Mocking builtins

Mocking or stubbing out builtin functions, such as open(), can be slightly tricky. It is not always obvious when the builtin function you are trying to mock might be internally called by the test runner and cause unexpected behavior in the test. As a result, the recommended way to mock out builtin functions is to always specify a fall-through with should_call() first and use with_args() to limit the scope of your mock or stub to just the specific invocation you are trying to replace:

mock = flexmock(__builtins__)
mock.should_call("open")  # set the fall-through
mock.should_receive("open").with_args("file_name").and_return(
    flexmock(read=lambda: "some data")
)