Extract specific strings from a single long line

Regex is suited for this task:

import re

text = 'Authentication success on Interface Gi1/0/20 AuditSessionID 0000000XXXXXXXXXX, Authentication success on Interface Gi1/0/24 AuditSessionID 0000000XXXXXXXXXX, Authentication not succeed on Interface Fi1/0/10 AuditSessionID 0000000XXXXXXXXXX'
re.findall('Interface (.*?) ', text)

The re.findall() will return a list containing what you wanted.

['Gi1/0/20', 'Gi1/0/24', 'Fi1/0/10']

The pattern 'Interface (.*?) ' works by matching Everything beginning with the word "Interface", followed by a space, then something or nothing, then another space. That aforementioned something or nothing is represented by (.*?), which captures (i.e. it gets added to the output of re.findall()) whatever is matched by .*?, which is any character (.), any number of times (*), as few times as necessary to match (?). You can play around with regexes on sites like https://regex101.com/, which will allow you to run Python regexes, as well as explain them (better than I can).