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Showing content from https://www.python.org/dev/peps/pep-0289/../pep-0202/ below:

PEP 202 – List Comprehensions

Author:

Barry Warsaw <barry at python.org>

Status:

Final

Type:

Standards Track

Created:

13-Jul-2000

Python-Version:

2.0

Post-History:

• Introduction
• The Proposed Solution
• Rationale
• Examples
• Reference Implementation
• BDFL Pronouncements
• References

This PEP describes a proposed syntactical extension to Python, list comprehensions.

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.

List comprehensions provide a more concise way to create lists in situations where map() and filter() and/or nested loops would currently be used.

>>> 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')]
>>> print [i for i in zip(nums, fruit) if i[0]%2==0]
[(2, 'Peaches'), (4, 'Bananas')]

List comprehensions become part of the Python language with release 2.0, documented in [1].

• The syntax proposed above is the Right One.
• The form [x, y for ...] is disallowed; one is required to write [(x, y) for ...].
• The form [... for x... for y...] nests, with the last index varying fastest, just like nested for loops.

Source: https://github.com/python/peps/blob/main/peps/pep-0202.rst

Last modified: 2025-02-01 08:55:40 GMT

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