pretend
Pretend is a library to make stubbing with Python easier.
What is stubbing?
Stubbing is a technique for writing tests. You may hear the term mixed up with mocks, fakes, or doubles. Basically a stub is an object that returns pre-canned responses, rather than doing any computation.
Martin Fowler does a good job explaining the terms in his Mocks Aren't Stubs article.
pretend
?
How do I install It's easy with pip
!
$ pip install pretend
pretend
?
How do I use It's easy, the stub
function makes it easy to create a stub:
>>> from pretend import stub
>>> x = stub(country_code="US")
>>> some_function(x)
Here x
will be an object with a single attribute country_code
which has
the value "US"
. Unlike mocks, x
will not respond to any other attribute
or methods, nor does it have any methods for making assertions about what you
accessed.
If you want to add a method to the stub, simply provide a function to it:
>>> from pretend import stub
>>> x = stub(country_code=lambda: "US")
>>> x.country_code()
'US'
It's important to note that functions on stubs do not take a self
argument, this is because stubs should be returning pre-canned values, not
doing computations.
pretend
Exceptions with Sometimes a method you want to stub doesn't return a value, but instead raises
an exception. To make this easy, pretend
provides a helper function,
raiser
, it can be used like so:
>>> from pretend import stub, raiser
>>> x = stub(func=raiser(ValueError))
>>> x.func()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "pretend.py", line 74, in inner
raise exc
ValueError
Why is stubbing better?
Ideally stubbing tests how your system responds to a particular input, rather than which API is used. Stubbing still requires you to write tests that check the results of a computation, rather than looking for side effects. This doesn't always work though, so you do sometimes still need mocking (e.g. sometimes you really want to check for a side effect.)
How do I get my stub into place?
If you come from other mocking libraries you're probably used to a patch
method to put a mock in place. pretend
doesn't include anything like this,
a) we believe it's better, where possible, to pass stubs as arguments rather
than monkey patch them into place, b) we believe that when you do need to
monkey patch something into place you should use something provided by your
testing tool. py.test
includes such a tool.
What if I really need to record the calls?
If you really really need to, pretend
includes a call_recorder
utility:
>>> from pretend import call_recorder, call
>>> f = call_recorder(lambda a: a + 2)
>>> f(3)
5
>>> assert f.calls == [call(3)]
Who wrote this?
pretend
is by Alex Gaynor, who was just tired of not having a good stubbing
tool for Python. The name is from Idan Gazit.