Using python s mock patch object to change the return value of a method called within another method

Question

Is it possible to mock a return value of a function called within another function I am trying to test  I would like the mocked method  which will be called in many methods I m testing  to returned my specified variables each time it is called  For example    class Foo      def method 1           results   uses some other method       def method n           results   uses some other method     In the unit test  I would like to use mock to change the return value of uses some other method   so that any time it is called in Foo  it will return what I defined in  patch object

User · Answer

This can be done with something like this:

# foo.py
class Foo:
    def method_1():
        results = uses_some_other_method()


# testing.py
from mock import patch

@patch('Foo.uses_some_other_method', return_value="specific_value"):
def test_some_other_method(mock_some_other_method):
    foo = Foo()
    the_value = foo.method_1()
    assert the_value == "specific_value"

Here's a source that you can read: Patching in the wrong place

User · Answer

There are two ways you can do this  with patch and with patch object  Patch assumes that you are not directly importing the object but that it is being used by the object you are testing as in the following   foo py def some fn        return  some fn   class Foo object       def method 1 self           return some fn      bar py import foo class Bar object       def method 2 self           tmp   foo Foo           return tmp method 1      test case 1 py import bar from mock import patch   patch  foo some fn   def test bar mock some fn       mock some fn return value    test-val-1      tmp   bar Bar       assert tmp method 2       test-val-1      mock some fn return value    test-val-2      assert tmp method 2       test-val-2    If you are directly importing the module to be tested  you can use patch object as follows    test case 2 py import foo from mock import patch   patch object foo   some fn   def test foo test some fn       test some fn return value    test-val-1      tmp   foo Foo       assert tmp method 1       test-val-1      test some fn return value    test-val-2      assert tmp method 1       test-val-2    In both cases some fn will be  un-mocked  after the test function is complete   Edit  In order to mock multiple functions  just add more decorators to the function and add arguments to take in the extra parameters   patch object foo   some fn    patch object foo   other fn   def test foo test other fn  test some fn             Note that the closer the decorator is to the function definition  the earlier it is in the parameter list

User · Answer

To add to Silfheed s answer  which was useful  I needed to patch multiple methods of the object in question  I found it more elegant to do it this way  Given the following function to test  located in module a function to test py  from some other module import SomeOtherClass  def add results        my object   SomeOtherClass  some contextual parameters       result a   my object method a       result b   my object method b            return result a   result b  To test this function  or class method  it doesn t matter   one can patch multiple methods of the class SomeOtherClass by using patch object   in combination with sys modules   patch object sys modules  module a function to test     SomeOtherClass   def test  should add results self  mocked other class     mocked other class   method a return value   4   mocked other class   method b return value   7    self assertEqual add results    11   This works no matter the number of methods of SomeOtherClass you need to patch  with independent results  Also  using the same patching method  an actual instance of SomeOtherClass can be returned if need be   patch object sys modules  module a function to test     SomeOtherClass   def test  should add results self  mocked other class     other class instance   SomeOtherClass  some controlled parameters     mocked other class return value   other class instance

User · Answer

Let me clarify what you re talking about  you want to test Foo in a testcase  which calls external method uses some other method  Instead of calling the actual method  you want to mock the return value   class Foo      def method 1           results   uses some other method       def method n           results   uses some other method     Suppose the above code is in foo py and uses some other method is defined in module bar py  Here is the unittest   import unittest import mock  from foo import Foo   class TestFoo unittest TestCase        def setup self           self foo   Foo         mock patch  foo uses some other method       def test method 1 self  mock method           mock method return value   3         self foo method 1  args    kwargs           mock method assert called with  args    kwargs    If you want to change the return value every time you passed in different arguments  mock provides side effect

[python] Using python's mock patch.object to change the return value of a method called within another method

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