I have a list that looks like this:
lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
I'd like to generate a filtered list that looks like this:
filtered_lst = [2, 6, 7, 9, 10, 13]
Does Python provide a convention for custom slicing. Something such as:
lst[1, 5, 6, 8, 9, 12] # slice a list by index
from operator import itemgetter
itemgetter(1, 5, 6, 8, 9, 12)(lst)
Demo:
>>> from operator import itemgetter
>>> lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
>>> itemgetter(1, 5, 6, 8, 9, 12)(lst)
(2, 6, 7, 9, 10, 13)
This returns a tuple; cast to a list with list(itemgetter(...)(lst)) if a that is a requirement.
Note that this is the equivalent of a slice expression (lst[start:stop]) with a set of indices instead of a range; it can not be used as a left-hand-side slice assignment (lst[start:stop] = some_iterable).
list type), which does allow for a [index1, index2, .., indexn] notation. The result of a slice expression is a new list, and that's what I think the OP is after. :-) The other use of a slice is as an assignment target, which is what I think you mean, and that is very much out of scope here. Just getting our terminology sorted out here, I think! :-)Numpy arrays have this kind of slicing syntax:
In [45]: import numpy as np
In [46]: lst = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13])
In [47]: lst[[1, 5, 6, 8, 9, 12]]
Out[47]: array([ 2, 6, 7, 9, 10, 13])
It's easily and straightforwardly done using a list comprehension.
lst = range(1, 14)
indices = [1, 5, 6, 8, 9, 12]
filtered_lst = [lst[i] for i in indices]
I'd go with the operator.itemgetter() method that Martijn Pieters has suggested, but here's another way (for completeness)
In [23]: lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
In [24]: indices = set([1, 5, 6, 8, 9, 12])
In [25]: [n for i,n in enumerate(lst) if i in indices]
Out[25]: [2, 6, 7, 9, 10, 13]
A Python slice allows you to make the slice the target of an assignment. And the Python slicing syntax does not allow for slices with irregular patterns of indices. So, if you want to make your "custom" slice the target of an assignment, that's not possible with Python slice syntax.
If your requirements are met by taking a copy of the specified elements, then operator.itemgetter() meets your needs. If you need slice assignment, then numpy slices are a good option.
