case-insensitive list sorting, without lowercasing the result?

Question

I have a list of strings like this:

['Aden', 'abel']

I want to sort the items, case-insensitive. So I want to get:

['abel', 'Aden']

But I get the opposite with sorted() or list.sort(), because uppercase appears before lowercase.

How can I ignore the case? I've seen solutions which involves lowercasing all list items, but I don't want to change the case of the list items.

Solution 1

In Python 3.3+ there is the str.casefold method that's specifically designed for caseless matching:

sorted_list = sorted(unsorted_list, key=str.casefold)

In Python 2 use lower():

sorted_list = sorted(unsorted_list, key=lambda s: s.lower())

It works for both normal and unicode strings, since they both have a lower method.

In Python 2 it works for a mix of normal and unicode strings, since values of the two types can be compared with each other. Python 3 doesn't work like that, though: you can't compare a byte string and a unicode string, so in Python 3 you should do the sane thing and only sort lists of one type of string.

>>> lst = ['Aden', u'abe1']
>>> sorted(lst)
['Aden', u'abe1']
>>> sorted(lst, key=lambda s: s.lower())
[u'abe1', 'Aden']

Solution 2

>>> x = ['Aden', 'abel']
>>> sorted(x, key=str.lower) # Or unicode.lower if all items are unicode
['abel', 'Aden']

In Python 3 str is unicode but in Python 2 you can use this more general approach which works for both str and unicode:

>>> sorted(x, key=lambda s: s.lower())
['abel', 'Aden']

Solution 3

You can also try this to sort the list in-place:

>>> x = ['Aden', 'abel']
>>> x.sort(key=lambda y: y.lower())
>>> x
['abel', 'Aden']

Solution 4

This works in Python 3 and does not involves lowercasing the result (!).

values.sort(key=str.lower)

Solution 5

In python3 you can use

list1.sort(key=lambda x: x.lower()) #Case In-sensitive             
list1.sort() #Case Sensitive

View more solutions

Comments

[ady]

This tutorial is very helpful: docs.python.org/3/howto/sorting.html#sortinghowto

[Admin]

Thank you. I know I should have mentioned this before, but I've heard there's a problem with using this method on a unicode string (Py2). Do you know anything about that?

[Admin]

They are all unicode. Thanks! One more question, how to do it on a list like this: [['Aden'], ['abel']]

[jamylak]

Does each list only have one item? If so just modify it a bit to: sorted(x,key=lambda i:i[0].lower())

[Admin]

Well, it might have some other stuff as well, which should not be used for sorting though.

[jamylak]

Nevermind, it appears i was wrong, the sorting does work for a mix of both string and unicode, i was confused with a previous question where tuples were also included in the sort.

[IceArdor]

This solution is overkill and unreadable when a one-liner suffices. This might be more acceptable in a language other than Python.

[Daniel Andersson]

One can avoid the lambda function roundtrip by (Python 3) using the general str.lower function as sorted(lst, key=str.lower) or (Python 2) using the lower method of the string module as sorted(lst, key=string.lower). One can also use str.lower for strings in Python 2, but would then have to use unicode.lower for unicode objects, whereas string.lower accepts both (which, as you put it, is probably not really a "sane" mode of operation, though).

[matth]

howto sorting: docs.python.org/3/howto/sorting.html#key-functions list.sort: docs.python.org/3/library/stdtypes.html#list.sort

[Enterprise]

This would not work for a list like ['Z', 'B', 'a', 'b', 'A'], which sorts to ['a', 'A', 'B', 'b', 'Z']. The capital 'B' appears before the lowercase 'b' because Python's sort() and sorted() preserve the original order when strings match. In this case the capital 'B' is considered to match the lowercase 'b' when using casefold. This always happens if you convert case in order to compare: sorted(spam, key=str.lower) or sorted(spam, key=str.upper) or sorted(spam, key=str.casefold).

[Enterprise]

Try this solution instead: stackoverflow.com/a/1098160/10668287. It will sort ['Aden', 'aden'] correctly as ['aden', 'Aden'].

[Mike Q]

The biggest issue with this solution is that it violates the rule of using the built-in tested tooling for a home grown solution. It is an interesting response though, I'll give you that.

[Mike Q]

The biggest issue with this solution is that it violates the rule of using the built-in tested tooling for a home grown solution. It is an interesting response though, I'll give you that.