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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.
This commit is contained in:
Barry Warsaw 2000-07-31 15:52:45 +00:00
parent 208ff7b554
commit 88002f143d
1 changed files with 32 additions and 20 deletions
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52
pep-0201.txt
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@ -10,14 +10,14 @@ Post-History: 27-Jul-2000
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 @@ Motivation
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 @@ Lockstep For-Loops
[(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 @@ References
[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
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