PEP 201 – Lockstep Iteration
- Lockstep Iteration
- barry at python.org (Barry Warsaw)
- Standards Track
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.
Standard for-loops in Python iterate over every element in a sequence until the sequence is exhausted . 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 fashion. In other words, in a way such that the 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. This PEP
proposes a standard way of performing such iterations by introducing a
new builtin function called
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 are non-nested iterations over two or more sequences, such that at each pass through the loop, one element from each sequence is taken to compose the target. This behavior can already be accomplished in Python through the use of the map() built- in function:
>>> a = (1, 2, 3) >>> b = (4, 5, 6) >>> for i in map(None, a, b): print i ... (1, 4) (2, 5) (3, 6) >>> map(None, a, b) [(1, 4), (2, 5), (3, 6)]
The for-loop simply iterates over this list as normal.
While the map() idiom is a common one in Python, it has several disadvantages:
- It is non-obvious to programmers without a functional programming background.
- The use of the magic
Nonefirst argument is non-obvious.
- It has arbitrary, often unintended, and inflexible semantics when
the lists are not of the same length: the shorter sequences are
>>> c = (4, 5, 6, 7) >>> map(None, a, c) [(1, 4), (2, 5), (3, 6), (None, 7)]
For these reasons, several proposals were floated in the Python 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 202).
The Proposed Solution
The proposed solution is to introduce a new built-in sequence
generator function, available in the
__builtin__ module. This
function is to be called
zip and has the following signature:
zip(seqa, [seqb, [...]])
zip() takes one or more sequences and weaves their elements
together, just as
map(None, ...) does with sequences of equal
length. The weaving stops when the shortest sequence is exhausted.
zip() returns a real Python list, the same way
Here are some examples, based on the reference implementation below:
>>> a = (1, 2, 3, 4) >>> b = (5, 6, 7, 8) >>> c = (9, 10, 11) >>> d = (12, 13) >>> zip(a, b) [(1, 5), (2, 6), (3, 7), (4, 8)] >>> zip(a, d) [(1, 12), (2, 13)] >>> zip(a, b, c, d) [(1, 5, 9, 12), (2, 6, 10, 13)]
Note that when the sequences are of the same length,
>>> a = (1, 2, 3) >>> b = (4, 5, 6) >>> x = zip(a, b) >>> y = zip(*x) # alternatively, apply(zip, x) >>> z = zip(*y) # alternatively, apply(zip, y) >>> x [(1, 4), (2, 5), (3, 6)] >>> y [(1, 2, 3), (4, 5, 6)] >>> z [(1, 4), (2, 5), (3, 6)] >>> x == z 1
It is not possible to reverse zip this way when the sequences are not all the same length.
Here is a reference implementation, in Python of the zip() built-in function. This will be replaced with a C implementation after final approval:
def zip(*args): if not args: raise TypeError('zip() expects one or more sequence arguments') ret =  i = 0 try: while 1: item =  for s in args: item.append(s[i]) ret.append(tuple(item)) i = i + 1 except IndexError: return ret
Note: the BDFL refers to Guido van Rossum, Python’s Benevolent Dictator For Life.
- The function’s name. An earlier version of this PEP included an
open issue listing 20+ proposed alternative names to
zip(). In the face of no overwhelmingly better choice, the BDFL strongly prefers
zip()due to its Haskell  heritage. See version 1.7 of this PEP for the list of alternatives.
zip()shall be a built-in function.
- Optional padding. An earlier version of this PEP proposed an
padkeyword argument, which would be used when the argument sequences were not the same length. This is similar behavior to the
map(None, ...)semantics except that the user would be able to specify pad object. This has been rejected by the BDFL in favor of always truncating to the shortest sequence, because of the KISS principle. If there’s a true need, it is easier to add later. If it is not needed, it would still be impossible to delete it in the future.
- Lazy evaluation. An earlier version of this PEP proposed that
zip()return a built-in object that performed lazy evaluation using
__getitem__()protocol. This has been strongly rejected by the BDFL in favor of returning a real Python list. If lazy evaluation is desired in the future, the BDFL suggests an
xzip()function be added.
zip()with no arguments. the BDFL strongly prefers this raise a TypeError exception.
zip()with one argument. the BDFL strongly prefers that this return a list of 1-tuples.
- Inner and outer container control. An earlier version of this PEP contains a rather lengthy discussion on a feature that some people wanted, namely the ability to control what the inner and outer container types were (they are tuples and list respectively in this version of the PEP). Given the simplified API and implementation, this elaboration is rejected. For a more detailed analysis, see version 1.7 of this PEP.
Subsequent Change to
In Python 2.4, zip() with no arguments was modified to return an empty
list rather than raising a TypeError exception. The rationale for the
original behavior was that the absence of arguments was thought to
indicate a programming error. However, that thinking did not
anticipate the use of zip() with the
* operator for unpacking
variable length argument lists. For example, the inverse of zip could
be defined as:
unzip = lambda s: zip(*s). That transformation
also defines a matrix transpose or an equivalent row/column swap for
tables defined as lists of tuples. The latter transformation is
commonly used when reading data files with records as rows and fields
as columns. For example, the code:
date, rain, high, low = zip(*csv.reader(file("weather.csv")))
rearranges columnar data so that each field is collected into individual tuples for straightforward looping and summarization:
print "Total rainfall", sum(rain)
zip(*args) is more easily coded if
zip(*) is handled
as an allowable case rather than an exception. This is especially
helpful when data is either built up from or recursed down to a null
case with no records.
Seeing this possibility, the BDFL agreed (with some misgivings) to have the behavior changed for Py2.4.
xzip()function discussed above was implemented in Py2.3 in the
itertools.izip(). This function provides lazy behavior, consuming single elements and producing a single tuple on each pass. The “just-in-time” style saves memory and runs faster than its list based counterpart,
itertoolsmodule also added
itertools.chain(). These tools can be used together to pad sequences with
None(to match the behavior of
zip(firstseq, chain(secondseq, repeat(None)))
Greg Wilson’s questionnaire on proposed syntax to some CS grad students http://www.python.org/pipermail/python-dev/2000-July/013139.html
This document has been placed in the public domain.
Last modified: 2022-01-21 11:03:51 GMT