Mocking python function based on input arguments

Question

We have been using Mock for python for a while    Now  we have a situation in which we want to mock a function  def foo self  my param        do something here  assign something to my result     return my result   Normally  the way to mock this would be  assuming foo being part of an object   self foo   MagicMock return value  mocked      Even  if i call foo   a couple of times i can use  self foo   MagicMock side effect   mocked once    mocked twice       Now  I am facing a situation in which I want to return a fixed value when the input parameter has a particular value  So if let s say  my param  is equal to  something  then I want to return  my cool mock   This seems to be available on mockito for python  when dummy  foo  something   thenReturn  my cool mock     I have been searching on how to achieve the same with Mock with no success    Any ideas

User · Answer

As indicated at Python Mock object with method called multiple times

A solution is to write my own side_effect

def my_side_effect(*args, **kwargs):
    if args[0] == 42:
        return "Called with 42"
    elif args[0] == 43:
        return "Called with 43"
    elif kwargs['foo'] == 7:
        return "Foo is seven"

mockobj.mockmethod.side_effect = my_side_effect

That does the trick

User · Answer

Although side effect can achieve the goal  it is not so convenient to setup side effect function for each test case  I write a lightweight Mock  which is called NextMock  to enhance the built-in mock to address this problem  here is a simple example  from nextmock import Mock  m   Mock    m with args 1  2  3  returns 123   assert m 1  2  3     123 assert m 3  2  1     123  It also supports argument matcher  from nextmock import Arg  Mock  m   Mock    m with args 1  2  Arg Any  returns 123   assert m 1  2  1     123 assert m 1  2   quot 123 quot      123  Hope this package could make testing more pleasant  Feel free to give any feedback

User · Answer

If side effect func is a function then whatever that function returns is what calls to the mock return  The side effect func function is called with the same arguments as the mock  This allows you to vary the return value of the call dynamically  based on the input   gt  gt  gt  def side effect func value           return value   1      gt  gt  gt  m   MagicMock side effect side effect func   gt  gt  gt  m 1  2  gt  gt  gt  m 2  3  gt  gt  gt  m mock calls  call 1   call 2     http   www voidspace org uk python mock mock html calling

User · Answer

Side effect takes a function  which can also be a lambda function   so for simple cases you may use   m   MagicMock side effect  lambda x  x 1

User · Answer

Just to show another way of doing it   def mock isdir path       return path in    var log     var log apache2     var log tomcat    with mock patch  os path isdir   as os path isdir      os path isdir side effect   mock isdir

User · Answer

You can also use partial from functools if you want to use a function that takes parameters but the function you are mocking does not  E g  like this   def mock year year       return datetime datetime year  11  28  tzinfo timezone utc     patch  django utils timezone now   side effect partial mock year  year 2020     This will return a callable that doesn t accept parameters  like Django s timezone now     but my mock year function does

User · Answer

I ve ended up here looking for  how to mock a function based on input arguments  and I finally solved this creating a simple aux function   def mock responses responses  default response None     return lambda input  responses input  if input in responses else default response   Now   my mock foo side effect   mock responses           x   42        y    1 2 3       my mock goo side effect   mock responses           hello    world          default response  hi        my mock foo  x       gt  42 my mock foo  y       gt   1 2 3  my mock foo  unknown       gt  None  my mock goo  hello       gt   world  my mock goo  ey       gt   hi    Hope this will help someone

User · Answer

If you  want to return a fixed value when the input parameter has a particular value   maybe you don t even need a mock and could use a dict along with its get method   foo     input1    value1    input2    value2   get  foo  input1      value1 foo  input2      value2   This works well when your fake s output is a mapping of input  When it s a function of input I d suggest using side effect as per Amber s answer   You can also use a combination of both if you want to preserve Mock s capabilities  assert called once  call count etc    self mock side effect     input1    value1    input2    value2   get

User · Answer

You can also use  mock patch object   Let s say a module my module py uses pandas to read from a database and we would like to test this module by mocking pd read sql table method  which takes table name as argument    What you can do is to create  inside your test  a db mock method that returns different objects depending on the argument provided   def db mock   kwargs       if kwargs  table name       table 1             return some DataFrame     elif kwargs  table name       table 2             return some other DataFrame   In your test function you then do   import my module as my module imported   mock patch object my module imported pd   read sql table   new callable lambda  db mock  def test my module mock read sql table         You can now test any methods from  my module   e g   foo  and any call this        method does to  read sql table  will be mocked by  db mock   e g      ret   my module imported foo table name  table 1          ret  is some DataFrame returned by  db mock

[python] Mocking python function based on input arguments

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