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Python State Machines for Humans

Project description

state machine for humans

There are two types of developers in this world: those who love state machines and those who will eventually.

I fall in the first camp. I think it is really important to have a declarative way to define the states of an object. That’s why I developed yasm.

Install

pip install yasm

Basic Usage

from collections import deque
import operator
import six
from string import whitespace, digits

from yasm import Event, state_machine
from yasm.utils import dispatch


@state_machine('calculator')
class Calculator(object):

    operators = {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
    }
    if six.PY3:
        operators['/'] = operator.truediv
    else:
        operators['/'] = operator.div

    def __init__(self):
        self.stack = deque()
        self.result = None

    def reset(self):
        self.stack.clear()
        self.result = None
        self.machine.reinit_instance(self)

    def calculate(self, string):
        self.reset()
        for char in string:
            dispatch(self, Event('parse', input=char))
        return self.result

    def start_building_number(self, state, event, instance):
        digit = event.input
        self.stack.append(int(digit))

    def build_number(self, state, event, instance):
        digit = event.input
        number = str(self.stack.pop())
        number += digit
        self.stack.append(int(number))

    def do_operation(self, state, event, instance):
        operation = event.input
        y = self.stack.pop()
        x = self.stack.pop()
        self.stack.append(self.operators[operation](float(x), float(y)))

    def do_equal(self, state, event, instance):
        number = self.stack.pop()
        self.result = number


def is_digit(state, event, instance):
    return event.input in digits


sm = Calculator.machine
sm.add_states(['initial', 'number', 'result'], initial='initial')

sm.add_transitions([
    {'from_state': 'initial', 'to_state': 'number', 'event': 'parse',
     'conditions': [is_digit], 'before': 'start_building_number'},
    {'from_state': 'number', 'to_state': 'number', 'event': 'parse',
     'conditions': [is_digit], 'before': 'build_number'},
    {'from_state': 'number', 'to_state': 'initial', 'event': 'parse',
     'conditions': [lambda state, evt, ins: evt.input in whitespace]},
    {'from_state': 'initial', 'to_state': 'initial', 'event': 'parse',
     'conditions': [lambda state, evt, ins: evt.input in '+-*/'],
     'before': 'do_operation'},
    {'from_state': 'initial', 'to_state': 'result', 'event': 'parse',
     'conditions': [lambda state, evt, ins: evt.input == '='],
     'before': 'do_equal'},
])


 calc = Calculator()
 for syntax, value in ((' 167 3 2 2 * * * 1 - =', 2003),
                       ('    167 3 2 2 * * * 1 - 2 / =', 1001.5),
                       ('    3   5 6 +  * =', 33),
                       ('        3    4       +     =', 7),
                       ('2 4 / 5 6 - * =', -0.5),):
     result = calc.calculate(syntax)
     assert result == value, (syntax, result, value)
     calc.reset()

Thank you

to aasm and ruby’s state_machine and jtushman’s jtushman/state_machine and all other state machines that I loved before

Project details


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