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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