Wait until a condition holds (or until a time out)

Often, in tests, you need to wait until some condition holds. This may be because you’re testing interaction with an external system or testing threaded (threads, processes, greenlet’s, etc.) interactions.

You can add sleeps to your tests, but it’s often hard to know how long to sleep.

zope.testing.wait provides a convenient way to wait until some condition holds. It will test a condition and, when true, return. It will sleep a short time between tests.

Here’s a silly example, that illustrates it’s use:

>>> from zope.testing.wait import wait
>>> wait(lambda : True)

Since the condition we passed is always True, it returned immediately. If the condition doesn’t hold, then we’ll get a timeout:

>>> wait((lambda : False), timeout=.01)
Traceback (most recent call last):
...
zope.testing.wait.Wait.TimeOutWaitingFor: <lambda>

wait has some keyword options:

timeout

How long, in seconds, to wait for the condition to hold

Defaults to 9 seconds.

wait

How long to wait between calls.

Defaults to .01 seconds.

message

A message (or other data) to pass to the timeout exception.

This defaults to None. If this is false, then the callable’s doc string or __name__ is used.

wait can be used as a decorator:

>>> @wait
... def ok():
...     return True

>>> @wait(timeout=.01)
... def no_way():
...     pass
Traceback (most recent call last):
...
zope.testing.wait.Wait.TimeOutWaitingFor: no_way

>>> @wait(timeout=.01)
... def no_way():
...     "never true"
Traceback (most recent call last):
...
zope.testing.wait.Wait.TimeOutWaitingFor: never true

Customization

wait is an instance of Wait. With Wait, you can create you’re own custom wait utilities. For example, if you’re testing something that uses getevent, you’d want to use gevent’s sleep function:

>>> import zope.testing.wait
>>> wait = zope.testing.wait.Wait(getsleep=lambda : gevent.sleep)

Wait takes a number of customization parameters:

exception

Timeout exception class

getnow

Function used to get a function for getting the current time.

Default: lambda : time.time

getsleep

Function used to get a sleep function.

Default: lambda : time.sleep

timeout

Default timeout

Default: 9

wait

Default time to wait between attempts

Default: .01