List comprehension vs filter()

Chris Angelico rosuav at gmail.com
Wed Apr 20 05:10:21 CEST 2011


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



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