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

.. more tests

    >>> import time
    >>> now = time.time()
    >>> @wait(timeout=.01, message='dang')
    ... def no_way():
    ...     "never true"
    Traceback (most recent call last):
    ...
    zope.testing.wait.Wait.TimeOutWaitingFor: dang

    >>> .01 < (time.time() - now) < .03
    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


.. more tests

    >>> def mysleep(t):
    ...     print_('mysleep', t)
    ...     time.sleep(t)

    >>> def mynow():
    ...     print_('mynow')
    ...     return time.time()

    >>> wait = zope.testing.wait.Wait(
    ...    getnow=(lambda : mynow), getsleep=(lambda : mysleep),
    ...    exception=ValueError, timeout=.1, wait=.02)

    >>> @wait
    ... def _(state=[]):
    ...     if len(state) > 1:
    ...        return True
    ...     state.append(0)
    mynow
    mysleep 0.02
    mynow
    mysleep 0.02

    >>> @wait(wait=.002)
    ... def _(state=[]):
    ...     if len(state) > 1:
    ...        return True
    ...     state.append(0)
    mynow
    mysleep 0.002
    mynow
    mysleep 0.002

    >>> @wait(timeout=0)
    ... def _(state=[]):
    ...     if len(state) > 1:
    ...        return True
    ...     state.append(0)
    Traceback (most recent call last):
    ...
    ValueError: _

    >>> wait = zope.testing.wait.Wait(timeout=0)
    >>> @wait(timeout=0)
    ... def _(state=[]):
    ...     if len(state) > 1:
    ...        return True
    ...     state.append(0)
    Traceback (most recent call last):
    ...
    zope.testing.wait.Wait.TimeOutWaitingFor: _
