how to test file write with print statement in python

Question

I would like to test the following statement

with open('test.txt', 'w') as f:
    print(['abc', 20], end='', file=f)

I tried

  from __future__ import print_function                                              
  from mock import patch, mock_open                                                  


  def f():                                                                           
      with open('test.txt', 'w') as f:                                               

          # f.write(str(['abc', 20]))                                                 
          print(['abc', 20], end='', file=f)                                         


  @patch('__builtin__.open', new_callable=mock_open)                                 
  def test(mock_f):                                                                  
      f()                                                                            
      mock_f.assert_called_with('test.txt', 'w')                                     
      handle = mock_f()                                                              
      handle.write.assert_called_once_with(str(['abc', 20]))

It complains that write is never called, which makes sense. In this case, what is the proper way to check the content for writing?

I also tried to use f.write(str(['abc', 20])) instead of the print statement, which passes the test. Is it just a bad idea to use print?

You could check that f.tell() is not zero; though it is a bit of a work-around — James
– James, Commented Sep 6, 2017 at 16:50
Seems like your mock open would need to emulate a context manager and return an instance of a mock file - then it would be the mock file that has the .write assertion(s). — wwii
– wwii, Commented Sep 6, 2017 at 18:49
There is an example in tthe docs that looks like what you are trying to do. — wwii
– wwii, Commented Sep 6, 2017 at 18:59
@wwii, I don't think that the example from docs, is what he wants. He is mocking the open method, and his assert is matching with print function. That is the error. — Mauro Baraldi
– Mauro Baraldi, Commented Sep 8, 2017 at 12:37

wwii · Accepted Answer · 2017-09-08 17:02:58Z

Python 3.6:

If you inspect the mock object you can see that the thing you are looking for is there.

from unittest.mock import mock_open, patch

def f():                                                                           
    with open('test.txt', 'w') as f:                                               

        print(['abc', 20], end='', file=f)                                         

@patch('__main__.__builtins__.open', new_callable=mock_open)                                 
def test(mock_f):
    f()
    mock_f.assert_called_with('test.txt', 'w')
    print('calls to open:\n', mock_f.mock_calls, end ='\n\n')
    handle = mock_f()
    print('calls to the file:\n', handle.mock_calls, end ='\n\n')
    print('calls to write:\n', handle.write.mock_calls, end ='\n\n')
    handle.write.assert_called_once_with(str(['abc', 20]))

>>> test()
calls to open:
 [call('test.txt', 'w'),
 call().__enter__(),
 call().write("['abc', 20]"),
 call().write(''),
 call().__exit__(None, None, None)]

calls to the file:
 [call.__enter__(),
 call.write("['abc', 20]"),
 call.write(''),
 call.__exit__(None, None, None)]

calls to write:
 [call("['abc', 20]"), call('')]

Traceback (most recent call last):
  File "<pyshell#98>", line 1, in <module>
    test()
  File "C:\Python36\lib\unittest\mock.py", line 1179, in patched
    return func(*args, **keywargs)
  File "C:/pyProjects33/test_tmp.py", line 30, in test
    handle.write.assert_called_once_with(str(['abc', 20]))
  File "C:\Python36\lib\unittest\mock.py", line 824, in assert_called_once_with
    raise AssertionError(msg)
AssertionError: Expected 'write' to be called once. Called 2 times.
>>>

.write() is being called a second time with '' because of the end argument you passed - you can verify this by playing around with that parameter.

How to test? You could use assert_any_call() :

    handle.write.assert_any_call(str(['abc', 20]))

or, if you want to validate the end argument, assert_has_calls():

from unittest.mock import call

....
        calls = [call(str(['abc', 20])), call('')]
        handle.write.assert_has_calls(calls)

You asked Is it just a bad idea to use print? | Maybe someone else can chime in but in my limited experience I have never consider using print() to write to a file.

Dammit! I was writing the answer, but took a break to lunch! It was pretty much similar to your answer. Good job!!

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