Problems with boolean statement and if statement in python

Question

def test():
    boo = True
    enter = int(raw_input("Enter something: "))
    if enter > 0:
        boo = False
        print "Did it work?!"
    else:
        print "Still working..."
    while boo == True:
        test()

test()

So this bit of code seems to only work if the first part of the if statement is met first try. aka if you enter -1 then 1 it continues to loop through the function. What am I doing wrong?

What are you trying to do exactly? Get a positive integer input from the user? — TheSoundDefense
– TheSoundDefense, Commented Jul 21, 2014 at 15:49
Did you realize that your function is being called in a loop and recursively? That seems a bit odd. — skrrgwasme
– skrrgwasme, Commented Jul 21, 2014 at 15:53
It's a bit odd, but it makes for a decent chuckle, anyway :) — stewSquared
– stewSquared, Commented Jul 21, 2014 at 16:00

Martijn Pieters · Accepted Answer · 2014-07-21 15:57:41Z

2

boo is a local variable. Every time you recursively call test(), that recursive call gets a new local variable boo that is independent of all parent functions calling it. As such, it is set to True at the beginning of the function.

Your calls look like this:

test(), boo = True, enter = 1, so while boo == True is true:
- test(), boo = True, enter = -1, so while boo == True is false, returning

at this level, boo == True is still true, so we call test() again.
- test(), boo = True, enter = -1, so while boo == True is false, returning

etc. ad nauseum.

You want to return boo:

def test():
    boo = True
    enter = int(raw_input("Enter something: "))
    if enter > 0:
        boo = False
        print "Did it work?!"
    else:
        print "Still working..."
    while boo:
        boo = test()
    return boo

Now when test() returns, you set boo in the calling function to False.

Note that == True is redundant, just test for boo itself in while boo.

It is really not a good idea to use recursion for user input. Just use a while loop:

while True:
    enter = int(raw_input("Enter something: "))
    if enter < 0:
        print "Did it work?!"
        break
    print "Still working..."

edited Jul 21, 2014 at 15:57

answered Jul 21, 2014 at 15:50

Martijn Pieters

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4 Comments

NPE Over a year ago

(+1) but I think it would be great to show that recursion is completely unnecessary here and only serves to obscure the logic.

Martijn Pieters Over a year ago

@NPE: I was busy adding a better option. I wanted to explain about where the OP had gone wrong first.

Joe Over a year ago

This is a great explanation. Thanks. I am new to python and will definitely be reading up on recursion. I think putting everything into a while loop will help a lot.

NPE Over a year ago

@MartijnPieters: Thanks for expanding the answer. I think it is great.

TheSoundDefense · Accepted Answer · 2014-07-21 15:52:56Z

Combining a potentially infinite while loop with a recursive function is a very dangerous game. When you get to subsequent calls of test(), even if they work and exit out right away, your original call to test() will still loop forever. boo is never changed in the original function call just because it's changed in later ones.

Your code could be fixed this way:

def test():
    boo = True
    enter = int(raw_input("Enter something: "))
    if enter > 0:
        boo = False
        print "Did it work?!"
    else:
        print "Still working..."
    if boo == True:
        test()

But it would be even better this way:

def test():
    enter = int(raw_input("Enter something: "))
    while enter <= 0:
        print "Still working..."
        enter = int(raw_input("Enter something: "))
    print "Did it work?!"

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