Python 3 urlopen context manager mocking - unit-testing

Question

I am new to testing and need some help here.
Assuming having this method:
from urllib.request import urlopen
def get_posts():
with urlopen('some url here') as data:
return json.loads(data.read().decode('utf-8'))
The question is how to test this method (using mock.patch decorator if possible)?
What I have now:
#mock.patch('mymodule.urlopen')
def test_get_post(self, mocked_urlopen):
mocked_urlopen.__enter__ = Mock(return_value=self.test_data)
mocked_urlopen.__exit__ = Mock(return_value=False)
...
But it does not seem to be working.
P.S. Is there any convenient way to work with data variable (which type is HTTPResponse) in test so it could just be simple string?

Answer 1

I was fighting with this as well, and finally figured it out. (Python 3 syntax):
import urllib.request
import unittest
from unittest.mock import patch, MagicMock
class TestUrlopen(unittest.TestCase):
#patch('urllib.request.urlopen')
def test_cm(self, mock_urlopen):
cm = MagicMock()
cm.getcode.return_value = 200
cm.read.return_value = 'contents'
cm.__enter__.return_value = cm
mock_urlopen.return_value = cm
with urllib.request.urlopen('http://foo') as response:
self.assertEqual(response.getcode(), 200)
self.assertEqual(response.read(), 'contents')
#patch('urllib.request.urlopen')
def test_no_cm(self, mock_urlopen):
cm = MagicMock()
cm.getcode.return_value = 200
cm.read.return_value = 'contents'
mock_urlopen.return_value = cm
response = urllib.request.urlopen('http://foo')
self.assertEqual(response.getcode(), 200)
self.assertEqual(response.read(), 'contents')
response.close()

Answer 2

here is my take on this
from urllib.request import urlopen
from unittest.mock import patch
class Mock():
def __init__(self, request, context):
return None
def read(self):
return self
def decode(self, arg):
return ''
def __iter__(self):
return self
def __next__(self):
raise StopIteration
with patch('urllib.request.urlopen', Mock):
# do whatever over here

Answer 3

with urlopen('some url here') as data is a context manager
Also, a file can be used as a context manager, so a better approach here is to use io.StringIO
import io
import json
import urllib.request
from unittest.mock import patch
def get_posts():
with urllib.request.urlopen('some url here') as data:
return json.load(data)
def test_get_posts():
data = io.StringIO('{"id": 123}')
with patch.object(urllib.request, 'urlopen', return_value=data):
assert get_posts() == {"id": 123}

Answer 4

Ok, so I have written simple class to simulate context manager.
class PatchContextManager:
def __init__(self, method, enter_return, exit_return=False):
self._patched = patch(method)
self._enter_return = enter_return
self._exit_return = exit_return
def __enter__(self):
res = self._patched.__enter__()
res.context = MagicMock()
res.context.__enter__.return_value = self._enter_return
res.context.__exit__.return_value = self._exit_return
res.return_value = res.context
return res
def __exit__(self, type, value, tb):
return self._patched.__exit__()
Usage:
with PatchContextManager('mymodule.method', 'return_string') as mocked:
a = mymodule.method(47) # a == 'return_string'
mocked.assert_called_with(47)
...