collections.defaultdict

defaultdict - подкласс словаря, который вызывает указанную функцию для подстановки несуществующих значений

class collections.defaultdict(default_factory=None, /[, ...])

In [2]: from collections import defaultdict

In [3]: data = "some very important text"

In [4]: d = defaultdict(int)

In [5]: d
Out[5]: defaultdict(int, {})

In [6]: for letter in data:
   ...:     d[letter] += 1
   ...:

In [7]: d
Out[7]:
defaultdict(int,
            {'s': 1,
             'o': 2,
             'm': 2,
             'e': 3,
             ' ': 3,
             'v': 1,
             'r': 2,
             'y': 1,
             'i': 1,
             'p': 1,
             't': 4,
             'a': 1,
             'n': 1,
             'x': 1})

In [9]: sorted(d.items(), key=lambda x: x[-1], reverse=True)
Out[9]:
[('t', 4),
 ('e', 3),
 (' ', 3),
 ('o', 2),
 ('m', 2),
 ('r', 2),
 ('s', 1),
 ('v', 1),
 ('y', 1),
 ('i', 1),
 ('p', 1),
 ('a', 1),
 ('n', 1),
 ('x', 1)]

Пример использования

config_sw1.txt

interface FastEthernet0/0
 switchport mode access
 switchport access vlan 10
!
interface FastEthernet0/1
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 100,200
 switchport mode trunk
!
interface FastEthernet0/2
 switchport mode access
 switchport access vlan 20
!
interface FastEthernet0/3
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 100,300,400,500,600
 switchport mode trunk

{'FastEthernet0/0': ['switchport mode access', 'switchport access vlan 10'],
 'FastEthernet0/1': ['switchport trunk encapsulation dot1q',
                     'switchport trunk allowed vlan 100,200',
                     'switchport mode trunk'],
 'FastEthernet0/2': ['switchport mode access', 'switchport access vlan 20'],
 'FastEthernet0/3': ['switchport trunk encapsulation dot1q',
                     'switchport trunk allowed vlan 100,300,400,500,600',
                     'switchport mode trunk'],
 'FastEthernet1/0': ['switchport mode access', 'switchport access vlan 20'],
 'FastEthernet1/1': ['switchport mode access', 'switchport access vlan 30'],
 'FastEthernet1/2': ['switchport trunk encapsulation dot1q',
                     'switchport trunk allowed vlan 400,500,600',
                     'switchport mode trunk']}

from pprint import pprint
from collections import defaultdict


def get_ip_from_cfg(filename):
    result = defaultdict(list)
    with open(filename) as f:
        for line in f:
            if line.startswith("interface"):
                intf = line.split()[-1]
            elif line.startswith(" switchport"):
                result[intf].append(line.strip())
    return result


if __name__ == "__main__":
    pprint(get_ip_from_cfg("config_sw1.txt"))