什么是封闭功能？

Question

According to the python tutorial , functions look for variable names in the symbol tables of enclosing functions before looking for global functions:

The execution of a function introduces a new symbol table used for the local variables of the function. More precisely, all variable assignments in a function store the value in the local symbol table; whereas variable references first look in the local symbol table, then in the local symbol tables of enclosing functions, then in the global symbol table, and finally in the table of built-in names. Thus, global variables cannot be directly assigned a value within a function (unless named in a global statement), although they may be referenced.

What exactly does "enclosing function" mean, and when is it used?

I see the following code prints 10 when called

def parent_function():
    y=10
    def child_function():
        print y
    child_function()

However, calling child_function() alone produces an error. Are enclosing functions used frequently?

Answer 1

The concept of an enclosing function is key in understanding the idea of closures. Because python does not have fully featured lambdas (they only allow expressions and not statements), having nested functions to pass on to other functions is a common use case:

def receiving_function(f):
    f()

def parent_function():
    y = 10

    def child_function():
        print(y)

    receiving_function(child_function)

will print 10 as before. This is a common instance of a closure, where the enclosing function "hands off" it's variables to the enclosed function. In the example above, this function is passed off to the receiving_function along with the non-local variable y .

Answer 2

The reason you cannot call child_function by itself, is that it is defined inside the parent_function . All python variable declarations use block scope, and declaring a function is no different.

Consider the following example.

>>> def parent_function():
...    y=10
...    def child_function():
...        print y
...    child_function()

>>> print y
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
NameError: name 'y' is not defined

The variable y is not accessible outside the parent_function . Why would you expect that child_function would be any different that y ?

Answer 3

Enclosing functions are functions nested in functions. We usually use it to get better encapsulation. That is enclosing functions are not visible for other functions.