Static Type Checking

Crochet comes with type hints for Python 3.6+. However, due to current limitations in Callable generic construction (see PEP 612 — Parameter Specification Variables), the arguments of a call to a @run_in_reactor-decorated function or method cannot be checked without giving type checkers some special help. Crochet ships with a plugin which fills this role when the mypy static type checker is used. It resides in crochet.mypy and must be configured as described in Configuring mypy to use plugins. For example, in a mypy.ini configuration file:

plugins = crochet.mypy

This type checking is intended primarily for code which calls the decorated function. As Twisted isn’t fully type-hinted yet, and in particular Deferred does not yet have a generic type argument so that the eventual result type can vary, the analysis of the return type of a @run_in_reactor function/method does not account for a Deferred result. This requires you to lie to the type checker when returning a Deferred; just cast it to the known, eventual result type using typing.cast. For example:

def get_time_in_x_seconds(delay: float) -> float:
    def get_time() -> float:
        return reactor.seconds()  # type: ignore

    if delay < 0.001:
        # Close enough; just return the current time.
        return get_time()
        d = Deferred()

        def complete():

        reactor.callLater(delay, complete)  # type: ignore
        return typing.cast(float, d)

If the mypy plugin is correctly installed, the client code will expect a float from the wait() of the EventualResult returned by a call to this function:

# OK
t1: float = get_time_in_x_seconds(2).wait(3)
print(f"The reactor time is {t1}")

# mypy error: Incompatible types in assignment
#   (expression has type "float", variable has type "str")
t2: str = get_time_in_x_seconds(2).wait(3)
print(f"The reactor time is {t2}")