Python recursion returning old value

Question

I'm trying to make a simple control in Python 2 to make sure that the user's input is smaller than a given constant. If this is not the case it should ask again for another integer until it is smaller.

limit_store = 12

def input_store():
    sel_store = int(raw_input("Which store do you want? "))
    if sel_store > limit_store:
        print "Store number %i is not valid" % sel_store
        input_store()  
    return sel_store

store = input_store()

This worked the first time I wrote it in an Ipython notebook. Now if the first input is 9, it returns 9 as expected. If the input is 999, which is larger than 12, the if statement is runned and we get to select another integer. Selecting a correct input after a wrong one will return the old wrong input instead of the newer input.

You would need to return input_store(), the value of sel_store will be stored on the first call to the function so if you enter 100 different numbers you will always get the first unless you return — Padraic Cunningham
– Padraic Cunningham, Commented Apr 22, 2015 at 21:39

TigerhawkT3 · Accepted Answer · 2015-04-22 22:25:38Z

2

You need to save the value after printing the "invalid" message.

if sel_store > limit_store:
    print "Store number %i is not valid" % sel_store
    sel_store = input_store() # save value here

returning the number instead of saving it would also work - as long as the number goes somewhere useful instead of being thrown away. Python doesn't have TCO, so the performance benefit is minimal. The difference is in what you find preferable: that your function has a single return statement, or that it returns immediately instead of saving the variable first.

It would be simpler as a loop anyway:

def input_store():
    while True:
        sel_store = int(raw_input("Which store do you want? "))
        if sel_store <= limit_store: break
        print "Store number %i is not valid" % sel_store
    return sel_store

edited Apr 22, 2015 at 22:25

answered Apr 22, 2015 at 21:21

TigerhawkT3

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7 Comments

bj0 Over a year ago

or just return input_store()

BAZA Over a year ago

So sorry. I forget to include that piece in the post. I'm editing now.

TigerhawkT3 Over a year ago

No - immediately after printing the "invalid" message, within the function.

BAZA Over a year ago

I think it worked. I still not sure why my version doesn't work but I understand why your does. Thanks.

TigerhawkT3 Over a year ago

It's because you saved the initial attempt, but then the returned value of the recursive call wasn't saved, so in effect you had a line that was just a statement containing some number, which the interpreter immediately threw away.

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