Function that takes a nested list of strings and returns a new nested list with all strings capitalized?

Question

This will capitalize them but only if there are no nested lists.

t = ['this','that', ['other']]

def capitalize_nested(t):
    res = []
    for s in t:
        res.append(s.capitalize())
    return res

print capitalize_nested(t)

I can't figure out how to get it to print out a nested list having all of the strings start with a capital letter. I must be missing something obvious, this just has me stumped.

Do you need to support arbitrary nested lists (unlimited depth)? — jfs, Dec 24 '12 at 17:57

David Robinson · Accepted Answer · 2012-12-24T18:03:56.810

12

Use a recursive solution (and using list comprehensions also helps make it more compact):

def capitalize_nested(t):
    if isinstance(t, list):
        return [capitalize_nested(s) for s in t]
    else:
        return t.capitalize()

For example:

print capitalize_nested(['this', 'that', ['other']])
# ['This', 'That', ['Other']]

edited Dec 24 '12 at 18:03

answered Dec 24 '12 at 17:57

David Robinson

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@Amber: good idea, changed (though the question does ask just for nested lists). – David Robinson Dec 24 '12 at 18:01
@Amber: Actually I just realized I can't do that at all, because `isinstance("this", collections.Sequence)` (calling on a string) returns True, so it will recurse infinitely on each of the strings. – David Robinson Dec 24 '12 at 18:04
True. Perhaps it would be safer to see if the thing in question is *not* a string. – Amber Dec 24 '12 at 18:06

inspectorG4dget · Answer 2 · 2012-12-24T19:35:43.520

3

def cap(L):
    for i,elem in enumerate(L):
         if isinstance(elem, str):
             L[i] = elem.capitalize()
         elif isinstance(elem, list):
             cap(L[i])

edited Dec 24 '12 at 19:35

answered Dec 24 '12 at 17:58

inspectorG4dget

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1

you should not return a non-None value if you modify the list inplace. – jfs Dec 24 '12 at 18:44
@J.F.Sebastian: very true. Modified – inspectorG4dget Dec 24 '12 at 19:27
The function is not expected to return a value so it is ok to remove `if:return/else` completely. `for i, elem, in enumerate(L)` will take care of empty lists. – jfs Dec 24 '12 at 19:34
@J.F.Sebastian: true. But I was trying to be explicit for OP's sake – inspectorG4dget Dec 24 '12 at 19:36

score 2 · Answer 3 · answered Dec 24 '12 at 17:58

Just check if s is a list then recursively call your capitalize_nested function:

t = ['this','that', ['other']]

def capitalize_nested(t):
    res = []
    for s in t:
        if type(s) == list:
            res.append(capitalize_nested(s))
        else:
            res.append(s.capitalize())
    return res

print capitalize_nested(t)

MostafaR · Answer 4 · 2012-12-24T19:19:42.247

1

The recursive solution is first solution and most beautiful solution, but not always the best solution, Check this iterative solution too:

def capitalize(t):
    lists = [t]
    while lists:
        l = lists.pop()
        for i, item in enumerate(l):
            if isinstance(item, list):
                lists.append(item)
            else:
                l[i] = item.capitalize()

edited Dec 24 '12 at 19:19

answered Dec 24 '12 at 18:30

MostafaR

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You are using `lists` as a stack by appending to it while iterating over it. I'm not sure whether such semantics is guaranteed by Python language but it works on Python 2.x, 3.x, Pypy, Jython. I've used explicit `while stack: lst = stack.pop()` in [the similar solution](http://stackoverflow.com/a/14024633/4279). – jfs Dec 24 '12 at 18:42
@J.F.Sebastian Thank you for your hint, I just improved the code. – MostafaR Dec 24 '12 at 18:45

score 1 · Answer 5 · answered Dec 24 '12 at 18:31

Here's a version that supports an arbitrary deep nested list:

from collections import MutableSequence

def capitalize_inplace(nested_list):
    stack = [nested_list]
    while stack:
        lst = stack.pop()
        for i, item in enumerate(lst):
            if isinstance(item, MutableSequence):
                stack.append(item)
            else:
                lst[i] = item.capitalize()

Example

L = ['this', 'that', ['other'], ['may',['be', ['nested'], 'further']]]
capitalize_inplace(L)
print(L)
# -> ['This', 'That', ['Other'], ['May', ['Be', ['Nested'], 'Further']]]

Function that takes a nested list of strings and returns a new nested list with all strings capitalized?

5 Answers5

Example