[Python-checkins] peps: Integrated many suggestions from c.l.py and others. Nothing (original) (raw)

georg.brandl python-checkins at python.org
Wed Mar 23 21:24:02 CET 2011

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http://hg.python.org/peps/rev/7775a6117a48 changeset: 68:7775a6117a48 user: Barry Warsaw <barry at python.org> date: Mon Jul 31 15:52:45 2000 +0000 summary: Integrated many suggestions from c.l.py and others. Nothing substantial w.r.t. the proposed solution, just clarifications, additional references, and an explanation of why most syntax changes don't work.

files: pep-0201.txt | 52 ++++++++++++++++++++++++--------------- 1 files changed, 32 insertions(+), 20 deletions(-)

diff --git a/pep-0201.txt b/pep-0201.txt --- a/pep-0201.txt +++ b/pep-0201.txt @@ -10,14 +10,14 @@

Introduction

This PEP describes the `lockstep iteration' proposal for Python
2.0. This PEP tracks the status and ownership of this feature,
slated for introduction in Python 2.0. It contains a description
of the feature and outlines changes necessary to support the
feature. This PEP summarizes discussions held in mailing list
forums, and provides URLs for further information, where
appropriate. The CVS revision history of this file contains the
definitive historical record.

This PEP describes the `lockstep iteration' proposal. This PEP
tracks the status and ownership of this feature, slated for
introduction in Python 2.0. It contains a description of the
feature and outlines changes necessary to support the feature.
This PEP summarizes discussions held in mailing list forums, and
provides URLs for further information, where appropriate. The CVS
revision history of this file contains the definitive historical
record.

Motivation @@ -25,13 +25,17 @@ Standard for-loops in Python iterate over every element in a sequence until the sequence is exhausted[1]. However, for-loops iterate over only a single sequence, and it is often desirable to

loop over more than one sequence, in a lock-step, "Chinese Menu"
type of way.

loop over more than one sequence in a lock-step fashion. In other
words, in a way such that nthe i-th iteration through the loop
returns an object containing the i-th element from each sequence.

The common idioms used to accomplish this are unintuitive and
inflexible. This PEP proposes a standard way of performing such
iterations by introducing a new builtin function called `zip'.

The common idioms used to accomplish this are unintuitive. This
PEP proposes a standard way of performing such iterations by
introducing a new builtin function called `zip'.
While the primary motivation for zip() comes from lock-step
iteration, by implementing zip() as a built-in function, it has
additional utility in contexts other than for-loops.

Lockstep For-Loops

@@ -70,13 +74,20 @@ [(1, 4), (2, 5), (3, 6), (None, 7)]

 For these reasons, several proposals were floated in the Python

2.0 beta time frame for providing a better spelling of lockstep
for-loops. The initial proposals centered around syntactic
changes to the for statement, but conflicts and problems with the
syntax were unresolvable, especially when lockstep for-loops were
combined with another proposed feature called `list
comprehensions' (see pep-0202.txt).

2.0 beta time frame for syntactic support of lockstep for-loops.
Here are two suggestions:
for x in seq1, y in seq2:
```
   # stuff
```
for x, y in seq1, seq2:
```
   # stuff
```
Neither of these forms would work, since they both already mean
something in Python and changing the meanings would break existing
code. All other suggestions for new syntax suffered the same
problem, or were in conflict with other another proposed feature
called `list comprehensions' (see pep-0202.txt).

The Proposed Solution

@@ -206,7 +217,8 @@ [1] http://www.python.org/doc/current/ref/for.html [2] http://www.haskell.org/onlinereport/standard-prelude.html#$vzip

TBD: URL to python-dev archives

Greg Wilson's questionaire on proposed syntax to some CS grad students
http://www.python.org/pipermail/python-dev/2000-July/013139.html

-- Repository URL: http://hg.python.org/peps

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