How to create def for sorted() from list of versioning files (filename+datetime)

[snippsat]

I understood that I need to use "key" function. This is the hardest part to understand.

You can use datetime.strptime to convert it to date format,the use it in sorted() with key.
Eg:

>>> from datetime import datetime
>>> from pprint import pprint

>>>
... data = '''\
... archive.pst 2016-10-1 080101.pst
... archive.pst 2016-10-20 080101.pst
... archive.pst 2016-10-8 080101.pst'''

>>> pst_files = []
>>> for line in data.split('\n'):
...     pst_files.append(line.split())

>>> pprint(pst_files)
[['archive.pst', '2016-10-1', '080101.pst'],
 ['archive.pst', '2016-10-20', '080101.pst'],
 ['archive.pst', '2016-10-8', '080101.pst']]


>>> pprint(sorted(pst_files, key=lambda x: datetime.strptime(x[1], "%Y-%m-%d"), reverse=True))
[['archive.pst', '2016-10-20', '080101.pst'],
 ['archive.pst', '2016-10-8', '080101.pst'],
 ['archive.pst', '2016-10-1', '080101.pst']]