nonlocal

python

装饰器,nonlocal

定义一个计算移动平均值的高阶函数：

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def make_averager():
    series = []
    def averager(new_value):
        series.append(new_value)
        total = sum(series)
        return total / len(series)
    return averager

>>>avg = make_averager()
>>>avg(10)
10.0
>>>avg(20)
15.0

series是make_averager函数的局部变量，是averager函数的自由变量，其名称保存在__code__属性中，绑定的值保存在__closure__属性中：

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>>>avg.__code__.co_varnames
('new_value', 'total')
>>>avg.__code__.co_freevars
('series',)
>>>avg.__closure__[0].cell_contents
[10, 20]

但是如果这样会报错：

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def make_averager():
    count = 0
    total = 0
    def averager(new_value):
        count += 1
        total += new_value
        return total / count
    return averager

>>>avg = make_averager()
>>>avg(10)
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-10-ace390caaa2e> in <module>----> 1 avg(10)

<ipython-input-8-3f2ada1aa04d> in averager(new_value)
	  3     total = 0
	  4     def averager(new_value):
----> 5    count += 1
	  6         total += new_value
	  7         return total / count

UnboundLocalError: local variable 'count' referenced before assignment

由于列表是一个可变对象，因此在averager函数中并未给series赋值，而是调用了append方法； 而数字是一个不可变对象，此时改变count的值会隐式创建局部变量，count就不再是自由变量了

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>>>avg.__code__.co_varnames
('new_value', 'count', 'total')

>>>avg.__code__.co_freevars
()

此时可以用nonlocal声明，将变量重新标记为自由变量：

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def make_averager():
    count = 0
    total = 0
    def averager(new_value):
        nonlocal count,total
        count += 1
        total += new_value
        return total / count
    return averager

>>>avg = make_averager()

>>>avg(10)
10.0

>>>avg(20)
15.0

>>>avg.__code__.co_varnames
('new_value',)

>>>avg.__code__.co_freevars
('count', 'total')

装饰器中的变量的作用域也是这样的：下面定义一个简单的装饰器，在函数每次调用时打印其调用次数:

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def decorator(func):
    count = 0
    def wrapper():
        nonlocal count
        count += 1
        result = func()
        print('{}函数共调用了：'.format(func),count,'次')
        return result
    return wrapper

@decoratordef cal():
    return 10

>>>for i in range(10):
    cal()
    
<function cal at 0x00000237C30E7168>函数共调用了：1 次<function cal at 0x00000237C30E7168>函数共调用了：2 次<function cal at 0x00000237C30E7168>函数共调用了：3 次<function cal at 0x00000237C30E7168>函数共调用了：4 次<function cal at 0x00000237C30E7168>函数共调用了：5 次<function cal at 0x00000237C30E7168>函数共调用了：6 次<function cal at 0x00000237C30E7168>函数共调用了：7 次<function cal at 0x00000237C30E7168>函数共调用了：8 次<function cal at 0x00000237C30E7168>函数共调用了：9 次<function cal at 0x00000237C30E7168>函数共调用了：10 次

@decoratordef cal_2():
    return 100

>>>for i in range(10):
    cal_2()
    
<function cal_2 at 0x00000237C30C0798>函数共调用了：1 次<function cal_2 at 0x00000237C30C0798>函数共调用了：2 次<function cal_2 at 0x00000237C30C0798>函数共调用了：3 次<function cal_2 at 0x00000237C30C0798>函数共调用了：4 次<function cal_2 at 0x00000237C30C0798>函数共调用了：5 次<function cal_2 at 0x00000237C30C0798>函数共调用了：6 次<function cal_2 at 0x00000237C30C0798>函数共调用了：7 次<function cal_2 at 0x00000237C30C0798>函数共调用了：8 次<function cal_2 at 0x00000237C30C0798>函数共调用了：9 次<function cal_2 at 0x00000237C30C0798>函数共调用了：10次