[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)
New submission from Karl Richter:
It would be useful to have a short statement in the docs (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions) that the expression in a list comprehension isn't put into a block, but evaluated against the same block where it is located because intuitively one might think that variables can be overridden in the statement.
This applies to both 2.7 and 3.4.1.
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assignee: docs@python
components: Documentation
messages: 220374
nosy: docs@python, krichter
priority: normal
severity: normal
status: open
title: Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)
type: enhancement
versions: Python 2.7
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Python tracker
R. David Murray added the comment: In 3.x a list comprehension (like a generator expression in 2.x) *is* a separate block:
[x for x in range(3)] [0, 1, 2] x Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'x' is not defined
I note that this is not in fact mentioned in the python3 version of the tutorial; there it still implies that it is completely equivalent to the unrolled if statement, which would imply it *was* in the same scope.
IMO the 2.7 tutorial is complete in the sense that the "is equivalent to" example is clearly in the same scope, while the python3 tutorial is missing a note that the comprehension is its own scope. Given the fact that it is *different* between the two, I wonder if it would be appropriate to add a note (footnote?) to that effect to the 2.7 tutorial. There is such a footnote in the corresponding part of the language reference.
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nosy: +r.david.murray
versions: +Python 3.4, Python 3.5
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Python tracker
Raymond Hettinger added the comment:
I wouldn't like to use the tutorial to highlight or draw attention a feature/bug that is going away.
I recommend leaving the tutorial as-is. For a decade, it has worked well for introducing people to list comprehensions. More complete implementation specs belong in the Language Reference or in the List Comp PEP.
The tutorial is mainly about showing the normal and correct usage of tools rather than emphasizing oddities that don't matter to most of the people, most of the time.
If you feel strongly compelled to "just add something", then consider a FAQ entry.
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assignee: docs@python -> rhettinger
nosy: +rhettinger
priority: normal -> low
type: enhancement ->
versions: -Python 3.4, Python 3.5
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Python tracker
R. David Murray added the comment:
OK, I have no objection to leaving the 2.7 tutorial alone. It seems to me that the 3.x tutorial should be fixed, though, because it currently says the unrolled loop is equivalent, but it isn't. The fact that this applies to all other comprehensions in python3 I think adds weight to the idea of including the information in the tutorial somehow.
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versions: +Python 3.4, Python 3.5 -Python 2.7
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Python tracker
Changes by Raymond Hettinger
Ezio Melotti added the comment:
If we don't want to go into the details of why it's not equivalent, using "roughly equivalent" might be enough.
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keywords: +easy
nosy: +ezio.melotti
stage: -> needs patch
type: -> enhancement
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Python tracker
Rose Ames added the comment:
Fwiw, I've seen a beginner be confused by this. Patch attached.
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keywords: +patch
nosy: +superluser
Added file: http://bugs.python.org/file36593/reword.patch
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Python tracker
R. David Murray added the comment:
Thanks!
While this patch does sort-of go into a detail, it seems to me like it does it in a tutorial-appropriate fashion. I'm +1 on applying this.
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stage: needs patch -> commit review
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Python tracker
Roundup Robot added the comment:
New changeset 84895d037258 by R David Murray in branch '3.4':
#21739: mention subtle difference between loops and listcomps in tutorial.
https://hg.python.org/cpython/rev/84895d037258
New changeset 8279017436a2 by R David Murray in branch 'default':
Merge #21739: mention subtle difference between loops and listcomps in tutorial.
https://hg.python.org/cpython/rev/8279017436a2
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nosy: +python-dev
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Python tracker
R. David Murray added the comment:
The only demure I got on this was from Ezio, who objected that the concept of 'scope' hadn't been introduced yet in the tutorial. So I reworded it to avoid the word 'scope'.
Thanks, Rose.
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resolution: -> fixed
stage: commit review -> resolved
status: open -> closed
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Python tracker
participants (6)
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Ezio Melotti -
Karl Richter
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R. David Murray
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Raymond Hettinger
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Rose Ames
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Roundup Robot