# 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):
    ...
    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):
    ...
    TimeOutWaitingFor: no_way

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