Sort list of strings ignoring upper/lower case

Question

I have a list which contains strings representing animal names. I need to sort the list. If I use sorted(list), it will give the list output with uppercase strings first and then lowercase.

But I need the below output.

Input:

var = ['ant','bat','cat','Bat','Lion','Goat','Cat','Ant']

Output:

['ant', 'Ant', 'bat', 'Bat', 'cat', 'Cat', 'Goat', 'Lion']

Answer 1

The sort() method and the sorted() function take a key argument:

var.sort(key=lambda v: v.upper())

The function named in key is called for each value and the return value is used when sorting, without affecting the actual values:

>>> var=['ant','bat','cat','Bat','Lion','Goat','Cat','Ant']
>>> sorted(var, key=lambda v: v.upper())
['ant', 'Ant', 'bat', 'Bat', 'cat', 'Cat', 'Goat', 'Lion']

To sort Ant before ant, you'd have to include a little more info in the key, so that otherwise equal values are sorted in a given order:

>>> sorted(var, key=lambda v: (v.upper(), v[0].islower()))
['Ant', 'ant', 'Bat', 'bat', 'Cat', 'cat', 'Goat', 'Lion']

The more complex key generates ('ANT', False) for Ant, and ('ANT', True) for ant; True is sorted after False and so uppercased words are sorted before their lowercase equivalent.

See the Python sorting HOWTO for more information.

Answer 2

New answer for Python 3, I'd like to add two points:

1. Use str.casefold for case-insensitive comparisons.
2. Use the method directly instead of inside of a lambda.

That is:

var = ['ant','bat','cat','Bat','Lion','Goat','Cat','Ant']

var.sort(key=str.casefold)

(which sorts in-place) and now:

>>> var
['ant', 'Ant', 'bat', 'Bat', 'cat', 'Cat', 'Goat', 'Lion']

Or, to return a new list, use sorted

>>> var = ['ant','bat','cat','Bat','Lion','Goat','Cat','Ant']
>>> sorted(var, key=str.casefold)
['ant', 'Ant', 'bat', 'Bat', 'cat', 'Cat', 'Goat', 'Lion']

Why is this different from str.lower or str.upper? According to the documentation:

Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. For example, the German lowercase letter 'ß' is equivalent to "ss". Since it is already lowercase, str.lower() would do nothing to 'ß'; casefold() converts it to "ss".

Answer 3

We can use the 'sorted' function according to Python Sorting HOW TO documentation.

a = sorted(Input, key=str.lower)print("Output1: ",a)

Output1:

['ant', 'Ant', 'bat', 'Bat', 'cat', 'Cat', 'Goat', 'Lion']