[Python-Dev] the role of assert in the standard library ?

[Terry Reedy]

On 4/28/2011 3:54 AM, Tarek Ziadé wrote:
> Hello
>
> I removed some assert calls in distutils some time ago because the
> package was not behaving correctly when people were using Python with
> the --optimize flag. In other words, assert became a full part of the
> code logic and removing them via -O was changing the behavior.
>
> In my opinion assert should be avoided completely anywhere else than
> in the tests. If this is a wrong statement, please let me know why :)

My understanding is that assert can be used in production code but only 
to catch logic errors by testing supposed invariants or postconditions. 
It should not be used to test usage errors, including preconditions. In 
other words, assert presence or absence should not affect behavior 
unless the code has a bug.

> So, I grepped the stdlib for assert calls, and I have found 177 of
> them and many of them are making Python acts differently depending on
> the -O flag,
>
> Here's an example on a randomly picked assert in the threading module:

This, to me is wrong:

    def __init__(self, group=None, target=None, name=None,
                  args=(), kwargs=None, verbose=None):
         assert group is None, "group argument must be None for now"

That catches a usage error and should raise a ValueError.

This

     def _wait(self, timeout):
         if not self._cond.wait_for(lambda : self._state != 0, timeout):
             #timed out.  Break the barrier
             self._break()
             raise BrokenBarrierError
         if self._state < 0:
             raise BrokenBarrierError
         assert self._state == 1

appears to be, or should be, a test of a postcondition that should 
*always* be true regardless of usage.


-- 
Terry Jan Reedy