Monkeypatching/mocking modules and environments

Sometimes tests need to invoke functionality which depends on global settings or which invokes code which cannot be easily tested such as network access. The monkeypatch fixture helps you to safely set/delete an attribute, dictionary item or environment variable or to modify sys.path for importing. See the monkeypatch blog post for some introduction material and a discussion of its motivation.

Simple example: monkeypatching functions

If you want to pretend that os.expanduser returns a certain directory, you can use the monkeypatch.setattr() method to patch this function before calling into a function which uses it:

# content of
import os.path
def getssh(): # pseudo application code
    return os.path.join(os.path.expanduser("~admin"), '.ssh')

def test_mytest(monkeypatch):
    def mockreturn(path):
        return '/abc'
    monkeypatch.setattr(os.path, 'expanduser', mockreturn)
    x = getssh()
    assert x == '/abc/.ssh'

Here our test function monkeypatches os.path.expanduser and then calls into a function that calls it. After the test function finishes the os.path.expanduser modification will be undone.

example: preventing “requests” from remote operations

If you want to prevent the “requests” library from performing http requests in all your tests, you can do:

# content of
import pytest
def no_requests(monkeypatch):

This autouse fixture will be executed for each test function and it will delete the method request.session.Session.request so that any attempts within tests to create http requests will fail.


Be advised that it is not recommended to patch builtin functions such as open, compile, etc., because it might break pytest’s internals. If that’s unavoidable, passing --tb=native, --assert=plain and --capture=no might help although there’s no guarantee.


Mind that patching stdlib functions and some third-party libraries used by pytest might break pytest itself, therefore in those cases it is recommended to use MonkeyPatch.context() to limit the patching to the block you want tested:

import functools

def test_partial(monkeypatch):
    with monkeypatch.context() as m:
        m.setattr(functools, "partial", 3)
        assert functools.partial == 3

See issue #3290 for details.

API Reference

Consult the docs for the MonkeyPatch class.