[Tutor] Recursive user input collection problem

Dave Angel davea at ieee.org
Tue Oct 13 23:02:32 CEST 2009

William Witteman wrote:
> I need to collect a couple of integers from a user, but I want to make
> sure that I actually get integers.  I tried this, but subsequent calls
> to the function don't update variable.  I'm not sure this is terribly
> clear - here's the code:
> num_of_articles = 0
> num_of_reviewers = 0
> def getinput(variable,prompt):
>   """
>   Get the input by prompting the user and collecting the response - if it is 
>   a non-integer, try again.
>   """
>   variable = 0
>   variable = raw_input(prompt)
>   try:
>     int(variable)
>     return variable
>   except ValueError:
>     print("We need an integer (number) here.")
>     getinput(variable,prompt)
> num_of_articles = getinput(num_of_articles,"Enter number of articles: ")
> num_of_reviewers = getinput(num_of_reviewers,"Enter number of reviewers: ")
> print(num_of_articles)
> print(num_of_reviewers)
> This works fine if I put in good input, but not if I pass in a bad
> value.  Can anyone show me where I have gone astray?  Thanks.
You called getinput() from inside getinput(), which is a recursive 
call.  So what happens is whenever the user makes a mistake, you have 
two copies of the function running.  And they get their own copies of 
variables, which isn't what you want in this case.  What you want 
instead is to loop back to the beginning, not to call.

In this simple case, you can simply enclose the entire function in a
   while True:

loop.  And when the exception happens, you loop around back to the 


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