calc2.py

import pyparsing as pp
from pyparsing import pyparsing_common as ppc

from repl import repl


variables = {}

var = ppc.identifier.copy()
def do_var(t):
    try:
        return variables[t[0]]
    except KeyError as err:
        raise pp.ParseFatalException(f'variable {err} is undefined')
var.addParseAction(do_var)

expr = pp.Forward()
expr.setName('expression')
atom = ppc.fnumber | var | pp.Suppress('(') + expr + pp.Suppress(')')

neg = '-' + atom
neg.addParseAction(lambda t: -t[1])
s_atom = neg | atom

term = s_atom + pp.ZeroOrMore((pp.Literal('*') | pp.Literal('/')) + s_atom)
def do_term(t):
    result = t[0]
    for i in range(2, len(t), 2):
        if t[i-1] == '*':
            result *= t[i]
        elif t[i-1] == '/':
            result /= t[i]
    return result
term.addParseAction(do_term)

expr <<= term + pp.ZeroOrMore((pp.Literal('+') | pp.Literal('-')) + term)
def do_expr(t):
    result = t[0]
    for i in range(2, len(t), 2):
        if t[i-1] == '+':
            result += t[i]
        elif t[i-1] == '-':
            result -= t[i]
    return result
expr.addParseAction(do_expr)

assign = ppc.identifier + '=' + expr
assign.setName('assignment')
def do_assign(t):
    variables[t[0]] = t[2]
    return t[2]
assign.addParseAction(do_assign)

stmt = assign | expr
repl(stmt)

repl.py

from prompt_toolkit import prompt
from prompt_toolkit.history import InMemoryHistory
from pyparsing import ParseBaseException


def sgr(*args):
    return '\033[{}m'.format(';'.join(str(i) for i in args))


def repl(parser):
    parser.validate()
    history = InMemoryHistory()
    while True:
        try:
            s = prompt('> ', history=history)
            result = parser.parseString(s, parseAll=True)[0]
            print(result)
        except ParseBaseException as err:
            print(f'{sgr(1, 31)}{err.__class__.__name__}{sgr(0)}: {err}')
        except KeyboardInterrupt:
            pass
        except EOFError:
            break