[Twisted-Python] Testing function with delayed calls

[Mashiat Sarker Shakkhar]

Hi

I have a function that uses callLater extensively to schedule a number of
different tasks. I don't want to get into the rationale behind such a
design, but here is a contrived example which will help me explain my
problem:


def b():

'''Do some work'''


def c():

'''Do some more work'''


def a(flag):

if flag:

return Reactor.callLater(300, b)

else:

return Reactor. callLater(100, c)


Now I want to test this function. Of course I can't wait for 5 minutes to
ensure that `b` or `c` will indeed be called. What I need is some sort of
mock clock which lets me fast forward time. Does any such thing exist in
Twisted / Trial? Or is there any other approach to test such code?


Regards

Shakkhar

[Ashwini Oruganti]

On Thu, Oct 1, 2015 at 1:40 PM, Mashiat Sarker Shakkhar <

Post by Mashiat Sarker Shakkhar
Hi
I have a function that uses callLater extensively to schedule a number of
different tasks. I don't want to get into the rationale behind such a
design, but here is a contrived example which will help me explain my
'''Do some work'''
'''Do some more work'''
return Reactor.callLater(300, b)
return Reactor. callLater(100, c)
Now I want to test this function. Of course I can't wait for 5 minutes to
ensure that `b` or `c` will indeed be called. What I need is some sort of
mock clock which lets me fast forward time. Does any such thing exist in
Twisted / Trial?

Yes there is! `MemoryReactorClock.advance`
https://twistedmatrix.com/documents/current/api/twisted.test.proto_helpers.MemoryReactorClock.html
does this for you.

An example usage can be seen in the tests for `HostnameEndpoint`
https://github.com/twisted/twisted/blob/trunk/twisted/internet/test/test_endpoints.py#L2064

-Ashwini

Post by Mashiat Sarker Shakkhar
Or is there any other approach to test such code?
Regards
Shakkhar
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[Mashiat Sarker Shakkhar]

Post by Ashwini Oruganti
On Thu, Oct 1, 2015 at 1:40 PM, Mashiat Sarker Shakkhar <

Post by Mashiat Sarker Shakkhar
Hi
I have a function that uses callLater extensively to schedule a number of
different tasks. I don't want to get into the rationale behind such a
design, but here is a contrived example which will help me explain my
'''Do some work'''
'''Do some more work'''
return Reactor.callLater(300, b)
return Reactor. callLater(100, c)
Now I want to test this function. Of course I can't wait for 5 minutes to
ensure that `b` or `c` will indeed be called. What I need is some sort of
mock clock which lets me fast forward time. Does any such thing exist in
Twisted / Trial?

Yes there is! `MemoryReactorClock.advance`
https://twistedmatrix.com/documents/current/api/twisted.test.proto_helpers.MemoryReactorClock.html
does this for you.

HI Ashwini

Thanks a lot for your response. I did come across MemoryReactorClock
before, but I could not figure out how to use it to test my code. I am
using trial as the test runner, which allows me to pass the name of the
reactor to be used in command line. But MemoryReactorClock is not
available, because it is not in twisted.internet.

The code that I am testing imports reactor from twisted.internet in the
global scope. Do I have to change that and pass a reactor explicitly (say,
as a function parameter) where it is needed? Is there any other way to
change the default reactor to MemoryReactorClock just for running tests?

Going with my previous example, do I have to rewrite the code like
following or is there a better way?

def b():

'''Do some work'''


def c():

'''Do some more work'''


def a(myreactor, flag):

if flag:

return myreactor.callLater(300, b)

else:

return myeactor. callLater(100, c)

Post by Ashwini Oruganti
New application code should prefer to pass and accept the reactor as a

parameter where it is needed, rather than relying on being able to import
this module to get a reference. This simplifies unit testing ...


But I'd prefer not to change the signature of existing functions.


Regards

Shakkhar

[Glyph Lefkowitz]

Post by Mashiat Sarker Shakkhar
Hi
'''Do some work'''
'''Do some more work'''
return Reactor.callLater(300, b)
return Reactor. callLater(100, c)
Now I want to test this function. Of course I can't wait for 5 minutes to ensure that `b` or `c` will indeed be called. What I need is some sort of mock clock which lets me fast forward time. Does any such thing exist in Twisted / Trial?
Yes there is! `MemoryReactorClock.advance` https://twistedmatrix.com/documents/current/api/twisted.test.proto_helpers.MemoryReactorClock.html <https://twistedmatrix.com/documents/current/api/twisted.test.proto_helpers.MemoryReactorClock.html> does this for you.
HI Ashwini
Thanks a lot for your response. I did come across MemoryReactorClock before, but I could not figure out how to use it to test my code. I am using trial as the test runner, which allows me to pass the name of the reactor to be used in command line. But MemoryReactorClock is not available, because it is not in twisted.internet.

Indeed, the MemoryReactorClock reactor is not suitable for installation in a global reactor.

Post by Mashiat Sarker Shakkhar
The code that I am testing imports reactor from twisted.internet in the global scope. Do I have to change that and pass a reactor explicitly (say, as a function parameter) where it is needed?

Yes.

Post by Mashiat Sarker Shakkhar
Is there any other way to change the default reactor to MemoryReactorClock just for running tests?

You can hack it using patch <http://twistedmatrix.com/documents/15.2.0/api/twisted.trial.unittest.SynchronousTestCase.html#patch <http://twistedmatrix.com/documents/15.2.0/api/twisted.trial.unittest.SynchronousTestCase.html#patch>>, by replacing the "reactor" attribute on your module. But this is much worse than passing a parameter, and should only be used if you _really_ can't change the signature of your code for some reason.

Post by Mashiat Sarker Shakkhar
Going with my previous example, do I have to rewrite the code like following or is there a better way?

You should rewrite code like the following, it would be better :). But hey, this is Python, nobody can stop you from doing anything you want :).

Post by Mashiat Sarker Shakkhar
'''Do some work'''
'''Do some more work'''
return myreactor.callLater(300, b)
return myeactor. callLater(100, c)

New application code should prefer to pass and accept the reactor as a parameter where it is needed, rather than relying on being able to import this module to get a reference. This simplifies unit testing ...

But I'd prefer not to change the signature of existing functions.

Passing parameters to functions that need them is not a bad thing. You should generally prefer that. It seems a lot of Python programmers are very resistant to passing arguments, and would instead prefer that everything be a global variable. I am honestly somewhat confused as to why :-). But it's a bad habit and you should try to shake it.

-glyph