86 lines
2.2 KiB
Plaintext
86 lines
2.2 KiB
Plaintext
PEP: 202
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Title: List Comprehensions
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Version: $Revision$
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Last-Modified: $Date$
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Author: barry@python.org (Barry Warsaw)
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Status: Final
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Type: Standards Track
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Created: 13-Jul-2000
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Python-Version: 2.0
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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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>>> 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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List comprehensions become part of the Python language with
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release 2.0, documented in [1].
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BDFL Pronouncements
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- The syntax proposed above is the Right One.
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- The form [x, y for ...] is disallowed; one is required to write
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[(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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References
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[1] http://docs.python.org/reference/expressions.html#list-displays
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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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