How do I sort a list of section numbers in Python? [duplicate]

Answer 1

Use a custom compare function that converts the strings into sub-lists of integers. Those will sort correctly without problems.

In [4]: ls = ['1.1', '1.10', '1.2', '1.2.3', '1.2.1', '1.9']

In [5]: def section(s):
   ...:     return [int(_) for _ in s.split(".")]
   ...:

In [6]: sorted(ls, key=section)
Out[6]: ['1.1', '1.2', '1.2.1', '1.2.3', '1.9', '1.10']

Answer 2

Book sections are usually numbered as x.x.x

Why not store the section numbers as tuples?

sections = [(2, 4, 1), (1, 10, 3),(1, 2, 1), (1, 1, 10), (1, 2, 3), (1, 4, 6)]

print(sorted(sections)) 

gives [(1, 1, 10), (1, 2, 1), (1, 2, 3), (1, 4, 6), (1, 10, 3), (2, 4, 1)]

Answer 3

As by your comments, float is not the datatype you need. In your case, you have an actual hierarchy of chapters/sections.

One simple (and remember, simple is better than complex) way is to represent the section numbers as tuples. Since tuples are sorted lexicographically, they naturally sort in the desired order:

>>> lf = [(1, ), (1, 1), (1, 10), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8), (2, ), (1, 9)]
>>> sorted(lf)
[(1, ), (1, 1), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8), (1, 9), (1, 10), (2, )]

As we can see, this also works for tuples with varying lengths.

If want to keep the sections as strings, natsort does a fine job of handling dotted values, too:

>>> s = ['1', '1.1', '1.10', '1.2']
>>> natsort.natsorted(s)
['1', '1.1', '1.2', '1.10']

You can also define your own SectionNumber class, but that's probably overkill.