【python】闭包

ref

• https://www.programiz.com/python-programming/closure
• https://www.geeksforgeeks.org/python-closures/
• https://zhuanlan.zhihu.com/p/26934085
• https://www.cnblogs.com/orion-orion/p/15504127.html

concept

• nested function（内嵌函数）: A function that is defined inside another function is known as a nested function.
• non-local variables

When do we have closures?

As seen from the above example, we have a closure in Python when a nested function references a value in its enclosing scope.

The criteria that must be met to create closure in Python are summarized in the following points.

We must have a nested function (function inside a function).
The nested function must refer to a value defined in the enclosing function.
The enclosing function must return the nested function.

example

def make_multiplier_of(n):
    def multiplier(x):
        return x * n
    return multiplier


# Multiplier of 3
times3 = make_multiplier_of(3)

# Multiplier of 5
times5 = make_multiplier_of(5)

# Output: 27
print(times3(9))

# Output: 15
print(times5(3))

# Output: 30
print(times5(times3(2)))

others

• 一般来说，当对象中只有一个方法时，这时使用闭包是更好的选择。
• All function objects have a closure attribute that returns a tuple of cell objects if it is a closure function. Referring to the example above, we know times3 and times5 are closure functions. 所有函数都有一个 closure属性，如果这个函数是一个闭包的话，那么它返回的是一个由 cell 对象 组成的元组对象。cell 对象的cell_contents 属性就是闭包中的自由变量。

make_multiplier_of.__closure__
times3.__closure__
(<cell at 0x0000000002D155B8: int object at 0x000000001E39B6E0>,)

The cell object has the attribute cell_contents which stores the closed value.

times3.__closure__[0].cell_contents
3
 times5.__closure__[0].cell_contents
5