Getting started
So what does flexmock actually help you do?
Setup
Flexmock features smooth integration with pretty much every popular test runner, so no special setup is necessary. Simply importing flexmock into your test module is sufficient to get started with any of the following examples.
from flexmock import flexmock
This will include flexmock in your test and make the necessary runner modifications so no further setup or cleanup is necessary.
Creating fake objects
Making a new object in Python requires defining a new class with all the fake methods and attributes you are interested in emulating and then instantiating it. For example, to create a FakePlane object to use in a test in place of a real Plane object, we would need to do something like this:
class FakePlane:
operational = True
model = "MIG-21"
def fly(self):
pass
plane = FakePlane() # this is tedious!
In other words, we must first create a class, make sure it contains all required attributes and methods, and finally instantiate it to create the object.
Flexmock provides an easier way to generate a fake object on the fly using the
flexmock()
function:
plane = flexmock(operational=True, model="MIG-21")
It is also possible to add methods to this object using the same notation and Python's handy lambda keyword to turn an attribute into a method:
plane = flexmock(operational=True, model="MIG-21", fly=lambda: None)
Stubs
While creating fake objects from scratch is often sufficient, many times it is easier to take an existing object and simply stub out certain methods or replace them with fake ones. Flexmock makes this easy as well:
flexmock(
Train, # this can be an instance, a class, or a module
get_destination="Tokyo",
get_speed=200,
)
By passing a real object (or class or module) into the flexmock()
function as
the first argument it is possible to modify that object in place and provide
default return values for any of its existing methods.
In addition to simply stubbing out return values, it can be useful to be able to call an entirely different method and substitute return values based on test-specific conditions:
flexmock(Train).should_receive("get_route").replace_with(
lambda x: custom_get_route()
)
Mocks
Expectations take many flavors, and flexmock has many different facilities and modes to generate them. The first and simplest is ensuring that a certain method is called:
flexmock(Train).should_receive("get_destination").once()
The .once()
modifier ensures that Train.get_destination()
is called at some
point during the test and will raise an exception if this does not happen.
Of course, it is also possible to provide a default return value:
flexmock(Train).should_receive("get_destination").and_return("Tokyo").once()
Or check that a method is called with specific arguments:
flexmock(Train).should_receive("set_destination").with_args(
"Tokyo"
).at_least().times(1)
In this example we used .times(1)
instead of .once()
and added the
.at_least()
modifier to demonstate that it is easy to match any number of
calls, including 0 calls or a variable amount of calls. As you've probably
guessed there is also an at_most()
modifier.
Spies
While replacing method calls with canned return values or checking that they are
called with specific arguments is quite useful, there are also times when you
want to execute the actual method and simply find out how many times it was
called. Flexmock uses should_call()
to generate this sort of expectations
instead of should_receive()
:
flexmock(Train).should_call("get_destination").once()
In the above case the real get_destination()
method will be executed, but
flexmock will raise an exception unless it is executed exactly once. All the
modifiers allowed with should_receive()
can also be used with should_call()
so it is possible to tweak the allowed arguments, return values, and call
counts.
flexmock(Train).should_call("set_destination").with_args(
object, str, int
).and_raise(Exception, re.compile("^No such dest.*")).once()
The above example introduces a handful of new capabilities -- matching exceptions, matching argument types (object naturally matches any argument type) and regex matching on string return values and arguments.
Summary
Flexmock has many other features and capabilities, but hopefully the above overview has given you enough of the flavor for the kind of things that it makes possible. For more details see the User Guide.