python closure part1
自己写的第一个 python 闭包
闭包之前在 javascript 中用过，写一个还不简单上代码：
#!/usr/bin/env python
def closure_counter():
count = 0
def counter():
count += 1
print count
return counter
counter1 = closure_counter()
counter1()
counter1()
counter2 = closure_counter()
counter2()
counter2()
``` output
Traceback (most recent call last):
File "./closure_test.py", line 11, in <module>
counter1()
File "./closure_test.py", line 6, in counter
count += 1
UnboundLocalError: local variable 'count' referenced before assignment
```
我错了，怎么会这样。万能的 google/stackoverflow 救救我吧。 Can you explain closures (as they relate to Python)? 中的一个示例代码用 nonlocal 解决上面的问题，但是 nonlocal 是 python 3.0 引入的 python 2.7 不支持。还好 Python Closures and the Python 2.7 nonlocal Solution 提供了解决方法。
def outer():
d = {'y' : 0}
def inner():
d['y'] += 1
return d['y']
return inner
“it’s kinda weird that we can’t access the non local variable but we can access the dictionary.” 是啊，太怪了，局部变量不行，字典却可以，具体原因待查待补充。
python closure counter 最终版本
通过字典这个解决方法看上去还是挺变扭的，想到 Understanding Python decorators 中有段例代码，一个 counter decorator 用到了闭包。
#!/usr/bin/env python
def closure_counter():
def counter():
counter.count += 1
return counter.count
counter.count = 0
return counter
c1 = closure_counter()
c2 = closure_counter()
print (c1(), c1(), c2(), c2())
``` output
(1, 2, 1, 2)
```
注意这里 counter.count = 0 不能放在 def counter(): 上面，否则又是个悲剧，具体原因待查待补充。
总结
原始版本跟最终版本相比，不说其中的错误，只说代码本身，测试代码更简洁，原始版本中还把 print 嵌入到功能代码中。代码是码出来的看看是不够的，要多码码，多跑跑，多比比 -:) 。
REF
- Can you explain closures (as they relate to Python)?
- Python Closures and the Python 2.7 nonlocal Solution
- Understanding Python decorators
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