Why doesn't program print value?

Question

I am wondering why the following program does not print 'b'. It is is very simple code; I think it must work; and do not know the reason why it doesn't.

def a():
    if b > 10:
        print 'b'
        sys.exit(1)

# main
while 1:
    a()
    b += 1

b is global variable. Actual code is more complicated but the structure is the same as mine. I guess when I call a() function and if b is greater than 10, it shows 'b'. However, it does not go inside if-statement.

Would you help me out how to solve?

Thanks.

Wait, how does it print 'b' if it does not enter the body of the if? Please clarify. — user395760
– user395760, Commented May 28, 2012 at 17:37
I have tried your code with globla b in front of def a(): and it works fine with 'b' printed. It did go inside if-statement. (Python 2.7.2 Win32) — Mayli
– Mayli, Commented May 28, 2012 at 17:37
@Mayli Absolutely not! It raises NameError: global name 'b' is not defined. The solution is not the global b. It's b = 0 before the loop starts! — jadkik94
– jadkik94, Commented May 28, 2012 at 17:40

Jakob Bowyer · Accepted Answer · 2012-05-28 17:34:39Z

4

Globals are horrid learn not to use them, try something like this

import sys
def a(value):
    if value > 10:
        print value
        print "Greater than 10!"
        sys.exit(0)
b = 0
while True:
    a(b)
    b += 1

answered May 28, 2012 at 17:34

Jakob Bowyer

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5 Comments

Sangwon Chae Over a year ago

I've tried to the same approach as you answered, but it does not work.

Jakob Bowyer Over a year ago

You need to delete the .pyc and restart the interpreter

user1342784 Over a year ago

While you are completely correct that globals should not be used, this doesn't actually answer the question.

Jakob Bowyer Over a year ago

@StevenBurnap The question is unclear and can't be reproduced

Sangwon Chae Over a year ago

Other functions work very well, but this function cannot work correctly. I do not know why it is not working.

Vidul · Accepted Answer · 2012-05-28 18:32:29Z

0

Global vars are not the usual way to go. You may prefer the usage of nested functions instead:

import sys

def outer(value=0):
    count = [value]
    print "started at:", count[0]
    def inner(x=1):
        print "current val:", count[0]
        count[0] += x
        if count[0] > 10:
            print "stopped at:", count[0]
            sys.exit(0)
    return inner

f = outer(5)
while True:
    f(1)

answered May 28, 2012 at 18:32

Vidul

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Comments

jadkik94 · Accepted Answer · 2012-05-28 17:43:47Z

Another answers suggests not using globals, and I agree. If you still want to use globals, you should define b outside of the loop first.(if you do, then please post the complete code, because apart from that, it should work (and it does)).

Now, global b in the function definition is not necessary, because python guesses it is a global variable when you try to access it before assigning. But since it is not defined it raises an NameError:

NameError: global name 'b' is not defined

If you don't see that, so there's something else, you're not showing the actual code that has a problem.

This gives you in the end, something similar:

import sys
def a():
    global b
    if b > 10:
        print 'b'
        sys.exit(1)

b = 0
# main
while 1:
    a()
    b += 1

This works correctly. Is copy pasting it not working? I don't think so... Do you get any exception/output/anything... What is your actual code doing? We really need more details to help you, because that is perfectly fine.

jgomo3 · Accepted Answer · 2012-05-28 17:45:50Z

0

You need to define b before "+="ing it.

def a():
    if b > 10:
        print 'b'
        sys.exit(1)

# main
b = 0 # HERE
while 1:
    a()
    b += 1

By the way, as many had told you: avoid globals.

answered May 28, 2012 at 17:45

jgomo3

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Comments

bpgergo · Accepted Answer · 2012-05-28 17:46:34Z

0

just my 2 cents: forget global variables.

anyway, this should work

def a():
    global b
    if b > 10:
        print 'b'
        sys.exit(1)

EDIT FUUUUUUUUUUUUUUU

Although I fully agree with Jackob on how you should use functions arguments and avoid globals, just for the record, here's the solution with global:

def a():
    global b
    if b > 10:
        print 'b'
        sys.exit(1)

# main
b = 0
while 1:
    a()
    b += 1

edited May 28, 2012 at 17:46

answered May 28, 2012 at 17:34

bpgergo

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1 Comment

user395760 Over a year ago

Here too: -1 That global is redundant as b is not assigned to. And otherwise that code is identical to OP's.

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