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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Scope of variable inside list comprehensions?
Date: Mon, 05 Dec 2011 19:10:08 +0100
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:16675

Roy Smith wrote:

> Consider the following django snippet.  Song(id) raises DoesNotExist if
> the id is unknown.
> 
>     try:
>         songs = [Song(id) for id in song_ids]
>     except Song.DoesNotExist:
>         print "unknown song id (%d)" % id
> 
> Is id guaranteed to be in scope in the print statement?  I found one
> thread
> (http://mail.python.org/pipermail/python-bugs-list/2006-April/033235.html)
> which says yes, but hints that it might not always be in the future.  Now
> that we're in the future, is that still true?  And for Python 3 also?
> 
> The current docs,
> http://docs.python.org/tutorial/datastructures.html#list-comprehensions,
> are mute on this point.

If you are using a generator expression id will already be out of scope in 
Python 2. In Python 3 list comprehensions have been changed to work the same 
way:

$ cat song.py
class DoesNotExist(Exception):
    pass

class Song:
    def __init__(self, id):
        if id == 2:
            raise DoesNotExist

ids = [1, 2]
try:
    songs = [Song(i) for i in ids]
except DoesNotExist as e:
    print("song #%d does not exist" % i)
$ python song.py
song #2 does not exist
$ python3 song.py
Traceback (most recent call last):
  File "song.py", line 11, in <module>
    songs = [Song(i) for i in ids]
  File "song.py", line 11, in <listcomp>
    songs = [Song(i) for i in ids]
  File "song.py", line 7, in __init__
    raise DoesNotExist
__main__.DoesNotExist

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "song.py", line 13, in <module>
    print("song #%d does not exist" % i)
NameError: name 'i' is not defined
$


$ cat song_gen.py
class DoesNotExist(Exception):
    pass

class Song:
    def __init__(self, id):
        if id == 2:
            raise DoesNotExist

ids = [1, 2]
try:
    songs = list(Song(i) for i in ids)
except DoesNotExist as e:
    print("song #%d does not exist" % i)
$ python  song_gen.py
Traceback (most recent call last):
  File "song_gen.py", line 13, in <module>
    print("song #%d does not exist" % i)
NameError: name 'i' is not defined

So you'd rather store the id in the exception.