[Python-bugs-list] [ python-Bugs-510384 ] Separate Scope for List Comprehensions

[noreply@sourceforge.net]

Bugs item #510384, was opened at 2002-01-29 14:13
You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=105470&aid=510384&group_id=5470

Category: Python Interpreter Core
Group: Feature Request
Status: Open
Resolution: None
Priority: 5
Submitted By: Raymond Hettinger (rhettinger)
Assigned to: Nobody/Anonymous (nobody)
Summary: Separate Scope for List Comprehensions

Initial Comment:
The variables in a list comprehension should not be in 
the enclosing scope.

i=10
temp = [str(i) for i in range(5)]
print i

Should print 10 instead of 4.

Implement the above as:

i=10
def _listcomp():
    for i in range(5):
        yield str(i)
temp = list(_listcomp())
print i

Note, I timed the difference between the existing and 
proposals implementations and found only a 4% decrease 
in speed.

In case someone is already relying on the list 
comprehension variables being in the local scope, a 
deprecation warning or from __future__ in warranted.

Also note, this implementation leaves open the 
possibility of creating generator comprehensions so 
that temp=[yield str(i) for i in range(5)] creates the 
same code as above except that the final 'list' 
coercion is eliminated:  temp=_listcomp



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Comment By: Nobody/Anonymous (nobody)
Date: 2002-01-30 11:34

Message:
Logged In: NO 

Sorry, the bug that was fixed was about global scope.

This was discussed when list comprehensions were designed, 
and it was a conscious decision that won't be changed.  
It's the same as for regular for loops.

--Guido (still can't log in)


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Comment By: Nobody/Anonymous (nobody)
Date: 2002-01-30 11:33

Message:
Logged In: NO 

Oops, I misread your complaint. The bug that was fixed was 
that sometimes the variables end up in the *global* scope.

This was discussed when it was designed and it was decided 
to do it this way.  It's similar to what a regular for loop 
does:

  for i in range(10): pass
  print i

prints 10.

I'll reject this when I can log in to SF again.

--Guido (again)


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Comment By: Raymond Hettinger (rhettinger)
Date: 2002-01-30 08:20

Message:
Logged In: YES 
user_id=80475

I verified that this has NOT been fixed in Python 2.2:


Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit 
(Intel)] on win32
Type "copyright", "credits" or "license" for more 
information.
IDLE 0.8 -- press F1 for help
>>> i=10
>>> temp = [str(i) for i in range(5)]
>>> print i
4

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Comment By: Nobody/Anonymous (nobody)
Date: 2002-01-30 07:52

Message:
Logged In: NO 

This is Guido; I can't log in here on the road.

I believe this has been fixed in Python 2.2, and possibly 
in 2.1.2.  Can you check that?

----------------------------------------------------------------------

You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=105470&aid=510384&group_id=5470