How to use mock_open with json.load()?

问题

I'm trying to get a unit test working that validates a function that reads credentials from a JSON-encoded file. Since the credentials themselves aren't fixed, the unit test needs to provide some and then test that they are correctly retrieved.

Here is the credentials function:

def read_credentials():
    basedir = os.path.dirname(__file__)
    with open(os.path.join(basedir, "authentication.json")) as f:
        data = json.load(f)
        return data["bot_name"], data["bot_password"]

and here is the test:

def test_credentials(self):
    with patch("builtins.open", mock_open(
        read_data='{"bot_name": "name", "bot_password": "password"}\n'
    )):
        name, password = shared.read_credentials()
    self.assertEqual(name, "name")
    self.assertEqual(password, "password")

However, when I run the test, the json code blows up with a decode error. Looking at the json code itself, I'm struggling to see why the mock test is failing because json.load(f) simply calls f.read() then calls json.loads().

Indeed, if I change my authentication function to the following, the unit test works:

def read_credentials():
    # Read the authentication file from the current directory and create a
    # HTTPBasicAuth object that can then be used for future calls.
    basedir = os.path.dirname(__file__)
    with open(os.path.join(basedir, "authentication.json")) as f:
        content = f.read()
        data = json.loads(content)
        return data["bot_name"], data["bot_password"]

I don't necessarily mind leaving my code in this form, but I'd like to understand if I've got something wrong in my test that would allow me to keep my function in its original form.

Stack trace:

Traceback (most recent call last):
  File "test_shared.py", line 56, in test_credentials
shared.read_credentials()
  File "shared.py", line 60, in read_credentials
data = json.loads(content)
  File "/home/philip/.local/share/virtualenvs/atlassian-webhook-basic-3gOncDp4/lib/python3.6/site-packages/flask/json/__init__.py", line 205, in loads
return _json.loads(s, **kwargs)
  File "/usr/lib/python3.6/json/__init__.py", line 367, in loads
return cls(**kw).decode(s)
  File "/usr/lib/python3.6/json/decoder.py", line 339, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.6/json/decoder.py", line 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

回答1:

I had the same issue and got around it by mocking json.load and builtins.open:

import json
from unittest.mock import patch, MagicMock

# I don't care about the actual open
p1 = patch( "builtins.open", MagicMock() )

m = MagicMock( side_effect = [ { "foo": "bar" } ] )
p2 = patch( "json.load", m )

with p1 as p_open:
    with p2 as p_json_load:
        f = open( "filename" )
        print( json.load( f ) )

Result:

{'foo': 'bar'}

来源：https://stackoverflow.com/questions/51138834/how-to-use-mock-open-with-json-load

标签

json

python-3.x

mocking