server number tracker

tracker.py

#server numbers
from collections import defaultdict

def next_server_number(numbers):
    for i,n in enumerate(sorted(numbers), 1):
        if i != n:
            return i
    return len(numbers) + 1

def split_letter_digit(s):
    alpha = ''
    for i,c in enumerate(s):
        if c.isalpha():
            alpha += c
        else:
            return alpha, int(s[i:])

class Tracker:
    def __init__(self):
        self.host_names = defaultdict(list)

    def allocate(self, s):
        for i, element in enumerate(self.host_names[s], 1):
            if not element:
                self.host_names[s][i] = True
                return s + str(i)
        self.host_names[s].append(True)
        return '{}{}'.format(s, len(self.host_names[s]))

    def deallocate(self, s):
        host_type, number = split_letter_digit(s)
        if host_type in self.host_names:
            val = self.host_names[host_type][number-1]
            if val:
                self.host_names[host_type][number-1] = False
            else:
                raise ValueError('Server name not allocated.')
        else:
            raise ValueError('No Such Host Type.')

tracker = Tracker()
print(tracker.allocate('taco'))
print(tracker.allocate('taco'))
tracker.deallocate('taco1')
tracker.deallocate('taco2')