How can I patch a ConfigParser key/value?

Question

When I use the following code:

from unittest import mock
import configparser

configtext = '''
[SECTION]
whatever=True
'''

config = configparser.ConfigParser()
config.read_string(configtext)


def test_fails():
    expected_value = 'fnord'
    with mock.patch.dict(config, {'db': expected_value}):
        assert config['db'] is expected_value

My test fails because AttributeError: 'str' object has no attribute 'items' .

This is not at all what I expected. Obviously I would expect it to set the value like I want... but apparently config is only dict-ish, unfortunately.

How can I patch this so config['db'] is the value I want, just for the lifetime of my test?

Answer 1

It appears that the problem is that I had a slight misunderstanding. Although ConfigParser looks dict-like, it isn't actually. The stack trace holds evidence of this:

    def test_fails():
        expected_value = 'whatever'
>       with mock.patch.dict(config, {'db': expected_value}):

test.py:15: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/usr/lib/python3.5/unittest/mock.py:1593: in __enter__
    self._patch_dict()
/usr/lib/python3.5/unittest/mock.py:1619: in _patch_dict
    in_dict[key] = values[key]
/usr/lib/python3.5/configparser.py:969: in __setitem__
    self.read_dict({key: value})
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <configparser.ConfigParser object at 0x7f1be6d20f98>, dictionary = {'db': 'whatever'}
source = '<dict>'

Note that it's trying to do a read_dict here. That's because it's expecting to have a section-ish format:

>>> parser = configparser.ConfigParser()
>>> parser.read_dict({'section1': {'key1': 'value1',
...                                'key2': 'value2',
...                                'key3': 'value3'},
...                   'section2': {'keyA': 'valueA',
...                                'keyB': 'valueB',
...                                'keyC': 'valueC'},
...                   'section3': {'foo': 'x',
...                                'bar': 'y',
...                                'baz': 'z'}
... })

^{_{From the docs}}

Having a single-key access is not possible. To get this example to work, you have to do the following:

with mock.patch.dict(config, {'db': {'db': expected_value}}):
    # rest of code

Note: Values will be converted to their stringish counterparts. So if you are trying to store an actual database connection (or similar) here, it won't work.