Finite State Machine#

A finite-state machine (FSM) or finite-state automaton (FSA, plural: automata), finite automaton, or simply a state machine, is a mathematical model of computation.

It is an abstract machine that can be in exactly one of a finite number of states at any given time. The FSM can change from one state to another in response to some inputs; the change from one state to another is called a transition.

An FSM is defined by a list of its states, its initial state, and the inputs that trigger each transition.


Source: WikiPedia

Usage example#

Not all functionality of the bot can be implemented as single handler, for example you will need to collect some data from user in separated steps you will need to use FSM.

FSM Example

Let’s see how to do that step-by-step

Step by step#

Before handle any states you will need to specify what kind of states you want to handle

15    ReplyKeyboardRemove,
16)
17
18form_router = Router()

And then write handler for each state separately from the start of dialog

Here is dialog can be started only via command /start, so lets handle it and make transition user to state Form.name

21class Form(StatesGroup):
22    name = State()
23    like_bots = State()
24    language = State()
25
26
27@form_router.message(Command(commands=["start"]))

After that you will need to save some data to the storage and make transition to next step.

48    await message.answer(
49        "Cancelled.",
50        reply_markup=ReplyKeyboardRemove(),
51    )
52
53
54@form_router.message(Form.name)
55async def process_name(message: Message, state: FSMContext) -> None:
56    await state.update_data(name=message.text)
57    await state.set_state(Form.like_bots)
58    await message.answer(
59        f"Nice to meet you, {html.quote(message.text)}!\nDid you like to write bots?",
60        reply_markup=ReplyKeyboardMarkup(
61            keyboard=[
62                [
63                    KeyboardButton(text="Yes"),

At the next steps user can make different answers, it can be yes, no or any other

Handle yes and soon we need to handle Form.language state

77        "Not bad not terrible.\nSee you soon.",
78        reply_markup=ReplyKeyboardRemove(),
79    )
80    await show_summary(message=message, data=data, positive=False)
81
82
83@form_router.message(Form.like_bots, F.text.casefold() == "yes")
84async def process_like_write_bots(message: Message, state: FSMContext) -> None:

Handle no

66            ],
67            resize_keyboard=True,
68        ),
69    )
70
71
72@form_router.message(Form.like_bots, F.text.casefold() == "no")
73async def process_dont_like_write_bots(message: Message, state: FSMContext) -> None:
74    data = await state.get_data()

And handle any other answers

87    await message.reply(
88        "Cool! I'm too!\nWhat programming language did you use for it?",
89        reply_markup=ReplyKeyboardRemove(),

All possible cases of like_bots step was covered, let’s implement finally step

 92
 93@form_router.message(Form.like_bots)
 94async def process_unknown_write_bots(message: Message, state: FSMContext) -> None:
 95    await message.reply("I don't understand you :(")
 96
 97
 98@form_router.message(Form.language)
 99async def process_language(message: Message, state: FSMContext) -> None:
100    data = await state.update_data(language=message.text)
101    await state.clear()
102    text = (

And now you have covered all steps from the image, but you can make possibility to cancel conversation, lets do that via command or text

30    await message.answer(
31        "Hi there! What's your name?",
32        reply_markup=ReplyKeyboardRemove(),
33    )
34
35
36@form_router.message(Command(commands=["cancel"]))
37@form_router.message(F.text.casefold() == "cancel")
38async def cancel_handler(message: Message, state: FSMContext) -> None:
39    """
40    Allow user to cancel any action
41    """
42    current_state = await state.get_state()
43    if current_state is None:
44        return
45

Complete example#

  1import asyncio
  2import logging
  3import sys
  4from os import getenv
  5from typing import Any, Dict
  6
  7from aiogram import Bot, Dispatcher, F, Router, html
  8from aiogram.filters import Command
  9from aiogram.fsm.context import FSMContext
 10from aiogram.fsm.state import State, StatesGroup
 11from aiogram.types import (
 12    KeyboardButton,
 13    Message,
 14    ReplyKeyboardMarkup,
 15    ReplyKeyboardRemove,
 16)
 17
 18form_router = Router()
 19
 20
 21class Form(StatesGroup):
 22    name = State()
 23    like_bots = State()
 24    language = State()
 25
 26
 27@form_router.message(Command(commands=["start"]))
 28async def command_start(message: Message, state: FSMContext) -> None:
 29    await state.set_state(Form.name)
 30    await message.answer(
 31        "Hi there! What's your name?",
 32        reply_markup=ReplyKeyboardRemove(),
 33    )
 34
 35
 36@form_router.message(Command(commands=["cancel"]))
 37@form_router.message(F.text.casefold() == "cancel")
 38async def cancel_handler(message: Message, state: FSMContext) -> None:
 39    """
 40    Allow user to cancel any action
 41    """
 42    current_state = await state.get_state()
 43    if current_state is None:
 44        return
 45
 46    logging.info("Cancelling state %r", current_state)
 47    await state.clear()
 48    await message.answer(
 49        "Cancelled.",
 50        reply_markup=ReplyKeyboardRemove(),
 51    )
 52
 53
 54@form_router.message(Form.name)
 55async def process_name(message: Message, state: FSMContext) -> None:
 56    await state.update_data(name=message.text)
 57    await state.set_state(Form.like_bots)
 58    await message.answer(
 59        f"Nice to meet you, {html.quote(message.text)}!\nDid you like to write bots?",
 60        reply_markup=ReplyKeyboardMarkup(
 61            keyboard=[
 62                [
 63                    KeyboardButton(text="Yes"),
 64                    KeyboardButton(text="No"),
 65                ]
 66            ],
 67            resize_keyboard=True,
 68        ),
 69    )
 70
 71
 72@form_router.message(Form.like_bots, F.text.casefold() == "no")
 73async def process_dont_like_write_bots(message: Message, state: FSMContext) -> None:
 74    data = await state.get_data()
 75    await state.clear()
 76    await message.answer(
 77        "Not bad not terrible.\nSee you soon.",
 78        reply_markup=ReplyKeyboardRemove(),
 79    )
 80    await show_summary(message=message, data=data, positive=False)
 81
 82
 83@form_router.message(Form.like_bots, F.text.casefold() == "yes")
 84async def process_like_write_bots(message: Message, state: FSMContext) -> None:
 85    await state.set_state(Form.language)
 86
 87    await message.reply(
 88        "Cool! I'm too!\nWhat programming language did you use for it?",
 89        reply_markup=ReplyKeyboardRemove(),
 90    )
 91
 92
 93@form_router.message(Form.like_bots)
 94async def process_unknown_write_bots(message: Message, state: FSMContext) -> None:
 95    await message.reply("I don't understand you :(")
 96
 97
 98@form_router.message(Form.language)
 99async def process_language(message: Message, state: FSMContext) -> None:
100    data = await state.update_data(language=message.text)
101    await state.clear()
102    text = (
103        "Thank for all! Python is in my hearth!\nSee you soon."
104        if message.text.casefold() == "python"
105        else "Thank for information!\nSee you soon."
106    )
107    await message.answer(text)
108    await show_summary(message=message, data=data)
109
110
111async def show_summary(message: Message, data: Dict[str, Any], positive: bool = True) -> None:
112    name = data["name"]
113    language = data.get("language", "<something unexpected>")
114    text = f"I'll keep in mind that, {html.quote(name)}, "
115    text += (
116        f"you like to write bots with {html.quote(language)}."
117        if positive
118        else "you don't like to write bots, so sad..."
119    )
120    await message.answer(text=text, reply_markup=ReplyKeyboardRemove())
121
122
123async def main():
124    bot = Bot(token=getenv("TELEGRAM_TOKEN"), parse_mode="HTML")
125    dp = Dispatcher()
126    dp.include_router(form_router)
127
128    await dp.start_polling(bot)
129
130
131if __name__ == "__main__":
132    logging.basicConfig(level=logging.INFO, stream=sys.stdout)
133    asyncio.run(main())

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