121 lines
3.4 KiB
Plaintext
121 lines
3.4 KiB
Plaintext
PEP: 202
|
||
Title: List Comprehensions
|
||
Version: $Revision$
|
||
Owner: tpeters@beopen.com (Tim Peters)
|
||
Python-Version: 2.0
|
||
Status: Incomplete
|
||
|
||
|
||
Introduction
|
||
|
||
This PEP describes a proposed syntactical extension to Python, list
|
||
comprehensions.
|
||
|
||
|
||
|
||
The Proposed Solution
|
||
|
||
It is proposed to allow conditional construction of list literals using
|
||
for and if clauses. They would nest in the same way for loops and if
|
||
statements nest now.
|
||
|
||
|
||
|
||
Rationale
|
||
|
||
List comprehensions provide a more concise way to create lists in
|
||
situations where map() and filter() and/or nested loops would currently
|
||
be used.
|
||
|
||
|
||
Examples
|
||
|
||
>>> print [i for i in range(10)]
|
||
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
|
||
|
||
>>> print [i for i in range(20) if i%2 == 0]
|
||
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
|
||
|
||
>>> nums = [1,2,3,4]
|
||
>>> fruit = ["Apples", "Peaches", "Pears", "Bananas"]
|
||
>>> print [i,f for i in nums for f in fruit]
|
||
[(1, 'Apples'), (1, 'Peaches'), (1, 'Pears'), (1, 'Bananas'),
|
||
(2, 'Apples'), (2, 'Peaches'), (2, 'Pears'), (2, 'Bananas'),
|
||
(3, 'Apples'), (3, 'Peaches'), (3, 'Pears'), (3, 'Bananas'),
|
||
(4, 'Apples'), (4, 'Peaches'), (4, 'Pears'), (4, 'Bananas')]
|
||
>>> print [i,f for i in nums for f in fruit if f[0] == "P"]
|
||
[(1, 'Peaches'), (1, 'Pears'),
|
||
(2, 'Peaches'), (2, 'Pears'),
|
||
(3, 'Peaches'), (3, 'Pears'),
|
||
(4, 'Peaches'), (4, 'Pears')]
|
||
>>> print [i,f for i in nums for f in fruit if f[0] == "P" if i%2 == 1]
|
||
[(1, 'Peaches'), (1, 'Pears'), (3, 'Peaches'), (3, 'Pears')]
|
||
>>> def zip(*args):
|
||
... return apply(map, (None,)+args)
|
||
...
|
||
>>> print [i for i in zip(nums,fruit) if i[0]%2==0]
|
||
[(2, 'Peaches'), (4, 'Bananas')]
|
||
|
||
|
||
Reference Implementation
|
||
|
||
Please refer to
|
||
|
||
https://sourceforge.net/patch/?func=detailpatch&patch_id=100654&group_id=5470
|
||
|
||
for a patch that adds list comprehensions to Python.
|
||
|
||
|
||
|
||
Open Issues
|
||
|
||
Syntax
|
||
|
||
Several people proposed connecting or separating syntax between the
|
||
various clauses, for example, requiring a semicolon between them to
|
||
set them apart:
|
||
|
||
[i,f; for i in nums; for f in fruit; if f[0]=="P"; if i%2==1]
|
||
|
||
To minimize strain on the Python parser, Guido has suggested
|
||
requiring parentheses around the initial tuple:
|
||
|
||
[(i,f) for i in nums for f in fruit if f[0]=="P" if i%2==1]
|
||
|
||
Semantics
|
||
|
||
The semantics of multiple for clauses is not obvious to many
|
||
people. Currently, it nests, so that
|
||
|
||
[i,f for i in nums for f in fruit]
|
||
|
||
is functionally equivalent to
|
||
|
||
tmp = []
|
||
for i in nums:
|
||
for f in fruit:
|
||
tmp.append((i,f))
|
||
|
||
Other people would read it as if it executed
|
||
|
||
map(None, nums, fruit)
|
||
|
||
It's not clear that this is necessary. The newly proposed zip()
|
||
builtin takes care of that case.
|
||
|
||
Stability of the Implementation
|
||
|
||
The current reference implementation is simply an adaptation of Greg
|
||
Ewing's original demonstration of the concept. Other than tracking
|
||
changes to the source code to keep it a valid patch, reindenting the
|
||
code and switching to function prototypes, nothing has been done to
|
||
it. This obviously raises some questions about how stable the code
|
||
is. It has not had a lot of exercise, though the patch does include
|
||
a few test cases.
|
||
|
||
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
End:
|