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 function argument 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.
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 an function that calls it. After the test function finishes the os.path.expanduser modification will be undone.
monkeypatch.setattr/delattr/delitem/delenv() all by default raise an Exception if the target does not exist. Pass raising=False if you want to skip this check.