[Twisted-Python] Global reactor unit tests in the Twisted test suite

Christopher Armstrong radix at twistedmatrix.com
Tue Nov 1 17:37:24 EDT 2011

On Tue, Nov 1, 2011 at 5:29 PM, Anton Gyllenberg <anton at iki.fi> wrote:

> On Tue, Nov 1, 2011 at 20:48, Glyph <glyph at twistedmatrix.com> wrote:
> > On Nov 1, 2011, at 12:14 PM, Phil Mayers wrote:
> >> I find the "pass reactor as 1st argument to everything" API pattern
> >> messy. I'm sure there's a good reason. What is it?
> >
> > This pattern is a solution to the problem, but I agree that it is
> possibly not the optimal solution.  It sort of points in a direction where
> every possible module that might be imported becomes an argument to your
> function.  After all, there are plenty of other modules which have to be
> mocked for testing, why not just make everyone's __init__ method take
> sys.modules as an argument too, and never import anything?  In more complex
> systems this can definitely turn into a bit of a mess.
> I've often wondered whether there is a *real* use of the reactor
> argument other than unit testing. I haven't had other uses for it, and
> haven't seen (or perhaps not understood) real application code making
> use of it. Now, I've sometimes made an argument for unit testing,
> other than the obvious quality assurance, that unit tested code will
> be more reusable and have better interfaces as it already is used for
> at least two things: the real application and the unit test. However,
> if accommodating unit testing requires sacrificing the natural
> interface, then that kind of takes the edge off that argument.
> Thanks to everybody for the discussion and input. Very informative!
Here's the thing: not only does removing the global reactor make *our* unit
tests a lot nicer, it makes the unit tests of our users much easier to
write, since they will want to use our APIs in their unit tests in a way
that doesn't require ugly global hacking. I don't know how many times I
have raged at some crappy library whose code is incompatible with consumer
unit testing. Having testable interfaces is a really valuable feature we
can provide to our users.

Christopher Armstrong
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