138 lines
3.9 KiB
Plaintext
138 lines
3.9 KiB
Plaintext
PEP: 202
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Title: List Comprehensions
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Version: $Revision$
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Author: tpeters@beopen.com (Tim Peters)
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Status: Draft
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Type: Standards Track
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Python-Version: 2.0
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Created: 13-Jul-2000
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Post-History:
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Introduction
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This PEP describes a proposed syntactical extension to Python,
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list comprehensions.
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The Proposed Solution
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It is proposed to allow conditional construction of list literals
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using for and if clauses. They would nest in the same way for
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loops and if statements nest now.
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Rationale
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List comprehensions provide a more concise way to create lists in
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situations where map() and filter() and/or nested loops would
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currently be used.
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Examples
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>>> print [i for i in range(10)]
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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>>> print [i for i in range(20) if i%2 == 0]
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[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
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>>> nums = [1,2,3,4]
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>>> fruit = ["Apples", "Peaches", "Pears", "Bananas"]
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>>> print [(i,f) for i in nums for f in fruit]
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[(1, 'Apples'), (1, 'Peaches'), (1, 'Pears'), (1, 'Bananas'),
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(2, 'Apples'), (2, 'Peaches'), (2, 'Pears'), (2, 'Bananas'),
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(3, 'Apples'), (3, 'Peaches'), (3, 'Pears'), (3, 'Bananas'),
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(4, 'Apples'), (4, 'Peaches'), (4, 'Pears'), (4, 'Bananas')]
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>>> print [(i,f) for i in nums for f in fruit if f[0] == "P"]
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[(1, 'Peaches'), (1, 'Pears'),
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(2, 'Peaches'), (2, 'Pears'),
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(3, 'Peaches'), (3, 'Pears'),
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(4, 'Peaches'), (4, 'Pears')]
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>>> print [(i,f) for i in nums for f in fruit if f[0] == "P" if i%2 == 1]
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[(1, 'Peaches'), (1, 'Pears'), (3, 'Peaches'), (3, 'Pears')]
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>>> def zip(*args):
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... return apply(map, (None,)+args)
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...
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>>> print [i for i in zip(nums,fruit) if i[0]%2==0]
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[(2, 'Peaches'), (4, 'Bananas')]
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Reference Implementation
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SourceForge contains a patch that adds list comprehensions to Python[1].
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BDFL Pronouncements
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Note: the BDFL refers to Guido van Rossum, Python's Benevolent
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Dictator For Life.
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- The syntax proposed above is the Right One.
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- The form [x, y for ...] should be disallowed; one should be
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required to write [(x, y) for ...].
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- The form [... for x... for y...] nests, with the last index
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varying fastest, just like nested for loops.
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Open Issues
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Syntax
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Several people proposed connecting or separating syntax
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between the various clauses, for example, requiring a
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semicolon between them to set them apart:
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[i,f; for i in nums; for f in fruit; if f[0]=="P"; if i%2==1]
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To minimize strain on the Python parser, Guido has suggested
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requiring parentheses around the initial tuple:
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[(i,f) for i in nums for f in fruit if f[0]=="P" if i%2==1]
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Semantics
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The semantics of multiple for clauses is not obvious to many
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people. Currently, it nests, so that
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[i,f for i in nums for f in fruit]
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is functionally equivalent to
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tmp = []
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for i in nums:
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for f in fruit:
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tmp.append((i,f))
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Other people would read it as if it executed
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map(None, nums, fruit)
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It's not clear that this is necessary. The newly proposed
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zip() builtin[2] takes care of that case.
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Stability of the Implementation
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The current reference implementation is simply an adaptation
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of Greg Ewing's original demonstration of the concept. Other
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than tracking changes to the source code to keep it a valid
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patch, reindenting the code and switching to function
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prototypes, nothing has been done to it. This obviously
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raises some questions about how stable the code is. It has
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not had a lot of exercise, though the patch does include a few
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test cases.
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References
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[1] https://sourceforge.net/patch/?func=detailpatch&patch_id=100654&group_id=5470
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[2] Lockstep Iteration, pep-0201.txt
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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