFoo2(self):
        f = Foo2()
        return f.Get()

and in foo2.py, I have:

class Foo2(object):

    def __init__(self):
        self.myvar = "eeee"

    def Get(self):
        return self.myvar

In test_foo.py,

import os, sys, json, pytest
from mock import *
from foo import Foo

def test_foo_ok(monkeypatch):

    monkeypatch.setattr(Foo, "GetFoo2", lambda x:"abc")
    f = Foo()
    result = f.GetFoo2()
    assert result == "abc"

So, in test_foo.py, I am able to mock or monkeypatch the method GetFoo2(). However, instead of doing that, how can I mock the return value of Foo2's Get() method from test_foo.py without changing the code in foo.py and foo2.py?

Answer 1

Whenever you import using from, you can patch that module's namespace. Here is an example for your test_foo_ok function:

import os, sys, json, pytest
from mock import *
from foo import Foo

def test_foo_ok(monkeypatch):
    monkeypatch.setattr("foo.Foo2.Get", lambda x:"abc")
    f = Foo()
    result = f.GetFoo2()
    assert result == "abc"

See the where to patch section of the Mock library's documentation for more details. The same rules apply to pytest's monkeypatch.