Fake new instances

Occasionally you will want a class to create fake objects when it is being instantiated. Flexmock makes it easy and painless.

Your first option is to simply replace the class with a function:

flexmock(some_module).should_receive("NameOfClass").and_return(fake_instance)
# fake_instance can be created with flexmock as well

The downside if this is that you may run into subtle issues since the class has now been replaced by a function.

Flexmock offers another alternative using the .new_instances() method:

Warning

Usage of .new_instances() method is discouraged due to a bug in CPython which prevents proper teardown of the mock. Due to this bug, the mock leaks into other tests and can prevent creating new instances of the class. More information in issue #16.

>>> class Group: pass
>>> fake_group = flexmock(name="fake")
>>> flexmock(Group).new_instances(fake_group)
>>> Group().name == "fake"
True

It is also possible to return different fake objects in a sequence:

>>> class Group: pass
>>> fake_group1 = flexmock(name="fake")
>>> fake_group2 = flexmock(name="real")
>>> flexmock(Group).new_instances(fake_group1, fake_group2)
>>> Group().name == "fake"
True
>>> Group().name == "real"
True

Another approach, if you're familiar with how instance instatiation is done in Python, is to stub the __new__ method directly:

>>> flexmock(Group).should_receive("__new__").and_return(fake_group)
>>> # or, if you want to be even slicker
>>> flexmock(Group, __new__=fake_group)

In fact, the new_instances command is simply shorthand for should_receive("__new__").and_return() under the hood.