Non-English Resources

Add an event to this calendar.

Times are shown in UTC/GMT.

Add an event to this calendar.

PEP: 201 Lockstep Iteration b58467baacaa 2008-10-02 12:51:05 +0000 (Thu, 02 Oct 2008) Barry Warsaw Final Standards Track 13-Jul-2000 2.0 27-Jul-2000

Introduction

```    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

```    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 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.  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

```    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 `None' first 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 padded with `None'.

>>> 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.

```

Return Value

```    zip() returns a real Python list, the same way map() does.

```

Examples

```    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, zip() is
reversible:

>>> 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.

```

Reference Implementation

```    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

```

BDFL Pronouncements

```    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[2] 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
optional `pad' keyword 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 zip()

```    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 straight-forward looping and summarization:

print "Total rainfall", sum(rain)

Using 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.

```

Other Changes

```    - The xzip() function discussed above was implemented in Py2.3 in
the itertools module as 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, zip().

- The itertools module also added itertools.repeat() and
itertools.chain().  These tools can be used together to pad
sequences with None (to match the behavior of map(None, seqn)):

zip(firstseq, chain(secondseq, repeat(None)))

```

References

```    [1] http://docs.python.org/reference/compound_stmts.html#for
[2] http://www.haskell.org/onlinereport/standard-prelude.html#\$vzip

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

```

Copyright

```    This document has been placed in the public domain.

```