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 test_module.py
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 conftest.py
import pytest
@pytest.fixture(autouse=True)
def no_requests(monkeypatch):
monkeypatch.delattr("requests.sessions.Session.request")
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.
Note
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.
Note
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.