How to mock input arguments into a method in python

Question

def generate_file(config: dict): -> None:
    ...
    my_obj = create_my_obj(config)
    my_obj_binary = my_obj.export()
    write_file(my_obj_binary, "some_path", mode="wb")

def create_my_obj(config: dict) -> MyObj:
   # some configuration checking etc.
   ...
   ...
   
   return MyObj(opt1="something", opt2="something_else", value="VALUE_TO_MOCK")


# TEST
def test_my_obj():
   config = ... # get config data from json or whatever...

   generate_file(config)
   
   # load a golden sample binary
   ...
   # load binary generated by my file
   ...

   # compare those two
   assert golden_sample_binary == generated_binary

Above is a snippet of my code I'm testing. My code generates a binary file and I need to compare this binary file with a golden sample to test, whether my code generates the file properly. I need to "mock" one input argument to a very specific value in MyObj() invoked in create_my_obj(). I'm using unittest library, unfortunately I'm not able to figure this out.

Thanks for any hints

When you say I need to "mock" one input argument to a very specific value in MyObj() invoked in create_my_obj() By value do you just mean mock an attribute? if so you can do an arbitrary assignment to the attribute using MyObj.attribute = "value" inline — Kieran Wood
– Kieran Wood, Commented Aug 23, 2021 at 5:09
From "test_my_obj( )" I don't have access to "MyObj" class to be able to do so. By default MyObj( ) set's "value" to None, whereas I need it to be let's say '1234'. I could do what you propose in "create_my_obj", but that would modify my code permanently, not just for testing. — ST Renegade
– ST Renegade, Commented Aug 23, 2021 at 5:15
I see, so my_obj is being serialized and then you are checking the file in the test. If you know what value should be you can add a line after my_obj = create_my_obj(config) that just has my_obj.value = "1234" which means it will be baked into the export for you. — Kieran Wood
– Kieran Wood, Commented Aug 23, 2021 at 5:18
Still the same problem, generate_file( ) can't be modified this way, as it's part of the "API" to say. I need to do it from test_my_obj. So my idea was, "redefine" MyObj in a way, where value wouldn't be None by default, but '1234' and I thought mocking MyObj could work, but can't figure out how. — ST Renegade
– ST Renegade, Commented Aug 23, 2021 at 5:23
I see, you can use mock objects with the mock library (docs.python.org/3/library/unittest.mock.html#the-mock-class). I haven't done it with a class so I unfortunately can't help you, but it's probably the right place to start looking. Otherwise the Patch the function's __defaults__ attribute section of this article might help mdawar.dev/blog/mock-python-default-function-argument — Kieran Wood
– Kieran Wood, Commented Aug 23, 2021 at 5:28

ST Renegade · Accepted Answer · 2021-08-23 06:16:04Z

So based on @Kiearan Wood link, the solution is to patch the defaults of the init method using mock.patch.object.

import unittest.mock as mock

def test_my_obj():
   config = ... # get config data from json or whatever...

   new_defaults = list(MyObj.__init__.__defaults__)
   # The value to mock is the last in the list...
   new_defaults[-1] = "value to mock"
   new_defaults = tuple(new_defaults)

   with mock.patch.object(MyObj.__init__, "__defaults__", new_defaults):
       generate_file(config)
   
   # load a golden sample binary
   ...
   # load binary generated by my file
   ...

   # compare those two
   assert golden_sample_binary == generated_binary

Thank you very much for the link, which helped a lot!! :-)

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