what is a closure?

[Fredrik Lundh]

Nick Trout wrote:
> Sorry, this is probably not the place to ask this but you lot are pretty
> friendly :-)

So you don't want me to flame you for not knowing about
the free on-line dictionary of computing (foldoc)? ;-)

    http://www.foldoc.org/

> I have seen the term "closure" used... what is it exactly? Is it just a
> class member function?

    http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?query=closure

    a closure is a data structure that holds an expression
    and an environment of variable bindings in which that
    expression is to be evaluated. The variables may be
    local or global

In Python, you have to use explicity syntax to move variables
from an outer scope (environment) to the inner scope.

In the following example, the "a=a" default argument binding
binds add's local name "a" to the value of the "a" argument
from make_adder.

    def make_adder(a):
        def add(b, a=a):
            return a + b
        return add

    >>> m = make_adder(10)
    >>> m(20)
    30

In a language with full lexical scoping, you could leave it to the
compiler to figure this out.

But in today's Python, it's probably better to use a class instead.

</F>

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