import vk_bot

bot = vk_bot.VkBot('YOUR_ACCESS_TOKEN')

from vk_client import GroupEventType

@bot.on([GroupEventType.GROUP_JOIN])
def group_join_handler(obj):
    bot.vk.Message.send(
        peer=obj.user,
        message=f'Welcome, {obj.user.first_name}!'
    )

from vk_bot.ext import CmdHandler

command = CmdHandler(bot)

@command(pass_msg=True)
def whoami(msg):
    """Print effective user name."""
    return f'{msg.sender.first_name} {msg.sender.last_name}'

import operator

OPS = {
    'add': operator.add,
    'sub': operator.sub,
    'mul': operator.mul,
    'div': operator.truediv
}

@command(args={
    'op': {'choices': list(OPS)},
    'a': {'type': int},
    'b': {'type': int}
})
def calc(op, a, b):
    """Simple calculator."""
    try:
        return f'Result: {OPS[op](a, b)}'
    except ArithmeticError as e:
        return f'Error: {e}'

bot.run()