从settings.py导入的变量的UnboundLocalError

Question

I have a file settings.py which has the following variables defined:

DEBUG_MODE = True
WRAP_UP = False

I am importing settings into my script using:

from settings import *

I am able to run this without any problem:

if(DEBUG_MODE):
    # Do something

But when I run this:

if(WRAP_UP == False):
    # Do something

it gives me the following error:

UnboundLocalError: local variable 'WRAP_UP' referenced before assignment.

Why does one variable work and the other doesn't? How do I fix this?

Answer 1

In Python, inside a function, any variables you don't explicitly declare global or nonlocal (closure) are considered local if you assign to them anywhere in the function, non-local (closure, global, or builtin) if you don't.

So, assuming you do the from settings import * globally (if not, you have bigger problems…), that creates global variables named DEBUG_MODE and WRAP_UP , so you can do this:

def spam():
    if DEBUG_MODE:
        # Do something

… and that works. But if you do this:

def eggs():
    if WRAP_UP:
        # Do something
    WRAP_UP = True

… that won't work. The assignment means that eggs has a local variable named WRAP_UP , which hides the global variable of the same name. So, the first line is trying to access that local variable, which doesn't have a value yet.

The solution is an explicit global statement, which will force eggs to use the global variable even though it has an assignment:

def eggs():
    global WRAP_UP
    if WRAP_UP:
        # Do something
    WRAP_UP = True

Of course that's assuming that you want eggs to reassign the global, but I suspect that's what you want.