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From: ram@zedat.fu-berlin.de (Stefan Ram)
Newsgroups: comp.lang.python
Subject: Re: how to discover what values produced an exception?
Date: 3 May 2024 14:26:58 GMT
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Johanne Fairchild <jfairchild@tudado.org> wrote or quoted:
>How to discover what values produced an exception?

  An exception isn't always tied to a value. I could, like, write:

if 3 > 2: "".call()

  And I'd get an AttributeError exception. Can I now say the
  values 3 and 2 caused this exception? Or was it the empty
  string? Or the lack of a call method?

  Let me try your example here: In the console:

|>>> ( 0, 0 )< 4
|TypeError: '<' not supported between instances of 'tuple' and 'int'

  In this case, the values aren't important, one could even
  say they're a distraction. What matters are the types of the
  values, and those were given. However, the values are visible
  in the console as they were typed in. In a script:

|Traceback (most recent call last):
|  File "Main.py", line 1, in <module>
|    ( 0, 0 )< 4
|TypeError: '<' not supported between instances of 'tuple' and 'int'

  . Now I can see the offending line, which in this case even spills
  the beans on the values.

  Sometimes you really need those values. If they're values of global
  names, you can kind of get them in the IDLE shell up to a point.

  So I write this script in the IDLE editor and then run it:

a = 4
( 0, 0 )< a

  . Now I get this in the IDLE console:

|Traceback (most recent call last):
|  File "Main.py", line 2, in <module>
|    ( 0, 0 )< a
|TypeError: '<' not supported between instances of 'tuple' and 'int'

  . Then I can type

a 

  into the console, hit Return, and I see the value 4. 

  This script here has local variables, and that trick ain't
  gonna fly no more:

def main():
    a = 4
    ( 0, 0 )< a

main()

  Now you got to add a debug print statement and run the script again.

def main():
    a = 4
    print( a )
    ( 0, 0 )< a

main()

  In a bigger script there's all kinds of values and variables,
  and sometimes the Python implementation can't know which ones
  are critical for an exception, so the programmer's got to step
  in and write those debug print statements.

  (You could also use the IDLE debugger to see local variables,
  but I prefer debug print statements.)