Python Sort unexpected behaviour

I have a data structure such that:

   [(1, '(A) Begin plans for world domination'), 
    (2, 'Listen to Symphony for the New World'), 
    (3, '(A) Change world'), 
    (4, '(D) Listen to Symphony for the New World'), 
    (5, '(C) Hello, World!'), (4, 'Improve Python todo project'), 
    (6, 'Begin plans for world domination'), 
    (7, '(A) Improve Python todo project')]

And I would like to sort first by the contents of the parentheses (if they have parentheses), and then by number.

I'm currently filtering for keywords which works fine, my sort is incorrect:

        filtered_items = [
            (i+1, line.strip()) for i, line in enumerate(items)
            if not args.filter_string or args.filter_string.lower() in line.lower()
        ]

        sorted_items = sorted(filtered_items, key=lambda item: (
            not item[1].startswith('('),
            item[1][:3],
            item[0],
        ))

this current sort will yield the output:

1 (A) Begin plans for world domination
3 (A) Change world
7 (A) Improve Python todo project
5 (C) Hello, World!'), (4, 'Improve Python todo project
4 (D) Listen to Symphony for the New World
6 Begin plans for world domination
2 Listen to Symphony for the New World

The last 2 are the wrong way round.

Can anyone explain why this is?

Your key tuple is (Whether there is parenthesis, first three letters, item number), so when there is no parenthesis, it sorts by alphabetical order based on the first three letters. You want your second tuple to be conditional based on presence of parenthesis:

sorted_items = sorted(
    filtered_items,
    key=lambda item: (not item[1].startswith("("), item[1][0:3] if item[1].startswith("(") else item[0]),
)

You're using the first 3 characters of the string as part of the sort key, even if the string doesn't start with a parenthesis. The last 2 items are sorted that way because 'Beg' comes before 'Lis'.

Your key tuple needs to be conditional. The way it's constructed right now the first three letters of the string are higher priority than the number even if there are no parentheses. Try this:

key=lambda item: (0, item[1][1], item[0]) if item[1].startswith("(") else (1, item[0])

Here's another way to do this.

Caveat is that the value in parentheses is a single uppercase letter

data = [
    (1, '(A) Begin plans for world domination'), 
    (2, 'Listen to Symphony for the New World'), 
    (3, '(A) Change world'), 
    (4, '(D) Listen to Symphony for the New World'), 
    (5, '(C) Hello, World!'),
    (4, 'Improve Python todo project'), 
    (6, 'Begin plans for world domination'), 
    (7, '(A) Improve Python todo project')]

def k(t):
    i, s = t
    return ord(s[1]) if s[0] == '(' else i + ord('Z'), s

print(*sorted(data, key=k), sep='\n')

Output:

(1, '(A) Begin plans for world domination')
(3, '(A) Change world')
(7, '(A) Improve Python todo project')
(5, '(C) Hello, World!')
(4, '(D) Listen to Symphony for the New World')
(2, 'Listen to Symphony for the New World')
(4, 'Improve Python todo project')
(6, 'Begin plans for world domination')

...which, based on the description in the question, is what I believe the output should look like