tokens = [('number', 2), ('binop+',), ('number', 3), ('binop*',),
          ('(',), ('number', 4), ('binop+',), ('unop-',), ('number', 3), (')',)]


def eval_multiplies():
    global tokens

    print("Started eval_multiplies: tokens={}".format(tokens))

    val = eval_adds()
    while len(tokens) > 0:
        if tokens[0][0] == 'binop*':
            tokens = tokens[1:]
            val2 = eval_adds()
            val = val * val2
        elif tokens[0][0] == 'binop/':
            tokens = tokens[1:]
            val2 = eval_adds()
            val = val / val2
        else:
            break

    print("Returning {}".format(val))
    return val

def eval_adds():
    global tokens

    print("Started eval_adds:       tokens={}".format(tokens))

    val = eval_unops()
    while len(tokens) > 0:
        if tokens[0][0] == 'binop+':
            tokens = tokens[1:]
            val2 = eval_unops()
            val = val + val2
        elif tokens[0][0] == 'binop-':
            tokens = tokens[1:]
            val2 = eval_unops()
            val = val - val2
        else:
            break

    print("Returning {}".format(val))
    return val

def eval_unops():
    global tokens

    print("Started eval_unops:      tokens={}".format(tokens))

    inverse = False
    while len(tokens) > 0:
        if tokens[0][0] == 'unop-':
            inverse = not inverse
            tokens = tokens[1:]
        else:
            break

    val = eval_factor()
    if inverse: val = -val
    print("Returning {}".format(val))
    return val

def eval_factor():
    global tokens

    print("Started eval_factor:     tokens={}".format(tokens))

    if tokens[0][0] == 'number':
        val = tokens[0][1]
        tokens = tokens[1:]
        print("Returning {}".format(val))
        return val

    if tokens[0][0] == '(':
        tokens = tokens[1:]
        val = eval_multiplies()
        assert tokens[0][0] == ')' # Report error, missing )
        tokens = tokens[1:]
        print("Returning {}".format(val))
        return val

    assert False # Report error, weird expression found





val = eval_multiplies()
print(val)