pytest mock.patch not working (tested fixes)

If mock.patch has no effect, you patched the wrong name: after from requests import get, patch "yourmodule.get", not "requests.get". fixture 'mocker' not found means pytest-mock is not installed in the environment running pytest.

Most "mock not working" bugs in pytest are one of four things: patching the wrong name, the mocker fixture missing, a typo in the target or assertion, or a call that differs from what you assert. Measured on pytest 9.1.1, pytest-mock 3.17.0, requests 2.34.2, Python 3.13.5.

1. Patch where the name is looked up, not where it is defined

$ cat weather.py
import requests
from requests import get

def temp_module(city):
    return requests.get(f"https://api.example/{city}").json()["t"]

def temp_from(city):
    return get(f"https://api.example/{city}").json()["t"]
$ cat test_weather.py
from unittest import mock
import weather

def test_patch_library():                       # patches requests.get
    with mock.patch("requests.get") as g:
        g.return_value.json.return_value = {"t": 21}
        assert weather.temp_module("oslo") == 21
        assert weather.temp_from("oslo") == 21   # 'from requests import get' kept the real one

def test_patch_where_used():                    # patches the name weather looks up
    with mock.patch("weather.get") as g:
        g.return_value.json.return_value = {"t": 21}
        assert weather.temp_from("oslo") == 21
$ pytest -q --tb=line | tail -2
FAILED test_weather.py::test_patch_library - requests.exceptions.ConnectionEr...
1 failed, 1 passed in 0.08s

mock.patch("requests.get") replaces the attribute on the requests module. temp_module looks up requests.get at call time, so it sees the mock. temp_from did from requests import get, which bound the real function into weather at import time, so the test made a real HTTP request (here it failed to resolve the host). Patch "weather.get", the module that uses it.

2. fixture 'mocker' not found

$ cat test_m.py
def test_m(mocker):
    mocker.patch("os.getcwd", return_value="/x")
$ pytest -q -p no:pytest_mock | grep -E "not found|passed|failed|error"
E       fixture 'mocker' not found
1 error in 0.00s
$ pytest -q | tail -1
1 passed in 0.00s

mocker comes from the pytest-mock plugin, not pytest itself. -p no:pytest_mock above disables the installed plugin to show what you get when it is missing. Install pytest-mock into the same environment that runs pytest (python -m pip install pytest-mock), or use unittest.mock.patch directly.

3. AttributeError: does not have the attribute, not a valid assertion, autospec TypeError

$ cat test_typos.py
from unittest import mock
import json

def test_wrong_target():
    with mock.patch("json.lods"):
        pass

def test_typo_assert():
    m = mock.Mock()
    m.save(1)
    m.asert_called_once_with(2)

def test_autospec():
    with mock.patch("json.loads", autospec=True) as m:
        json.loads("{}", "extra", "args")
$ pytest -q | grep -E "^E  |passed|failed"
E           AttributeError: <module 'json' from '/usr/lib/python3.13/json/__init__.py'> does not have the attribute 'lods'
E               AttributeError: 'asert_called_once_with' is not a valid assertion. Use a spec for the mock if 'asert_called_once_with' is meant to be an attribute.. Did you mean: 'assert_called_once_with'?
E                       TypeError: too many positional arguments
3 failed in 0.17s

A misspelled patch target raises AttributeError ... does not have the attribute (add create=True only if the attribute really should not exist yet). A misspelled assertion method on a Mock raises instead of silently passing. With autospec=True the mock keeps the real signature, so a wrong call fails with TypeError instead of being accepted.

4. AssertionError: expected call not found

$ cat test_called.py
from unittest import mock

def test_called():
    m = mock.Mock()
    m.save("a.txt", mode="w")
    m.save.assert_called_once_with("a.txt", mode="wb")
$ pytest -q | grep -E "^E  |passed|failed" | head -4
E       AssertionError: expected call not found.
E       Expected: save('a.txt', mode='wb')
E         Actual: save('a.txt', mode='w')
E

The message prints Expected and Actual calls; positional and keyword arguments must match exactly. Use m.save.call_args or m.mock_calls to see what really happened, or mock.ANY for arguments you do not care about.

Related: pytest fixture not found, pytest error importing plugin, pytest caplog.

Written with AI assistance. Measured on 2026-10-23 with pytest 9.1.1, pytest-mock 3.17.0 and requests 2.34.2 on Python 3.13.5 in a clean virtualenv. Each block was run verbatim and the output pasted unedited. PlainHeaders is our extension; Python and pytest are free.