varnisher.py

@contextmanager
def assertingBeforeAfter(test, pre_post_f, *args, **kwargs):
    """>>> x = [1, 2, 3]
    >>> with assertingBeforeAfter(lambda o, n: o != n, lambda: len(x))
    ...    x.append(1)
    >>>
    >>> x = [1, 2, 3]
    >>> with assertingBeforeAfter(lambda o, n: o != n, lambda: len(x))
    ...     print "hi"
    Traceback (most recent call last):
    File "<stdin>", line 2, in <module>
    File "/usr/lib64/python2.6/contextlib.py", line 23, in __exit__
      self.gen.next()
    File "<stdin>", line 8, in assertingBeforeAfter
    AssertionError
    """
    old = pre_post_f(*args, **kwargs)
    try:
        yield
    finally:
        new = pre_post_f(*args, **kwargs)
        assert test(old, new), "test was not true for args=%r, kwargs=%r" % \
            (args, kwargs)