Function

what the function really is in python

  • Besides that function is an object, even everything in Python (Yes! Even classes) are objects.
  • Various different names can be bound to the same function object.
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def first(msg):
    print(msg)

first("Hello")

second = first
second("Hello")
  • Functions can be passed as arguments to another function.
  • a function can return another function, like the closure
  • In fact, any object which implements the special method __call__() is termed callable. So, in the most basic sense, a decorator is a callable that returns a callable.

Closure

nested function&enclosing function

参考代码

  • We can see that the nested function printer() was able to access the non-local variable msg of the enclosing function.

What the closure really is

closure is a tech by which some data in the enclosing scope gets attached to the nested function scope, that is we have a closure in Python when a nested function references a value in its enclosing scope.

当我们谈论“闭包函数”时，到底指的而是enclosing function还是nested function？其实，都不是，在下面的代码中 times3 才是”闭包函数”

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

# Multiplier of 3
times3 = make_multiplier_of(3)

#closure function have a __closure__ attribute that returns a tuple of cell objects
>>> times3.__closure__
(<cell at 0x0000000002D155B8: int object at 0x000000001E39B6E0>,)

How to create a closure

  1. 在python中创建closure，必须要满足 三个条件
  • 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.
  1. Examples of defining a Closure Function

使用闭包的场景

  1. 在什么时候，会让人想起要使用闭包呢？——2个条件
  • 为了可复用性，要用一个函数来封装某一个功能，例如列表排序
  • 在封装过程中，不得不定义另外一个“辅助函数”(nested function)

e.g. 优先级排序

  1. 如果nested function的状态要保持，即，enclosing function要用这个状态，还是把nested function定义为类，并令其实现__call__方法，如此，类实例就能被调用了。”effective python”p33-1，def sort_priority3()和class Sorter()

closure v.s. class

  • 为了替代类。众所周知，class封装了属性和方法，方法除了使用被调用时传入的形参，就只能访问class中定义的属性了。closure就是为了实现类似“类方法”的作用。
  • Closures can avoid the use of global values and provides some form of data hiding.

Q：一个class可以定义多个方法，那么在一个enclosing function中是否可以定义多个nested function(closure)？如何决定返回哪一个呢？

A:But when the number of attributes and methods get larger, better implement a class.

How does a closure work

使用闭包的客户端代码要经过两次函数调用：

  1. call the enclosing function with some parameters, so the closure can take the passing parameters and the nonlocal variable as Initialization Parameters just like which are passed when creating an object but has not been called yet
  2. call the closure to run the closure body

Generator function

参见 Generator