A simple python calculator to demo recursive descent parsing. Execute the script to use the calculator. It accepts only well formed input. Use parentheses to specify operator precedence.

calc.py

"""
exp ::= term  | exp + term | exp - term
term ::= factor | factor * term | factor / term
factor ::= number | ( exp )
"""

class Calculator():
    def __init__(self, tokens):
        self._tokens = tokens
        self._current = tokens[0]
    
    def exp(self):
        result = self.term()
        while self._current in ('+', '-'):
            if self._current == '+':
                self.next()
                result += self.term()
            if self._current == '-':
                self.next()
                result -= self.term()
        return result

    def factor(self):
        result = None
        if self._current[0].isdigit() or self._current[-1].isdigit():
            result = float(self._current)
            self.next()
        elif self._current is '(':
            self.next()
            result = self.exp()
            self.next()
        return result

    def next(self): 
        self._tokens = self._tokens[1:]
        self._current = self._tokens[0] if len(self._tokens) > 0 else None

    def term(self):
        result = self.factor()
        while self._current in ('*', '/'):
            if self._current == '*':
                self.next()
                result *= self.term()
            if self._current == '/':
                self.next()
                result /= self.term()
        return result

if __name__ == '__main__':
    while True:
        lst = list(raw_input('> ').replace(' ', ''))
        tokens = []
        for i in range(len(lst)):
            if lst[i].isdigit() and i > 0 and  (tokens[-1].isdigit() or tokens[-1][-1] is '.'):
                tokens[-1] += lst[i]          
            elif lst[i] is '.' and i > 0 and tokens[-1].isdigit():
                tokens[-1] += lst[i]
            else:
                tokens.append(lst[i])
        print Calculator(tokens).exp() 

I didn't try it, but looking at the code I'd say that it cannot handle things like "1+2-3+4-5" because expects a sequences of zero or more "+" followed by a sequence of zero or more "-". Ditto for "*" and "/".

Instead of two consecutive while loops should do one while loop accepting all operators of the same precedence.

Fixed.

Should it not be
"exp ::= term | term + term | term - term"
instead of
"exp ::= term | exp + term | exp - term" in the docstring?

 ...
 while True:
        tokens = re.findall(r'[\d.]+|[+-/*()]', input('> ').replace(' ', ''))
        print Calculator(tokens).exp() 

It cannot handle with things like 12 x 21 / 13