Mocking a class Mock or patch

Question

I am using mock with Python and was wondering which of those two approaches is better  read  more pythonic     Method one   Just create a mock object and use that  The code looks like   def test one  self       mock   Mock       mock method return value   True      self sut something mock    This should called mock method and checks the result       self assertTrue mock method called    Method two   Use patch to create a mock   The code looks like    patch  MyClass   def test two  self  mock       instance   mock return value     instance method return value   True     self sut something instance    This should called mock method and checks the result       self assertTrue instance method called    Both methods do the same thing   I am unsure of the differences    Could anyone enlighten me

User · Answer

I ve got a YouTube video on this   Short answer  Use mock when you re passing in the thing that you want mocked  and patch if you re not   Of the two  mock is strongly preferred because it means you re writing code with proper dependency injection   Silly example     Use a mock to test this  my custom tweeter twitter api  sentence       sentence replace  cks   x       We re cool and hip      twitter api send sentence     Use a patch to mock out twitter api  You have to patch the Twitter   module class    and have it return a mock  Much uglier  but sometimes necessary  my badly written tweeter sentence       twitter api   Twitter user  XXX   password  YYY       sentence replace  cks   x        twitter api send sentence

User · Answer

Key points which explain difference and provide guidance upon  working with unittest mock   Use Mock if you want to replace some interface elements passing args  of the object under test Use patch if you want to replace internal call to some objects and imported modules of the object under test Always provide spec from the object you are mocking  With patch you can always provide autospec With Mock you can provide spec Instead of Mock  you can use create autospec  which intended to create Mock objects with specification     In the question above the right answer would be to use Mock  or to be more precise create autospec  because it will add spec to the mock methods of the class you are mocking   the defined spec on the mock will be helpful in case of an attempt to call method of the class which doesn t exists   regardless signature   please see some from unittest import TestCase from unittest mock import Mock  create autospec  patch   class MyClass            staticmethod     def method foo  bar           print foo    def something some class  MyClass       arg   1       Would fail becuase of wrong parameters passed to methd      return some class method arg    def second some class  MyClass       arg   1     return some class unexisted method arg    class TestSomethingTestCase TestCase       def test something with autospec self           mock   create autospec MyClass          mock method return value   True           Fails because of signature misuse          result   something mock          self assertTrue result          self assertTrue mock method called           def test something self           mock   Mock      Note that Mock spec MyClass  will also pass  because signatures of mock don t have spec          mock method return value   True                  result   something mock                   self assertTrue result          self assertTrue mock method called               def test second with patch autospec self           with patch f    name    MyClass   autospec True  as mock                Fails because of signature misuse              result   second mock          self assertTrue result          self assertTrue mock unexisted method called    class TestSecondTestCase TestCase       def test second with autospec self           mock   Mock spec MyClass            Fails because of signature misuse          result   second mock          self assertTrue result          self assertTrue mock unexisted method called           def test second with patch autospec self           with patch f    name    MyClass   autospec True  as mock                Fails because of signature misuse              result   second mock          self assertTrue result          self assertTrue mock unexisted method called           def test second self           mock   Mock           mock unexisted method return value   True                  result   second mock                   self assertTrue result          self assertTrue mock unexisted method called   The test cases with defined spec used fail because methods called from something and second functions aren t complaint with MyClass  which means - they catch bugs  whereas default Mock will display  As a side note there is one more option  use patch object to mock just the class method which is called with  The good use cases for patch would be the case when the class is used as inner part of function  def something        arg   1     return MyClass method arg   Then you will want to use patch as a decorator to mock the MyClass

User · Answer

mock patch is a very very different critter than mock Mock   patch replaces the class with a mock object and lets you work with the mock instance   Take a look at this snippet    gt  gt  gt  class MyClass object         def   init   self           print  Created MyClass  0   format id self         gt  gt  gt  def create instance          return MyClass         gt  gt  gt  x   create instance   Created MyClass 4299548304  gt  gt  gt    gt  gt  gt   mock patch    main   MyClass       def create instance2 MyClass         MyClass return value    foo        return create instance         gt  gt  gt  i   create instance2    gt  gt  gt  i  foo   gt  gt  gt  def create instance          print MyClass       return MyClass        gt  gt  gt  create instance2    lt mock Mock object at 0x100505d90 gt   foo   gt  gt  gt  create instance    lt class    main   MyClass  gt  Created MyClass 4300234128  lt   main   MyClass object at 0x100505d90 gt    patch replaces MyClass in a way that allows you to control the usage of the class in functions that you call   Once you patch a class  references to the class are completely replaced by the mock instance   mock patch is usually used when you are testing something that creates a new instance of a class inside of the test   mock Mock instances are clearer and are preferred   If your self sut something method created an instance of MyClass instead of receiving an instance as a parameter  then mock patch would be appropriate here

[python] Mocking a class: Mock() or patch()?

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