Skip to content


In order to discuss flexmock usage it is important to define the following terms:

  • Stub: fake object that returns a canned value.
  • Mock: fake object that returns a canned value and has an expectation, i.e. it includes a built-in assertion.
  • Spy: watches method calls and records/verifies if the method is called with required parameters and/or returns expected values/exceptions.


Flexmock declarations follow a consistent style of the following three forms:

  2. flexmock ( OBJECT [, ATTRIBUTE=VALUE, ...] )
  3. flexmock ( ATTRIBUTE=VALUE [, ATTRIBUTE=VALUE,...] )

OBJECT: Either a module, a class, or an instance of a class.

COMMAND: One of should_receive, should_call, or new_instances. These create the initial expectation object.

ATTRIBUTE: String name of an attribute.

MODIFIER: One of several Expectation modifiers, such as with_args, and_return, and_raise, or times.

VALUE: Anything


While the order of modifiers is unimportant to flexmock, there is a preferred convention that will make your tests more readable.

If using with_args(), place it before should_return(), and_raise(), and and_yield() modifiers:

flexmock(plane).should_receive("fly").with_args("up", "down").and_return("ok")

If using the times() modifier (or its aliases: once, twice, never), place them at the end of the flexmock statement: