a calc in python based on tutorial at Ruslan's blog: https://ruslanspivak.com/lsbasi-part3/

my_calc.py

NUMBER, OPER, NONE = "NUMBER", "OPER", "NONE"
VALID_OPERS = ["+", "-", "*", "/", "^"]

class Token(object):
    def __init__(self, type, value):
        self.type  = type
        self.value = value
    def __str__(self):
        return "{} {}".format(self.type, self.value)

# try to build a Token infering the type of the input
def build_token(tok):
    type = NUMBER
    try:
        value = float(tok)
    except ValueError:
        try:
            value = int(tok)
        except ValueError:
            if tok not in VALID_OPERS:
                raise Exception("UNRECOGNIZED TOKEN '{}'".format(tok))
            value = tok
            type = OPER

    return Token(type, value)

# Lexer
def get_tokens(stream):
    string = stream
    tok = ""
    for c in string:
        if c in VALID_OPERS:
            yield build_token(tok.strip())
            yield build_token(c)
            tok = ""
        else:
            tok += c

    if tok.strip():
        yield build_token(tok.strip())

    return Token(NONE,None)

from math import pow
oper = {
    '+' : lambda x,y: x+y,
    '-' : lambda x,y: x-y,
    '*' : lambda x,y: x*y,
    '/' : lambda x,y: x/y,
    '^' : lambda x,y: pow(x,y)
    }

# Parser
class Parser(object):
    def __init__(self, string):
        self.string = string
    def parse(self):
        gen = get_tokens(self.string)
        first_term = next(gen)
        result = first_term.value
        try:
            while True:
                operator  = next(gen)
                next_term = next(gen)
                result = oper[operator.value](first_term.value, next_term.value)
                first_term = build_token(str(result))
        except StopIteration:
            return result
        return None

def main():
    while True:
        try:
            text = input('calc> ')
        except IOError:
            break
        if not text:
            continue
        parser = Parser(text)
        result = parser.parse()
        print(result)

if __name__ == "__main__":
    main()