85 lines
2.2 KiB
Plaintext
85 lines
2.2 KiB
Plaintext
PEP: 202
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Title: List Comprehensions
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Author: Barry Warsaw <barry@python.org>
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Status: Final
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Type: Standards Track
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Content-Type: text/x-rst
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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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============
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This PEP describes a proposed syntactical extension to Python, list
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comprehensions.
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The Proposed Solution
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=====================
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It is proposed to allow conditional construction of list literals using for and
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if clauses. They would nest in the same way for loops and if statements nest
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now.
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Rationale
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=========
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List comprehensions provide a more concise way to create lists in situations
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where ``map()`` and ``filter()`` and/or nested loops would currently be used.
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Examples
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========
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::
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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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========================
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List comprehensions become part of the Python language with release 2.0,
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documented in [1]_.
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BDFL Pronouncements
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===================
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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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==========
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.. [1] http://docs.python.org/reference/expressions.html#list-displays
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