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.
object keeping a record of setattr/item/env/syspath changes.
set attribute name on obj to value, by default raise AttributeEror if the attribute did not exist.
delete attribute name from obj, by default raise AttributeError it the attribute did not previously exist.
set dictionary entry name to value.
delete name from dict, raise KeyError if it doesn’t exist.
set environment variable name to value. if prepend is a character, read the current environment variable value and prepend the value adjoined with the prepend character.
delete name from environment, raise KeyError it not exists.
prepend path to sys.path list of import locations.
change the current working directory to the specified path path can be a string or a py.path.local object
undo previous changes. This call consumes the undo stack. Calling it a second time has no effect unless you do more monkeypatching after the undo call.
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.