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Alternative iterator syntax

Marcin 'Qrczak' Kowalczyk qrczak at knm.org.pl
Wed Feb 21 16:46:33 EST 2001 
Wed, 21 Feb 2001 03:32:03 -0800 (PST), Huaiyu Zhu <hzhu at users.sourceforge.net> pisze:

> Following suggestion by Jeff Petkau <jpet at eskimo.com>,
> (http://mail.python.org/pipermail/python-list/2001-February/029944.html),
> here is a different proposal for iterators.

I dislike it for the following reasons:

* It requires "magic attributes", i.e. implementing __getattr__ in
  each user class that wants to be a sequence with the same interface
  as list. Python already has a way used for things like obtaining
  iterators: method calls. Use that. Using d.keys to mean a flavor
  of d.keys() is weird.

* It is too complex. There are many special methods, and syntactic
  sugar like for loops expands to one of many forms depending on
  what operations are provided. Python succeeded being quite simple.
  Please don't ruin that.

* The __arity__ thing does not fit dynamically typed Python at all.
  When you want some static typing, go for a more general type system
  (which would be optional in the context of Python) instead of
  adding a kludge like this.


What do I propose instead? Choose one:

1. Design a simple iterator protocol and add methods like
   xitems(), xkeys() and xvalues() which produce such iterators.
   I mean simple protocol, used only for 'for' loops! A single
   method for __next__ suffices (it can be __call__).

2. Design a lazy list framework. Let items(), keys(), values(),
   range(), readlines() etc. return lazy lists. Laziness is not as easy
   in imperative languages as in functional ones. The framework should
   take care to keep the current behavior of
       k = d.keys()
       d[spam] = eggs
       use(k)
   It can be done by putting weak references (avoiding reference loops)
   to iterators obtained from a mutable object inside the object, to
   notify iterators when the object is about to mutate, so they suck
   all elements immediately before the mutation. Perhaps design a more
   general mutability notification framework before that.

-- 
 __("<  Marcin Kowalczyk * qrczak at knm.org.pl http://qrczak.ids.net.pl/
 \__/
  ^^                      SYGNATURA ZASTĘPCZA
QRCZAK
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