"filtered view" upon lists?

Paul McGuire ptmcg at austin.rr._bogus_.com
Tue Sep 12 16:12:13 CEST 2006


"Jorge Godoy" <jgodoy at gmail.com> wrote in message 
news:878xkpb5wa.fsf at gmail.com...
> Wildemar Wildenburger <wildemar at freakmail.de> writes:
>
>> I don't know how else to call what I'm currently implementing: An object 
>> that
>> behaves like a list but doesn't store it's own items but rather pulls 
>> them
>> from a larger list (if they match a certain criterion).
>> Changes to the filter are instantly reflected in the underlying list.
>> Clear enough?
>
> It looks like you're implementing a callable to me.  This is a method that
> returns results based on some input -- here your original list and filter.
>
> Then you'll use this method wherever you need that filtered list.
>
>> Ok, so I figured that this is generic enough to be found in some standard
>> module already (I've had this often enough: Painfully implementing s/th 
>> and
>> then finding it in the libs some weeks later.).
>
> I don't believe it is generic.  Nobody knows your data specs or filtering
> needs.
>
>> Any pointers?
>
> Use of list comprehension might make it easier to code this:
>
>
> def myCallable(my_list, filter):
>    filtered_list = [(item) for item in my_list if filter(item)]
>    return filtered_list
>
>
> Example of full code:
>
>>>> test_list = range(10)
>>>> filter = lambda x: not x%2
>>>> def myCallable(list, filter):
> ...     filtered_list = [(item) for item in list if filter(item)]
> ...     return filtered_list
> ...
>>>> myCallable(test_list, filter)
> [0, 2, 4, 6, 8]
>>>> for item in myCallable(test_list, filter):
> ...     print "See?  I'm", item
> ...
> See?  I'm 0
> See?  I'm 2
> See?  I'm 4
> See?  I'm 6
> See?  I'm 8
>>>>
>
>
> -- 
> Jorge Godoy      <jgodoy at gmail.com>

Functionally-speaking, there is the filter built-in that does this same 
thing:

>>> data = range(10)
>>> odds = filter(lambda x : x % 2, data)
>>> evens = filter(lambda x : x % 2 != 1, data)
>>> odds
[1, 3, 5, 7, 9]
>>> evens
[0, 2, 4, 6, 8]


-- Paul





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