When Closure get external variable's value?

[Huayang Xia]

My understanding was:

Closure is a nested function first. If it refers free variable, then it
is closure. If it doesn't refer free variable, it doesn't have to be
nested. That is probably the reason, the free variable is emphasized.
Normally it makes sense to return a closure, but not a non-closure
nested function.

I don't understand why while a nested function perfectly matches the
definition of closure, it is not closure simply because it is not used
by external world.

BTW, IMHO, The meaning of  the "most important sentence" was contained
in the original quote. That was the reason I skipped it.

On Dec 19, 4:14 pm, Bruno Desthuilliers
<bdesth.quelquech... at free.quelquepart.fr> wrote:
> 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.