From: Paul Prescod <paul@...>
Date: 2003-03-21T14:28:05+09:00
Subject: Re: Strong advantages over Python

Joel VanderWerf wrote:
 > ...
> icon = Icon.new(...)
> 
> SliderControl.new("Horizontal position") do |new_x|
>   icon.move_to(new_x, icon.y)
> end
> 
> Could this be made so elegant in Python?

It's almost identical in Python.


icon = Icon(...)

slider = SliderControl("Horizontal position",
     lambda new_x: icon.move_to(new_x, icon.y))

==

The only limitation of Python's closures is _overwriting_ variables 
inherited from an intermediate scope. Note that there are several 
unusual things that need to happen to run into problems:

1. You need to define a function in a function.

someglobal = someClass()
def a():
     def b():
         ....
     return b

But that's not enough. This code works.

2. The inner function must do an assignment to something outside of its 
scope.

someglobal = someClass()
def a():
     def b():
         someglobal.x = 5
     return b

But that's not enough. This code works.

3. The assignment must be to something in an intermediate scope.

def a():
     someinter = someClass()
     def b():
         someinter.x = 5
     return b

Not enough.

4. The intermediate assignment must be to a variable, not a property of 
an object or a method call. Even though Python is an object oriented 
language, so most non-local assignments are to object properties.

def a():
     someinter = 0
     def b():
         someinter = 5
     return b

Now you'll run into the problem. And the workaround adds roughly 10 
characters to the program.

  Paul Prescod