kram1032 - 1 year ago 75
Python Question

# traverse nested list with single for

I have a bunch of lists in lists. The nesting depth is determined at run-time and I only want to access them to a specific (run-time decided) depth to manipulate what ever is at that level in an arbitrary way.

Ideally I'd like to be able to do this as simply as:

``````for x in access_list(nested_list, d)
# do stuff at nesting-depth d
``````

What
`access_list`
should do:

``````>>> mylist = [[[0, 1], [2, 3]], [[4, 5], [6, 7]]]
>>> for d in range(4):
...     for l in access_list(mylist, d):
...         print((d, l))
(0, [[[0, 1], [2, 3]], [[4, 5], [6, 7]]])
(1, [[[0, 1], [2, 3]])
(1, [[4, 5], [6, 7]]])
(2, [0, 1])
(2, [2, 3])
(2, [4, 5])
(2, [6, 7])
(3, 0)
(3, 1)
(3, 2)
(3, 3)
(3, 4)
(3, 5)
(3, 6)
(3, 7)
``````

My attempt turned out to do essentially nothing:

``````def access_list(lists, d):
if not d:
return lists
return [access_list(_list, d-1) for _list in lists]
``````

It just returns the whole list structure again.
What could I do to make this work?

This generator function should work for nested lists and saves memory as it doesn't build a list itself, but lazily produces the items:

``````def access_list(nested_list):
if not isinstance(nested_list, list):
# if not isinstance(nested_list, (list, set)): you get the idea
yield nested_list
else:
for item in nested_list:
for x in access_list(item):
yield x
# in Python 3, you can replace that loop by:
# yield from access_list(item)
return

> l = [1, 2, [3, [4, 5], 6]]
> list(access_list(l))
[1, 2, 3, 4, 5, 6]
``````

If you want access to nesting depth, the following will produce pairs `(item, depth)`:

``````def access_list(nested_list, d=0):
if not isinstance(nested_list, list):
yield nested_list, d
else:
for item in nested_list:
for x in access_list(item, d=d+1):
yield x
return

> l = [1, 2, [3, [4, 5], 6]]
> list(access_list(l))
[(1, 1), (2, 1), (3, 2), (4, 3), (5, 3), (6, 2)]
``````
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