Push ID implementation in Python

pushid.py

from datetime import datetime
import time
from random import random


class PushID(object):
    # Modeled after base64 web-safe chars, but ordered by ASCII.
    PUSH_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz'

    def __init__(self, server_time=0):
        # Timestamp of last push, used to prevent local collisions if you
        # pushtwice in one ms.
        self.last_push_time = server_time
 
        # We generate 72-bits of randomness which get turned into 12
        # characters and appended to the timestamp to prevent
        # collisions with other clients.  We store the last characters
        # we generated because in the event of a collision, we'll use
        # those same characters except "incremented" by one.
        self.last_rand_chars = []  # array

    def next_id(self):
        # Datetime now() in epoch
        now = int(time.time() * 1000)
        duplicate_time = (now == self.last_push_time)
        self.last_push_time = now

        timestamps_chars = []  # Use arrays
        for i in range(7, -1, -1):
            timestamps_chars.append(self.PUSH_CHARS[now % 64])
            now /= 64

        if now != 0:
            raise ValueError('We should have converted the entire timestamp.')

        if not duplicate_time:
            self.last_rand_chars = [int(random() * 64) for i in range(0, 12)]

        else:
            # If the timestamp hasn't changed since last push, use the
            # same random number, except incremented by 1.
            for i in range(11, -1, -1):
                if self.last_rand_chars[i] == 63:
                    self.last_rand_chars[i] = 0
                else:
                    break
            self.last_rand_chars[i] += 1

        # Generate the UID
        uid = "".join(timestamps_chars) + "".join([self.PUSH_CHARS[i] for i in self.last_rand_chars])

        if len(uid) != 20:
            raise ValueError('Length should be 20.')
        return uid


if __name__ == '__main__':
    pid = PushID()