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

Question

Book sections are usually numbered as x.x.x, such as 1.2.3. How do I sort a list of section numbers?

Store section numbers as a list of strings.

# a list of strings, section numbers
ls = ['1.1', '1.10', '1.2', '1.2.3', '1.2.1', '1.9']    

lists = sorted([s.split('.') for s in ls], key=lambda x:map(int, x))    
# [['1', '1'], ['1', '2'], ['1', '2', '1'], ['1', '2', '3'], ['1', '9'], ['1', '10']]

r = ['.'.join(sublist) for sublist in lists]    
#['1.1', '1.2', '1.2.1', '1.2.3', '1.9', '1.10']

However, my expecting result is,

['1.1', '1.10', '1.2', '1.2.1', '1.2.3', '1.9']

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']