New submission from Matt Long <[email protected]>:
The description of nesting list comprehensions in section 5.1.5 of the main
Python tutorial
(http://docs.python.org/tutorial/datastructures.html#nested-list-comprehensions)
is misleading at best and arguably incorrect. Where it says "To avoid
apprehension when nesting list comprehensions, read from right to left." This
is incorrect and conflicts directly with the comment at the bottom of PEP 202
(http://www.python.org/dev/peps/pep-0202/), which says the last index varies
fastest.
Take the following example:
matrix = [[1,2],[3,4],[5,6]]
my_list = []
for row in matrix:
for number in row
my_list.append(number)
The current documentation would suggest I do the following to achieve the same
result with list comprehensions since the for statements should be read from
right to left:
matrix = [[1,2],[3,4],[5,6]]
my_list = [number for number in row for row in matrix]
Running this code will result in an error. The correct form is:
matrix = [[1,2],[3,4],[5,6]]
[number for row in matrix for number in row]
Clearly the nesting order only matters when the inner loop depends on the outer
loop as in my example above. There is no such dependency in the documentation's
example, which is why it is does not result in an Error.
----------
assignee: docs@python
components: Documentation
messages: 148994
nosy: docs@python, mattlong
priority: normal
severity: normal
status: open
title: Incorrect nested list comprehension documentation
type: behavior
versions: Python 2.6, Python 2.7
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Python tracker <[email protected]>
<http://bugs.python.org/issue13549>
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