UnBoundLocalError:赋值之前引用的局部变量(Python) [英] UnBoundLocalError: local variable referenced before assignment (Python)

问题描述

我正在尝试创建一个返回值servo_quadrant的函数servo_to_quadrant.

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

与此问题类似的问题涉及到函数外部的全局变量存在问题.在这种情况下，我认为这不是问题所在，因为仅从函数内部需要变量(尽管我可能是错的).

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).

代码:

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)

错误:

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

推荐答案

这是因为您在函数中的前面if条件之一下分配了变量servo_quadrant，并且如果没有条件返回True，则表示您将没有任何servo_quadrant.为了解决这个问题，您需要在函数中初始化此变量.

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.

您可以将servo_quadrant = 0放在函数的顶层，或者可以在返回任何内容之前检查servo_quadrant的值:

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

还注意，您需要重新分配变量servo_val:

Also Note that you need to reassign variable servo_val :

if servo_val < 0: servo_val=360 + servo_val

演示:

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

结果:

4

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