Python closure concepts and examples

Closure: In programming versions, a closure (also lexical closure or function closure) is a function or reference to a function together with a referencing environment-a table storing a reference to each of the non-local variables (also called free variables or upvalues) of that function.
In my understanding, closures are entities combined by functions and their referenced environments. The closure function references non-local variables, which are bound to the values of these non-local variables in the reference environment.

Let's look at several examples.

1. What is the output of the following code?

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Flist = [] For I in xrange (3 ): Def func (x ): Return x * I Flist. append (func)   For f in flist: Print f (2)

In fact, every printing is 4, because here is a closure, I forms a closure together in the reference environment of the func function, and I is gradually changed to 2, func is referenced by I, therefore, when f (2) is called, the value of I is 2, and f (2) returns 4.

2. What is the output of the following code?

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Def generate_power_func (n ): Def nth_power (x ): Return x ** n Return nth_power   If _ name _ = '_ main __': Raised_to_4 = generate_power_func (4) Del generate_power_func Print raised_to_4 (2)

The output is 16. because n is part of the reference environment in the closure, even if the life cycle of generate_power_func ends, the closure still can be called to n = 4 in raise_to_4 (2.

If you do not want to use the closure feature, you can define external variables as a container, such as dict and list, as shown in the following example:

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Def outer (): D = {'Y': 0}   Def inner (): D ['Y'] + = 1 Return d ['Y'] Return inner     Def foo (): A = [1,]   Def bar (): A [0] = a [0] + 1 Return a [0] Return bar     If _ name _ = '_ main __': Outer = outer () Print outer () Foo = foo () Print foo ()

In python3, a variable can be modified with the nonlocal keyword in the internal function to identify a variable as a non-local variable.

The sample code for this article is in: https://github.com/smilejay/python/blob/master/py2014/closure.py