装饰器运行错误：“UnboundLocalError：赋值前引用了局部变量‘count’”

Question

I created two variables inside two different decorators, and one decorator works good but the other one shows an error.

def running_average(func):
    data = {'total': 0, 'count': 0}
    def wrapper(*args, **kwargs):
        print(data)
        val = func(*args, **kwargs)
        data['total'] += val
        data['count'] += 1
        print('Average of {} so far: {:.01f}'.format(func.__name__,
              data['total']/data['count']))
        return func(*args, **kwargs)
    return wrapper

@running_average
def foo(x):
    return x + 2

This foo function works.

def countcalls(func):
    count = 0
    print(count)
    def wrapper(*args, **kwargs):
        print(count)
        count += 1
        print('# of calls: {}'.format(count))
        return func(*args, **kwargs)
    return wrapper

@countcalls
def boo(x):
    return x + 2

But this boo function shows error: UnboundLocalError: local variable 'count' referenced before assignment .

From my understanding, these two functions follow the same pattern. In both decorators, a variable is defined in the decorator, and the variable is used in the wrapper function. However, this foo function works good, but this boo function doesn't work. I was wondering what is the reason?

Answer 1

The difference is that in the first example, the name data always refers to the same object; it is never reassigned. The dictionary data refers to is itself mutated (changed), but it is always the same object.

In contrast, when you do count += 1 in your second example, that changes the value that the name count refers to. That makes count a local variable. When the interpreter sees this, it notices that you reference the local variable count before assigning to it, and fails.

The solution is to tell the interpreter to use the count from the enclosing scope by declaring it nonlocal

def countcalls(func):
    count = 0
    print(count)
    def wrapper(*args, **kwargs):
        nonlocal
        print(count)
        count += 1
        print('# of calls: {}'.format(count))
        return func(*args, **kwargs)
    return wrapper