Very simple shunting-yard parser for converting in-fix to post-fix.

expr.py

def postfix(tokens):
    output_stack = []
    op_stack = []
    for token in tokens:
        if is_number(token):
            output_stack.append(token)
        elif token in ['+', '-', '*', '/']:
            while len(op_stack) > 0:
                if (token in ['+', '-'] and op_stack[-1] in ['*', '/']) or (token in ['*', '/'] and op_stack[-1] in ['*', '/']):
                    output_stack.append(op_stack.pop())
                else:
                    break
            op_stack.append(token)
    output_stack += reversed(op_stack)
    return output_stack


def is_number(s):
    try:
        float(s)
        return True
    except ValueError:
        return False

print postfix(['3', '+', '4', '*', '2', '-', '5'])