How do I mock an open used in a with statement using the Mock framework in Python

Question

How do I test the following code with unittest mock  def testme filepath       with open filepath  as f          return f read

User · Answer

The top answer is useful but I expanded on it a bit.

If you want to set the value of your file object (the f in as f) based on the arguments passed to open() here's one way to do it:

def save_arg_return_data(*args, **kwargs):
    mm = MagicMock(spec=file)
    mm.__enter__.return_value = do_something_with_data(*args, **kwargs)
    return mm
m = MagicMock()
m.side_effect = save_arg_return_array_of_data

# if your open() call is in the file mymodule.animals 
# use mymodule.animals as name_of_called_file
open_name = '%s.open' % name_of_called_file

with patch(open_name, m, create=True):
    #do testing here

Basically, open() will return an object and with will call __enter__() on that object.

To mock properly, we must mock open() to return a mock object. That mock object should then mock the __enter__() call on it (MagicMock will do this for us) to return the mock data/file object we want (hence mm.__enter__.return_value). Doing this with 2 mocks the way above allows us to capture the arguments passed to open() and pass them to our do_something_with_data method.

I passed an entire mock file as a string to open() and my do_something_with_data looked like this:

def do_something_with_data(*args, **kwargs):
    return args[0].split("\n")

This transforms the string into a list so you can do the following as you would with a normal file:

for line in file:
    #do action

User · Answer

If you don t need any file further  you can decorate the test method    patch  builtins open   mock open read data  data    def test testme        result   testeme       assert result     data

User · Answer

With the latest versions of mock  you can use the really useful mock open helper      mock open mock None  read data None        A helper function to create a   mock to replace the use of open  It works for open called directly or   used as a context manager       The mock argument is the mock object to configure  If None  the   default  then a MagicMock will be created for you  with the API   limited to methods or attributes available on standard file handles       read data is a string for the read method of the file handle to   return  This is an empty string by default     gt  gt  gt  from mock import mock open  patch  gt  gt  gt  m   mock open    gt  gt  gt  with patch     open  format   name     m  create True          with open  foo    w   as h             h write  some stuff     gt  gt  gt  m assert called once with  foo    w    gt  gt  gt  handle   m    gt  gt  gt  handle write assert called once with  some stuff

User · Answer

The way to do this has changed in mock 0 7 0 which finally supports mocking the python protocol methods  magic methods   particularly using the MagicMock   http   www voidspace org uk python mock magicmock html  An example of mocking open as a context manager  from the examples page in the mock documentation     gt  gt  gt  open name     s open      name    gt  gt  gt  with patch open name  create True  as mock open          mock open return value   MagicMock spec file              with open   some path    w   as f              f write  something        lt mock Mock object at 0x    gt   gt  gt  gt  file handle   mock open return value   enter   return value  gt  gt  gt  file handle write assert called with  something

User · Answer

I might be a bit late to the game  but this worked for me when calling open in another module without having to create a new file   test py  import unittest from mock import Mock  patch  mock open from MyObj import MyObj  class TestObj unittest TestCase       open    mock open       with patch object   builtin     open   open            ref   MyObj           ref save  myfile txt       assert open  call args list     call  myfile txt    wb      MyObj py  class MyObj object       def save self  filename           with open filename   wb   as f              f write  sample text     By patching the open function inside the   builtin   module to my mock open    I can mock writing to a file without creating one   Note  If you are using a module that uses cython  or your program depends on cython in any way  you will need to import cython s   builtin   module by including import   builtin   at the top of your file  You will not be able to mock the universal   builtin   if you are using cython

User · Answer

To use mock open for a simple file read    the original mock open snippet already given on this page is geared more for write    my text    some text to return when read   is called on the file object  mocked open function   mock mock open read data my text   with mock patch    builtin   open   mocked open function       with open  any string   as f          print f read     Note as per docs for mock open  this is specifically for read    so won t work with common patterns like for line in f  for example   Uses python 2 6 6   mock 1 0 1

User · Answer

Sourced from a github snippet to patch read and write functionality in python  The source link is over here import configparser import pytest  simpleconfig    quot  quot  quot  section  nkey   value n n quot  quot  quot   def test monkeypatch open read mockopen       filename    somefile txt      mockopen write filename  simpleconfig        parser   configparser ConfigParser       parser read filename      assert parser sections        section     def test monkeypatch open write mockopen       parser   configparser ConfigParser       parser add section  section       parser set  section    key    value         filename    somefile txt      parser write open filename   wb        assert mockopen read filename     simpleconfig

User · Answer

Python 3 Patch builtins open and use mock open  which is part of the mock framework  patch used as a context manager returns the object used to replace the patched one  from unittest mock import patch  mock open with patch  quot builtins open quot   mock open read data  quot data quot    as mock file      assert open  quot path to open quot   read       quot data quot      mock file assert called with  quot path to open quot    If you want to use patch as a decorator  using mock open   s result as the new  argument to patch can be a little bit weird  Instead  use patch s new callable  argument and remember that every extra argument that patch doesn t use will be passed to the new callable function  as described in the patch documentation   patch   takes arbitrary keyword arguments  These will be passed to the Mock  or new callable  on construction    patch  quot builtins open quot   new callable mock open  read data  quot data quot   def test patch mock file       assert open  quot path to open quot   read       quot data quot      mock file assert called with  quot path to open quot    Remember that in this case patch will pass the mocked object as an argument to your test function  Python 2 You need to patch   builtin   open instead of builtins open and mock is not part of unittest  you need to pip install and import it separately  from mock import patch  mock open with patch  quot   builtin   open quot   mock open read data  quot data quot    as mock file      assert open  quot path to open quot   read       quot data quot      mock file assert called with  quot path to open quot

User · Answer

To patch the built-in open   function with unittest   This worked for a patch to read a json config    class ObjectUnderTest      def   init   self  filename  str           with open filename   r   as f              dict content   json load f    The mocked object is the io TextIOWrapper object returned by the open   function   patch   lt src where object is used gt  open           return value io TextIOWrapper io BufferedReader io BytesIO b   test key    test value            def test object function under test self  mocker

[python] How do I mock an open used in a with statement (using the Mock framework in Python)?

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