retrieve item from nested list given index tuple

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Fri Aug 14 19:09:42 CEST 2009


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


> 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.



-- 
Steven



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