Python classmethod中的闭包问题

碰到一个 classmethod 中闭包的局部变量问题, 很有趣。

先来看下面的代码:

# test1.py
from functools import wraps

class A():
    ctx = {}
    
    @classmethod
    def cm_decorator(cls, value):
        def inner(fn):
            print("locals: ", locals())
            @wraps(fn)
            def wrapper(*args, **kwargs):
                kwargs["value"] = value
                return fn(*args, **kwargs)
            
            return wrapper
        return inner

@A.cm_decorator("1")
def test(*args, **kwargs):
    print(args, kwargs)

test()

执行结果:

> python3 test1.py
locals:  {'fn': <function test at 0x103320598>, 'value': '1'}
() {'value': '1'}

OK, 意料之中, 如果把 wrapper 中的 kwargs["value"] = value 去掉会是怎样的呢? 去掉后的代码:

# test2.py
from functools import wraps

class A():
    ctx = {}
    
    @classmethod
    def cm_decorator(cls, value):
        def inner(fn):
            print("locals: ", locals())
            @wraps(fn)
            def wrapper(*args, **kwargs):
                return fn(*args, **kwargs)
            
            return wrapper
        return inner

@A.cm_decorator("1")
def test(*args, **kwargs):
    print(args, kwargs)

test()

运行结果:

> python3 test2.py
locals:  {'fn': <function test at 0x103c05598>}
() {}

一眼就能发现, 之前的 value 没有了,

TODO

再看下面这个:

# test3.py
from functools import wraps

class A():
    ctx = {}
    
    @classmethod
    def cm_decorator(cls, value):
        def inner(fn):
            print("locals: ", locals())
            @wraps(fn)
            def wrapper(*args, **kwargs):
               
                ctx = cls.ctx
                 
                if ctx.get("value"):
                    value = ctx["value"]
                else:
                    ctx["value"] = value
                return fn(*args, **kwargs)
            
            return wrapper
        return inner

@A.cm_decorator("1")
def test(*args, **kwargs):
    print(args, kwargs)

test()

输出:

> python3 test2.py
locals:  {'fn': <function test at 0x103405598>, 'cls': <class '__main__.A'>}
Traceback (most recent call last):
  File "/Users/heaven/test/2.py", line 27, in <module>
    test()
  File "/Users/heaven/test/2.py", line 17, in wrapper
    kwargs["value"] = value
UnboundLocalError: local variable 'value' referenced before assignment