"filtered view" upon lists?

[Wildemar Wildenburger]

Jorge Godoy wrote:
 > 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'm not clear enough ;) .
I don't just want to extract certain elements from a list, I want an 
object that looks like a list, however all changes made to that object 
are automagically reflected in the original list. (I guess that is one 
of those 'if it's hard to explain, ...' cases.)

I should have included an example right away ... here goes:

# I have a list
l = [1, 2, 3, 4, 5, 6, 7]

# I then want to create a Filter instance
# (Filter beeing a *class* implemented by me)
# where isEven() returns True whenever an item of l
# should be included in f (in this case, even numbers).
# (I'm asking if something like this exists in the libs
# or elsewhere)
f = Filter(l, isEven)

# The desired behavior now goes something like this:
f
 >>> [2, 4, 6]
del f[1]
f
 >>> [2, 6]
l
 >>> [1, 2, 3, 5, 6, 7]
f.append(77)
f
 >>> [2, 6, 77]
# 77 being intentionally uneven
l
 >>> [1, 2, 3, 5, 6, 7, 77]
# could be [1, 2, 3, 5, 6, 77, 7] as well
# I don't care here

# and so forth ...

I think SQL views are the direct analog.

 > I don't believe it is generic.  Nobody knows your data specs or filtering
 > needs.
Hence the additional function in the Filter constructor ;) . You suggest 
the same thing below, so that is obviously no problem.

 > Use of list comprehension might make it easier to code this:
 >
 > <snip elaborate example>
I sort of wanted to avoid these. Though my lists shouldn't terribly 
long, so performance is not an issue so much. I simply want to avoid 
having two datasets that I have to sync. Major pain, I believe.


Coding all that really is quite straight forward, but it is a lot of 
mule-work. I hoped (no, I still hope) that there might be somethin like 
that around already.

A bit clearer now?

bye
wildemar