UnBoundLocalError: local variable referenced before assignment (Python)

Question

I'm trying to create a function servo_to_quadrant that returns the value servo_quadrant.

Questions similar to this one have involved there being an issue with a global variable outside of the function. I don't think that's the issue in this case, as the variable is only needed from within the function (although I could be wrong).

Code:

def servo_to_quadrant(servo_val):
    if servo_val < 0: 360 + servo_val
    if servo_val >= 360: servo_val = servo_val - 360
    if servo_val >= 0 and servo_val < 90: servo_quadrant = 1
    if servo_val >= 90 and servo_val < 180: servo_quadrant = 2
    if servo_val >= 180 and servo_val < 270: servo_quadrant = 3
    if servo_val >= 270 and servo_val < 360: servo_quadrant = 4
    return servo_quadrant

servo_val = -30
quadrant = servo_to_quadrant(servo_val)
print(quadrant)

Error:

Traceback (most recent call last):
  File "test2.py", line 11, in <module>
    quadrant = servo_to_quadrant(servo_val)
  File "test2.py", line 8, in servo_to_quadrant
    return servo_quadrant
UnboundLocalError: local variable 'servo_quadrant' referenced before assignment

Solution 1

It's because you have assigned the variable servo_quadrant under one of the preceding if conditions in your function, and if none of the conditions return True, you will haven't any servo_quadrant. For getting ride of this problem you need to initial this variable in your function.

You can put servo_quadrant = 0 on top level of your function or you can check the value of the servo_quadrant before you return anything :

if servo_quadrant :
    return servo_quadrant
return None

Also Note that you need to reassign variable servo_val :

if servo_val < 0: servo_val=360 + servo_val

Demo:

def servo_to_quadrant(servo_val):
    servo_quadrant=0
    if servo_val < 0: servo_val=360 + servo_val
    if servo_val >= 360: servo_val = servo_val - 360
    if servo_val >= 0 and servo_val < 90: servo_quadrant = 1
    if servo_val >= 90 and servo_val < 180: servo_quadrant = 2
    if servo_val >= 180 and servo_val < 270: servo_quadrant = 3
    if servo_val >= 270 and servo_val < 360: servo_quadrant = 4
    return servo_quadrant

servo_val = -30
quadrant = servo_to_quadrant(servo_val)
print quadrant

Result:

4

Solution 2

This is because you try to modify servo_quadrant which isn't defined in your function. Python uses global scope by default if you just read a variable. So if you don't modify it everything will work fine. If you need to modify it just add global servo_quadrant at the beginning of your function.