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

Steven D'Aprano steve at pearwood.info
Wed Jan 30 05:17:34 EST 2019

This thought is motivated by this bug report:


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>
  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.



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