逻辑和调用函数。 帮我理解这段代码

Question

I have some code that is basically a demo for how dict works. My problem is, I am not sure exactly when it is calling the process_order() function. It seems like the main loop ( go_shopping ) never calls it, but the script seems to work. The go_shopping function calls get_item , but that doesn't call process_order either. I am also having trouble with the line if not process_order(order, cart): . What does the if not part mean in this case? Is that where it is calling process_order ? It doesn't seem like it from the print statements, other wise it should print 'if not' when you add an item to the cart dictionary object.

Am I on the right track or missing something simple?

Code:

#!/usr/bin/python3.5

def get_item():
    print("[command] [item] (command is 'a' to add, 'd' to delete, 'q' to quit.)")
    line = input()

    command = line[:1]
    item = line[2:]

    return command, item

def add_to_cart(item, cart):
    if not item in cart:
        cart[item] = 0
    cart[item] += 1

def delete_from_cart(item, cart):
    if item in cart:
        if cart[item] <= 0:
            del cart[item]
        else:
            cart[item] -= 1

def process_order(order, cart):
    command, item = order

    if command == "a":
        add_to_cart(item, cart)
        print('added to cart')
    elif command == "d" and item in cart:
        delete_from_cart(item, cart)
    elif command == "q":
        return False
        print ('end process_order func')
    return True

def go_shopping():
    cart = dict()

while True:
    print ('start main loop')
    order = get_item()
    print ('exited process_order')
        if not process_order(order, cart):
            print ('if not')
            break
    print ('while loop end')
    print (cart)
    print ("Finished!")

go_shopping()

Answer 1

Thing is, I am not really sure of your problem. But you seem concerned with the moment when process_order method is called.

When you write

if not process_order(order, cart)

it must be seen as follows (just adding parentheses):

if (not process_order(order, cart))

So you're asking Python to do something if the condition not process_order(order, cart) is true. So, Python must know the boolean value of the expression not process_order(order, cart) .

This expression is composed of a unary operator, not , and a non-elementary expression, process_order(order, cart) . That latter expression needs to be evaluated, so Python to run the process_order method.

Therefore, when you write if not process_order(order, cart) , the process_order method is indeed executed.