calc.py

#expr = term + expr | term - expr | term
#term = term * factor | term / factor | factor
#factor = '(' expr ')' | number
#

def getch(expr, ch):
	while expr[0] == ' ':
		expr = expr[1:]

	if ch.find(expr[0]) == -1:
		raise "expected %s" % ch

	return expr[1:], expr[0]

def calc_factor(expr):
	s = 0
	if expr[0] == '(':
		expr,ch = getch(expr, '(')
		expr, e = calc_expr(expr)
		expr,ch = getch(expr, ')')
		return expr, e
	while expr[0] >= '0' and expr[0] <= '9':
		s = s * 10 + int(expr[0])
		expr = expr[1:]
	return expr,s

def calc_term(expr):
	expr, f = calc_factor(expr)
	if '*/'.find(expr[0]) != -1:
		expr,op = getch(expr, '*/')
		expr, f2 = calc_term(expr)
		if op == '*':
			return expr, f * f2
		elif op == '/':
			return expr, f / f2
	return expr,f

def calc_expr(expr):
	expr,sum =  calc_term(expr)
	if '+-'.find(expr[0]) != -1:
		expr,op = getch(expr, '+-')
		expr,t2 =  calc_expr(expr)
		if op == '+':
			return expr, sum +t2
		elif op == '-':
			return expr, sum - t2
	return expr, sum


def calc(expr):
	return calc_expr(expr + '#')[1]

print(calc("1+2+3+4+5+6+7+8+9+10"))
print(calc("(1+2)*4"))
print(calc("222/111"))