Python variable not changed during a function

Question

I came accross a problem 2 days ago and I couldn't find a way to make it work, it's simple and my solution seems "good" to me. Here is what I got :

leftKeyCounter = 0

def sendCommand():
    # I want to get the counter back to 0.
    leftKeyCounter = 0
    return leftKeyCounter

while True:
...
leftKeyCounter = leftKeyCounter + 1

The sendCommand() function is called automatically every 5 seconds with the "schedule" helper. In my terminal, the "leftKeyCounter" is not changed, for example; if it is 4, when the function is run, it tells me the variable is 0 now, but if i add one again, it's 5...

All the solutions I look for send me back to the "global variable" that has been deprecated, so I don't find a working solution...

Thanks for any help :)

You just need to add `global leftKeyCounter` in the `sendCommand()` function. — Farhan.K, Mar 24 '16 at 10:18

score 4 · Answer 1 · answered Mar 24 '16 at 10:19

The reason its doing this, is because the leftKeyCounter inside your method is different than the lefKeyCounter defined outside - even if they have the same name.

One way to make the method modify the leftKeyCounter that is defined outside, is to use the global keyword - so it knows that it is modifying the global version of the variable.

Another way is to pass the variable in, and return the modified value, and then save this return value:

def sendCommand(leftKeyCounter):
   # do something here
   if something_is_true:
       leftKeyCounter = 0
   return leftKeyCounter # this is the new value of leftKeyCounter

leftKeyCounter = 0 # this is the starting value
while True:
   # do something here
   leftKeyCounter = sendCommand(leftKeyCounter)

score 2 · Accepted Answer · edited May 23 '17 at 10:29

2

If you want to refer to global scope variables, and modify them, then you can do the following thing:

leftKeyCounter = 0

def sendCommand():
    # I want to get the counter back to 0.
    global leftKeyCounter
    leftKeyCounter = 0
    return leftKeyCounter

while True:
...
leftKeyCounter = leftKeyCounter + 1

This question had already been answered: Using global variables in a function other than the one that created them

edited May 23 '17 at 10:29

Community

1
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answered Mar 24 '16 at 10:16

Patrick Trentin

7,126
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I will try it right now, and I already came across this post you're refering to, but i thought it was deprecated, I've read it at so many places, that's why I didn't wan't to use the global var way... (the post is from 2009, so... ), thanks – ThaoD5 Mar 24 '16 at 10:20

score 2 · Answer 3 · edited May 23 '17 at 12:02

2

This is a scoping issue.

sendCommand -> leftKeyCounter is not the same as leftKeyCounter - it'll tell you it's 0 because within the scope of the function it is.

This stack overflow question has some excellent answers and information about how this works. Short Description of the Scoping Rules?

edited May 23 '17 at 12:02

Community

1
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answered Mar 24 '16 at 10:19

Callam Delaney

641
3
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Thanks ! Will take a look :) – ThaoD5 Mar 24 '16 at 10:24

Python variable not changed during a function

3 Answers3