Re: List comprehension vs filter()

Path	csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!news.albasani.net!weretis.net!feeder1.news.weretis.net!news.solani.org!.POSTED!not-for-mail
From	Peter Otten <__peter__@web.de>
Newsgroups	comp.lang.python
Subject	Re: List comprehension vs filter()
Followup-To	comp.lang.python
Date	Wed, 20 Apr 2011 12:41:18 +0200
Organization	None
Lines	48
Message-ID	<iomd86$f4r$1@solani.org> (permalink)
References	<mailman.603.1303269025.9059.python-list@python.org>
Mime-Version	1.0
Content-Type	text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding	7Bit
X-Trace	solani.org 1303296070 15515 eJwNysERACEIA8CWUJKg5Rwc9l+CfneWrqEKiAIPz5qVI21mmJrfo6QtlnslXu1tPzyI3iXYvBOMEGY= (20 Apr 2011 10:41:10 GMT)
X-Complaints-To	abuse@news.solani.org
NNTP-Posting-Date	Wed, 20 Apr 2011 10:41:10 +0000 (UTC)
X-User-ID	eJwNxckBwCAIBMCWUFiOclyU/ktI5jNQX95hDjcMxppT0v+bnrGPKfQGoUFeiaYUreJs87wdMgW+59j5Jld+XroVpQ==
Cancel-Lock	sha1:uZ4H1A5POevZnCW66CJZuNhpwQY=
X-NNTP-Posting-Host	eJwNy8ERACEIBMGUUFnUcG6BzT+E8zOPrhqsGJHbA+EQVPI0XeZuvshpJnjq25ExujSBE0ODswo0EUDfWYssRxfvjtMPqXf8JjIbgQ==
Xref	x330-a1.tempe.blueboxinc.net comp.lang.python:3683

Followups directed to: comp.lang.python

Show key headers only | View raw

Chris Angelico wrote:

> Context: Embedded Python interpreter, version 2.6.6
> 
> I have a list of dictionaries, where each dictionary has a "type"
> element which is a string. I want to reduce the list to just the
> dictionaries which have the same "type" as the first one.
> 
> lst=[{"type":"calc",...},{"type":"fixed",...},{"type":"calc",...},...]
> 
> I'm seeing a weird difference between two otherwise-equivalent-looking
> ways of doing the job.
> 
> type=lst[0]["type"].lower()
> 
> lst=filter(lambda x: x["type"].lower()==type,lst) # Restrict to that one
> type
> 
> lst=[i for i in lst if i["type"].lower()==type] # Restrict to that one
> type
> 
> If I use the filter() method, the resulting list is completely empty.
> If I use the list comprehension, it works perfectly. Oddly, either
> version works in the stand-alone interpreter.
> 
> I have no idea where to start looking for the problem. Hints, please!
> 
> Chris Angelico

The assignment writes to the local namespace, the lambda function reads from 
the global namespace; this will only work as expected if the two namespaces 
are the same:

>>> exec """type = 42; print filter(lambda x: x == type, [42])""" in {}, {}
[]
>>> ns = {}
>>> exec """type = 42; print filter(lambda x: x == type, [42])""" in ns
[42]

The list comprehension doesn't introduce another namespace, at least in 2.x:

$ python2.7 -c 'exec("type = 42; print([x for x in [42] if x == type])", {}, 
{})'
[42]
$ python3.2 -c 'exec("type = 42; print([x for x in [42] if x == type])", {}, 
{})'
[]

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

List comprehension vs filter() Chris Angelico <rosuav@gmail.com> - 2011-04-20 13:10 +1000
  Re: List comprehension vs filter() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-20 10:16 +0000
    Re: List comprehension vs filter() Chris Angelico <rosuav@gmail.com> - 2011-04-20 21:25 +1000
  Re: List comprehension vs filter() Peter Otten <__peter__@web.de> - 2011-04-20 12:41 +0200
    Re: List comprehension vs filter() Ian Kelly <ian.g.kelly@gmail.com> - 2011-04-20 08:44 -0600
    Re: List comprehension vs filter() Chris Angelico <rosuav@gmail.com> - 2011-04-21 04:03 +1000
    Re: List comprehension vs filter() Ian Kelly <ian.g.kelly@gmail.com> - 2011-04-20 12:35 -0600

csiph-web