Odtnhj

I have the following bit of code.

import yaml

def load_yaml_file(filename):
 with open(filename, 'rt') as f:
 data = yaml.load(f)

 return data

Is there a way to mock the open part so that f becomes '"hello":"world"' and thus I can assert that data is properly returned.

I tried to mock open with mock_open.return_value.__enter__.return_value = '"hello":"world"' and couldn't get it to work properly.

I'm using pytest and mocker.

DISCLAIMERS

• This solution does not use Mocker, but can be used with pytest.


• This solution works for Python >= 3.6

As you said you are using Mocker, I will assume that you are working on a very old codebase (<= 2.6). I highly recommend you port you code to any version >= 3.6

Since Python 3.3 mock is integrated to standard lib in unittest.mock and is a pretty much clone from old mock package

The mock lib has a feature called mock_open that does exactly what you need and has an example with exactly what you want.

with patch('__main__.open', mock_open(read_data='bibble')) as m:
 with open('foo') as h:
 result = h.read()

m.assert_called_once_with('foo')
assert result == 'bibble'

Adapting the solution to your need you can use this example

import yaml
from unittest.mock import patch, mock_open

def load_yaml_file(filename):
 with open(filename, 'rt') as f:
 data = yaml.load(f)

 return data

with patch('__main__.open', mock_open(read_data='"hello":"world"')) as m:
 res = load_yaml_file('foo')

assert res == "hello":"world"

You don't need to set __enter__ at all. Just pass in the data you want read as the read_data argument to mock_open():

mocked_open = mock.mock_open(read_data='"hello":"world"')
with mock.patch("yourmodule.open", mocked_open):
 result = load_yaml_file("foobar.yaml")

Demonstration:

>>> import yaml
>>> def load_yaml_file(filename):
... with open(filename, 'rt') as f:
... data = yaml.load(f)
... return data
...
>>> from unittest import mock
>>> mocked_open = mock.mock_open(read_data='"hello":"world"')
>>> with mock.patch("__main__.open", mocked_open):
... result = load_yaml_file("foobar.yaml")
...
>>> print(result)
'hello': 'world'
>>> mocked_open.mock_calls
[call('foobar.yaml', 'rt'),
 call().__enter__(),
 call().read(4096),
 call().read(4096),
 call().__exit__(None, None, None)]