When Closure get external variable's value?

[Bruno Desthuilliers]

Huayang Xia a écrit :
> I'm confused. What is the definition of closure.
 >
> I'm not sure if it's correct, I get the definition from wikipedia:
> 
> "A closure typically comes about when one function is declared entirely
> within the body of another, and the inner function refers to local
> variables of the outer function. At runtime, when the outer function
> executes, a closure is formed. It consists of the inner function's code
> and references to any variables in the outer function's scope that the
> closure needs."

You skipped the first and most important sentence:
"In programming languages, a closure is a function that refers to free 
variables in its lexical context."

IOW, a closure is a function that carry it's own environment. In the 
following code, the function returned by make_adder is a closure :

def make_adder(adding):
   def adder(num):
     return num + adding
   return adder


add_three = make_adder(3)
print add_three(4)
=> 7

> I agree it is not declaration, it's definition. However it's closure
> based on the above definition. It uses free variable.

Actually, it uses a variable defined in the enclosing scope. But as long 
as it's also executed in the same enclosing scope, it's just a nested 
function.

> Or you mean it's
> a closure only when the outer function returns it and be exposed to
> external world?

Bingo.