New submission from Brian McCutchon <bmccutc...@google.com>:
Currently, it is possible to make a basic single-threaded executor for unit
testing:
class FakeExecutor(futures.Executor):
def submit(self, f, *args, **kwargs):
future = futures.Future()
future.set_result(f(*args, **kwargs))
return future
def shutdown(self, wait=True):
pass
However, this evaluates the provided function eagerly, which may be undesirable
for tests. It prevents the tests from catching a whole class of errors (those
where the caller forgot to call .result() on a future that is only desirable
for its side-effects). It would be great to have an Executor implementation
where the function is only called when .result() is called so tests can catch
those errors.
I might add that, while future.set_result is documented as being supported for
unit tests, a comment in the CPython source says that Future.__init__() "Should
not be called by clients"
(https://github.com/python/cpython/blob/master/Lib/concurrent/futures/_base.py#L317),
suggesting that even the above code is unsupported and leaving me wondering
how I should test future-heavy code without using mock.patch on everything.
------ Alternatives that don't work ------
One might think it possible to create a FakeFuture to do this:
class FakeFuture(object):
def __init__(self, to_invoke):
self._to_invoke = to_invoke
def result(self, timeout=None):
return self._to_invoke()
However, futures.wait is not happy with this:
futures.wait([FakeFuture(lambda x: 1)]) # AttributeError: 'FakeFuture' object
has no attribute '_condition'
If FakeFuture is made to extend futures.Future, the above line instead hangs:
class FakeFuture(futures.Future):
def __init__(self, to_invoke):
super(FakeFuture, self).__init__()
self._to_invoke = to_invoke
def result(self, timeout=None):
return self._to_invoke()
I feel like I shouldn't have to patch out wait() in order to get good unit
tests.
----------
messages: 338576
nosy: Brian McCutchon
priority: normal
severity: normal
status: open
title: Add deferred single-threaded/fake executor to concurrent.futures
type: enhancement
versions: Python 3.7
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue36395>
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