Common Issues

When I double __new__, it breaks other tests, why?

This feature is deprecated, I recommend using the patch_class method, which fixes this issue and is much cleaner.

I get a VerifyingDoubleError “Cannot double method ... does not implement it”, whats going on?

Make sure you are using a version of doubles greater than 1.0.1. There was a bug prior to 1.0.1 that would not allow you to mock callable objects.

I get a VerifyingBuiltinDoubleArgumentError ”... is not a Python func”, what is going on?

Python does not allow doubles to look into builtin functions and asked them what their call signatures are. Since we can’t do this it is impossible to verify the arguments passed into a stubbed method. By default if doubles cannot inspect a function it raises a VerifyingBuiltinDoubleArgumentError, this is most common with builtins. To bypass this functionality for builtins you can do:

import functools

from doubles import no_builtin_verification, allow

with no_builtin_verification():

    # The test that uses this allowance must be within the context manager.


How can I make SomeClass(args, kwargs) return my double?

Use patch_class and allow_constructor:

from doubles import patch_class, allow_constructor

import myapp

def test_something_that_uses_user():
    patched_class = patch_class('myapp.user.User')
    allow_constructor(patched_class).and_return('Bob Barker')

    assert myapp.user.User() == 'Bob Barker'

patch_class creates a ClassDouble of the class specified, patches the original class and returns the ClassDouble. We then stub the constructor which controls what is is returned when an instance is created.

allow_constructor supports all of the same functionality as allow:

from doubles import patch_class, allow_constructor

import myapp

def test_something_that_uses_user():
    bob_barker = InstanceDouble('myapp.user.User')

    patched_class = patch_class('myapp.user.User')
    allow_constructor(patched_class).with_args('Bob Barker', 100).and_return(bob_barker)

    assert myapp.user.User('Bob Barker', 100) is bob_barker

How can I patch something like I do with mock?

Doubles also has patch but it isn’t a decorator:

from doubles import allow, patch

import myapp

def test_something_that_uses_user():
    patch('myapp.user.User', 'Bob Barker')

    assert myapp.user.User == 'Bob Barker'

Patches do not verify against the underlying object, so use them carefully. Patches are automatically restored at the end of the test.


How do I assert that a function is not called?

If you expect the send_mail method never to be called on user: