FIle object to put a character back into the input buffer

calc.py

from buffer import UngetableInput

token_stream = UngetableInput()

def expression():
    val = None
    ch = token_stream.getc()
    while ch == ' ':
        ch = token_stream.getc()
    if ch in {'-', '+', '.'} or ch.isdigit():
        token_stream.ungetc(ch)
        val = number()
        if type(val) == str:
            return 'Error'
    elif ch == '(':
        op = operator()
        exp1 = expression()
        exp2 = expression()
        if type(exp1) == str or type(exp2) == str:
            return 'Error'
        ch = token_stream.getc()
        while ch == ' ':
            ch = token_stream.getc()
        if ch != ')':
            return 'Invalid syntax.'
        val = calc(op, exp1, exp2)
    else:
        return 'Invalid token.'
    return val

def number():
    try:
        n = float(token_stream.getword())
    except ValueError:
        return 'Number expected.'
    return n

def operator():
    op = token_stream.getc()
    while op == ' ':
        op = token_stream.getc()
    if op in {'+', '-', '*', '/'}:
        return op
    else:
        return 'Operator expected.'

def calc(op, exp1, exp2):
    if op == '+':
        return exp1 + exp2
    elif op == '-':
        return exp1 - exp2
    elif op == '*':
        return exp1 * exp2
    elif op == '/':
        if exp2 == 0:
            return 'Division by zero not allowed.'
        return exp1 / exp2
    else:
        return 'Unknown operator'


while True:
    try:
        print('> ', end='')
        r = expression()
        try:
            v = float(r)
        except ValueError:
            print(r)
        else:
            print('=', v)
    except EOFError:
        exit()