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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::
[mypy]
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::
@run_in_reactor
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()
else:
d = Deferred()
def complete():
d.callback(get_time())
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}")
.. _PEP 612 — Parameter Specification Variables: https://www.python.org/dev/peps/pep-0612/
.. _Configuring mypy to use plugins: https://mypy.readthedocs.io/en/latest/extending_mypy.html#configuring-mypy-to-use-plugins
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