[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[Rose Ames]

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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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[R. David Murray]

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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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[Roundup Robot]

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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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[R. David Murray]

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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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[Karl Richter]

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.

--
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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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[R. David Murray]

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 "", line 1, in 
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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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[Raymond Hettinger]

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.

--
assignee: docs@python -> rhettinger
nosy: +rhettinger
priority: normal -> low
type: enhancement -> 
versions:  -Python 3.4, Python 3.5

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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[R. David Murray]

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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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[Raymond Hettinger]

Changes by Raymond Hettinger :


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assignee: rhettinger -> r.david.murray

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[issue21739] Add hint about expression in list comprehensions (https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions)

[Ezio Melotti]

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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