Steven D'Aprano wrote:
On Fri, 14 Aug 2009 15:54:54 +0000, Alan G Isaac wrote:
`lst` is a nested list
`tpl` is the indexes for an item in the list
What is the nice way to retrieve the item? (Speedy access is nice.)
Assuming you want to do this frequently, write a helper function, then
use it:
# Untested
def extract(nested, indexes):
for index in indexes:
nested = nested[index]
return nested
This looks OK for the first level of nesting. We are not told much about tpl
but suppose that:
lst= [a, [b, [c, d]], [e, f]] and that we wish to retrieve d and f from lst.
tpl would need to be something like [[1, 1, 1], [2, 1]].
If that is the requirement, then Untested is only a step along the road,
extract could be made recursive.
Colin W.
I don't want to use NumPy, but I'd like somehow to avoid an explicit
loop. I did consider using eval. E.g., eval('lst' +
'[%d]'*len(tpl)%tpl). It works but seems rather ugly.
And slow.
I kind of like
reduce(list.__getitem__, tpl, lst) but the reliance on reduce remains
controversial enough to see i removed from the Python 3 built-ins ...
It's just moved into functools.
lst = ['a', 'b', ['aa', ['aaa', 'bbb', 'ccc'], 'cc']]
from functools import reduce
lst = ['a', 'b', ['aa', ['aaa', 'bbb', 'ccc'], 'cc']]
reduce(list.__getitem__, (2, 1, 0), lst)
'aaa'
However, it doesn't work too well as soon as you mix sequence types:
reduce(list.__getitem__, (2, 1, 0, 0), lst)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: descriptor '__getitem__' requires a 'list' object but received
a 'str'
Try this instead:
from operator import getitem
reduce(getitem, (2, 1, 0), lst)
'aaa'
reduce(getitem, (2, 1, 0, 0), lst)
'a'
operator.getitem is less ugly too.
Colin W.
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