[Python-ideas] Stack traces ought to flag when a module has been changed on disk

[Anders Hovmöller]

I've been bitten by this and it always costs me several minutes of confusion.

+1

> On 30 Jan 2019, at 11:17, Steven D'Aprano <steve at pearwood.info> wrote:
> 
> This thought is motivated by this bug report:
> 
> https://bugs.python.org/issue35857
> 
> If you import a module, then edit the .py file that goes with it, and 
> then an exception occurs, the stack trace can show the wrong line.
> 
> It doesn't happen very often, but when it does happen, it can be very 
> perplexing. Here's a proposal:
> 
> When a stack trace is printed, before printing each line, the 
> interpreter checks whether the file's modification time is later than 
> the time recorded in the .pyc file. If the times are different, the 
> stack trace can flag the line and print an addition line stating that 
> the file may have changed and the stack trace may not be accurate.
> 
> Something like this perhaps?
> 
> 
> Traceback (most recent call last):
>  File "spam.py", line 99, in <spam>
>    eggs.foo()
>  File "eggs.py", line 123, in <eggs>
>    ? for obj in sequence:
>  File "cheese.py", line 456, in <cheese>
>    n = len(x)
> *** one or more files may have changed
> *** lines starting with ? may be inaccurate
> TypeError: object of type 'NoneType' has no len()
> 
> 
> I don't think performance will matter. Generating stack traces are 
> rarely performance critical, so I don't think that a few extra file 
> system checks will make any meaningful difference.
> 
> 
> Thoughts?
> 
> 
> 
> -- 
> Steve
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