Python - Mocking OS Error Exception

Question

Trying to mock a PermissionError exception in Python 3.6 using side_effect. Looks like my function is called and the EPERM exception is raised, but then it fails to run my except statements. Same code runs as expected for a 'real' OSError exception. My code:

#my_module.py
import os
import errno  
import sys
import inspect

def open_file(fname):
    try:
        with open('./' + fname, 'w') as f:
            print('never get here')
        return(0)

    except PermissionError as e:
        print('ERROR: \nIn function: ' + inspect.stack()[0][3])
        print('On line: {}'.format(sys.exc_info()[-1].tb_lineno), type(e).__name__, e)
        sys.exit(1)

My test:

#OpenFileMockTestCase.py
from unittest import TestCase
from unittest import mock
import errno
import my_module

class OpenFileMockTestCase(TestCase):

    @mock.patch('my_module.os.open')
    def test_2_open_file_mock_oserror(self, mock_oserror):
        with self.assertRaises(SystemExit):
            mock_oserror.my_module.open_file.side_effect = (OSError((errno.EPERM), 'Not Allowed'))
            print('starting open_file with testfile2.txt...')
            mock_oserror.my_module.open_file('testfile2.txt')

When I run:

C:\Users\mylib>coverage3 run -m unittest OpenFileMockTestCase.py -v
test_2_open_file_mock_oserror (OpenFileMockTestCase.OpenFileMockTestCase) ... starting open_file with testfile2.txt...

ERROR

======================================================================
ERROR: test_2_open_file_mock_oserror (OpenFileMockTestCase.OpenFileMockTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\users\xti027\appdata\local\programs\python\python36\lib\unittest\mock.py", line 1179, in patched
    return func(*args, **keywargs)
  File "C:\Users\xti027\Documents\DataTool-Git\DataTool\DataLoaderConfig\OpenFileMockTestCase.py", line 14, in test_2_open_file_mock_oserror
    mock_oserror.my_module.open_file('testfile2.txt')
  File "c:\users\xti027\appdata\local\programs\python\python36\lib\unittest\mock.py", line 939, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "c:\users\xti027\appdata\local\programs\python\python36\lib\unittest\mock.py", line 995, in _mock_call
    raise effect
PermissionError: [Errno 1] Not Allowed

----------------------------------------------------------------------
Ran 1 test in 0.031s

FAILED (errors=1)

I have read several SO questions and responses on Exceptions and mocking like How do I write a unit test for OSError? and looked at the Python doc: https://docs.python.org/3.6/library/unittest.mock.html#module-unittest.mock Am I mocking the right item at the right place?

Answer 1

You can just raise the PermissionError exception:

mock_oserror.side_effect = PermissionError

Note that we set the side-effect directly on the mocked open() call! I'd also mock the global open() name in your module-under-test, not os.open.

You also should call the function-under-test directly, not as an attribute of your mock_oserror object:

import my_module

# ....

@mock.patch('my_module.open')
def test_2_open_file_mock_oserror(self, mock_open):
    mock_open.side_effect = PermissionError
    print('starting open_file with testfile2.txt...')
    with self.assertRaises(SystemExit):
        my_module.open_file('testfile2.txt')

I used the name mock_open instead here, as that better reflects what is being mocked.

Demo:

>>> import os
>>> import errno
>>> import sys
>>> import inspect
>>> from unittest import mock
>>> def open_file(fname):
...     try:
...         with open('./' + fname, 'w') as f:
...             print('never get here')
...         return(0)
...     except PermissionError as e:
...         print('ERROR: \nIn function: ' + inspect.stack()[0][3])
...         print('On line: {}'.format(sys.exc_info()[-1].tb_lineno), type(e).__name__, e)
...         sys.exit(1)
...
>>> with mock.patch('__main__.open') as mock_oserror:
...     mock_oserror.side_effect = PermissionError
...     try:
...         open_file('testfile2.txt')
...     except SystemExit:
...         print('test passed, sys.exit() called')
...
ERROR:
In function: open_file
On line: 3 PermissionError
test passed, sys.exit() called