Why doesn't Python (error msg) tell me WHAT the actual (arg) values are ?

[Weatherby,Gerard]

I only use asserts for things I know to be true. Nothing is harder to debug than when something you know to be true turns out to be… not True. Because I’ll check everything else instead of the cause of the bug.

In other words, a failing assert means I have a hole in my program logic.

For that use, the default behavior –telling me which line the assert is on, is more than sufficient. Depending on the circumstance, I’ll re-run the code with a breakpoint or replace the assert with an informative f-string Exception.



From: Python-list <python-list-bounces+gweatherby=uchc.edu at python.org> on behalf of Peter J. Holzer <hjp-python at hjp.at>
Date: Saturday, February 25, 2023 at 9:22 AM
To: python-list at python.org <python-list at python.org>
Subject: Re: Why doesn't Python (error msg) tell me WHAT the actual (arg) values are ?
On 2023-02-25 09:10:06 -0500, Thomas Passin wrote:
> On 2/25/2023 1:13 AM, Peter J. Holzer wrote:
> > On 2023-02-24 18:19:52 -0500, Thomas Passin wrote:
> > > Sometimes you can use a second parameter to assert if you know what kind of
> > > error to expect:
[...]
> > > With type errors, assert may actually give you the information needed:
> > >
> > > > > > c = {"a": a, "b": 2}
> > > > > > assert a > c
> > > Traceback (most recent call last):
> > >    File "<stdin>", line 1, in <module>
> > > TypeError: '>' not supported between instances of 'list' and 'dict'
> >
> > Actually in this case it isn't assert which gives you the information,
> > it's evaluating the expression itself. You get the same error with just
> >      a > c
> > on a line by its own.
>
> In some cases.  For my example with an explanatory string, you wouldn't want
> to write code like that after an ordinary line of code, at least not very
> often.  The assert statement allows it syntactically.

Yes, but if an error in the expression triggers an exception (as in this
case) the explanatory string will never be displayed.

        hp

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