[Tutor] Generator next()

[spir]

On 12/29/2013 12:33 PM, Steven D'Aprano wrote:
> def adder_factory(n):
>      def plus(arg):
>          return arg + n
>      return plus  # returns the function itself
>
>
> If you call adder_factory(), it returns a function:
>
> py> adder_factory(10)
> <function adder_factory.<locals>.plus at 0xb7af6f5c>
>
> What good is this? Watch carefully:
>
>
> py> add_two = adder_factory(2)
> py> add_three = adder_factory(3)
> py> add_two(100)
> 102
> py> add_three(100)
> 103
>
>
> The factory lets you programmatically create functions on the fly.
> add_two() is a function which adds two to whatever argument it gets.
> add_three() is a function which adds three to whatever argument it gets.
> We can create an "add_whatever" function without knowing in advance what
> "whatever" is going to be:
>
> py> from random import randint
> py> add_whatever = adder_factory(randint(1, 10))
> py> add_whatever(200)
> 205
> py> add_whatever(300)
> 305
>
>
> So now you see the main reason for nested functions in Python: they let
> use create a function where you don't quite know exactly what it will do
> until runtime. You know the general form of what it will do, but the
> precise details aren't specified until runtime.

little complement:
Used that way, a function factory is the programming equivalent of parametric 
functions, or function families, in maths:

	add : v --> v + p

Where does p come from? it's a parameter (in the sense of maths), or an "unbound 
varaible"; while 'v' is the actual (bound) variable of the function. Add 
actually defines a family of parametric functions, each with its own 'p' 
parameter, adding p to their input variable (they each have only one). The 
programming equivalent of this is what Steven demonstrated above. However, there 
is much confusion due to (funny) programming terminology:
* a math variable is usually called parameter
* a math parameter is usually called "upvalue" (not kidding!)
* a parametric function, or function family (with undefinite param) is called 
function factory (this bit is actually semantically correct)
* a member of function family (with definite param) is called (function) closure
(every parametric expression is actually a closure, eg "index = index + offset", 
if offset is not locally defined)

Denis