Python 3.7 Bug

[DL Neil]

Bassam,
Greetings.


On 26/03/19 1:14 AM, Bassam Abdul-Baki wrote:
> Greetings,
> 
> In the following code, there's a bug on certain parameters.
> 
> ----------
> 
> def per(n, steps = 0):
>   digits = [int(i) for i in str(n)]
>   result = 1
>   for j in digits:
>    result *= j
>   steps += 1
>   print(steps, result, sep=" - ")
>   if result == 0:
>    print(result, str(result), len(str(result)), sep=" - ")
>   if len(str(result)) == 1:
>    print(" --- DONE ---")
>    return "DONE"
>   else:
>    per(result, steps)
> 
> ----------
> 
> What the program does:
> If I run per(X) and X is a multiple of 10, I should end up with 0 in a
> finite amount of steps.
> 
> The problem:
> If I run per(54), I do not get 'DONE' printed through the return
> statement.  WRONG!
> 
> If I run per(20), I do get 'DONE' printed through the return statement.
> CORRECT!
> 
> 20, 30, etc. are correct.  25, 45, etc. are not.
> 
> Is this a bug?


No - the Python 3.7 does exactly what you've asked it to do!

0
What is the objective of this piece of code? Other than "CORRECT!" and 
"WRONG!" there are few hints, especially the function name: "per". What 
does "per" mean - not everyone is a mathematician.

1
Had we been shown two entire output reports, eg one for 20 (correct) and 
another for 25 (incorrect), the answer would have been immediately apparent.

2
Is it a good idea/best practice to sometimes code the two values as 
"results" followed by "steps", and sometimes "steps" followed by 
"results"; or does this invite confusion?

3
Please express "len(str(result)) == 1" in English (or your preferred 
spoken-language). Is there an easier way to find numbers expressed in 
multiple digits? (and that's another overly-complicated way to say the 
same thing!)

4
If a 'debug print' is added at the beginning of the function, is the 
function being "called" more times than expected? Should/could more 
debug-print statements be added to check other sub-calculations along 
the way?

(PS 'debug-print' added to your code for debugging/testing and then 
removed for 'production-use' is a quick-and-dirty tactic. Please add a 
note to your Python ToDo list: "learn to use logging library instead of 
debug-print", otherwise wiser heads will criticise this advice, which 
works well when learning but perhaps not at the professional level)

5
Humility: that it might be a bug in your code rather than in Python 3.7.
Respect: It would be polite to mention if this is a student/homework 
assignment - or the text that you are following for self-improvement.
Charity begins at home: That people here will volunteer to help you, and 
can be more helpful if you help us - particularly by providing 
information which will save our (valuable) time.


Here's hoping you are now equipped to find the source of the problem...

-- 
Regards =dn