Confessions of a Python fanboy

Terry Reedy tjreedy at
Fri Jul 31 18:24:28 CEST 2009

Masklinn wrote:

> #each is simply a method that takes a function (called blocks in ruby). 
> One could call it a higher-order method I guess.
> It's an implementation of the concept of internal iteration: instead of 
> collections yielding iterator objects, and programmers using those 
> through specially-built iteration constructs (e.g. `for…in`), 
> collections control iteration over themselves (the iteration is 
> performed "inside" the collection, thus the "internal" part) and the 
> programmer provides the operations to perform at each iterative step 
> through (usually) a function.

Python's iterator protocol was developed in part to avoid the 
(inside-out) callback style of programming. Writing virtual collections 
as generator functions instead of iterator or iterable classes saves a 
lot of boilerplate code. The itertools modules shows how nicely 
iterators can be composed in a way that is much more awkward with callbacks.

> In Python (assuming we had anonymous defs and an each method on lists), 
> the following loop:
>     for item in some_list:
>         do_something(item)
>         do_something_else(item)
>     some_list.each((def (item):
>         do_something(item)
>         do_something_else(item)
>     ))

And how does Ruby do the equivalent of

def double(it):
   for i in it:
     yield 2*i

for i,j in zip(double(some_list), some_gen_func(args)):

Terry Jan Reedy

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