如何在python字典中编辑可变的键

Question

I am working on a code in python and relatively new at python so excuse me if the title of this post is unclear, am currently unable to figure out how to solve this issue where I have a blank dictionary

Cart={}

And I have a few lines of code that adds an variable to it if selected

if x==1:
   Cart['Milk']=[qnty],[total],[gst],[offer]
if x==2:
   Cart['Butter']=[qnty],[total],[gst],[offer]
if x==3:
   Cart['Cookies']=[qnty],[total],[gst],[offer]

Here's the part where I am unable to figure out, I would like to now edit the qnty part of the variable but as there is no 'fix' key in the dictionary i am unsure how to do it, I tried doing it the way below but will only give me the name and qnty and removes the other key.

if y == 2:
   print(Cart)
   item=str(input('key in an item to edit: '))
   for item in Cart:
         qnty=int(input('Key in the quantity of %s you want: '%item))
         Cart=[item],[qnty]
   print(shopping_basket)

Is there a way to edit the Qnty of the item only? I have also tried using list or assigning fixed key to the blank dictionary but just can't seem to work.

Thank you in advance feel free to ask me more if clarifications is needed.

Answer 1

Let's start with sample data in cart we have two items Cake and Milk

cart = {}
cart['Cake'] = {'qnty': 1, 'total': 1,'gst': None, 'offer': False}
cart['Milk'] = {'qnty': 3, 'total': 3,'gst': None, 'offer': True}

If you want to change qnty , you must set its variable by receiving an input from a user or set it internally

edit_item_attr = 'qnty'

Then you can ask which item to be edited, for example, Cake

# Item Selection
item_list = cart.keys()
print(f'Please select an item to Edit from {item_list}')
edit_item_name = 'Cake'

Finally, you can update the item's data

# Edit Method
cart.get(edit_item_name).update({edit_item_attr: 99999})

Result

'Cake': {'qnty': 99999, 'total': 1, 'gst': None, 'offer': False},
 'Milk': {'qnty': 3, 'total': 3, 'gst': None, 'offer': True}}

Answer 2

If I understood the problem correctly, the faulty line is Cart=[item],[qnty] . I guess you want to update the item to set the new quantity. If so, you could probably do it this way:

Cart[item] = (
   [qnty],
   Cart[item][1],
   Cart[item][2],
   Cart[item][3],
)

Here, you are working with tuples, which are immutable, so the best is to actually replace it completely, and just set the new value that you want

Answer 3

Your approach is flawed...

a) you use a tuple where you probably don't want a tuple. Tuples are immutable.

So rather than

Cart['Milk']= [qnty],[total],[gst],[offer]
             ^                            ^  you have invisible braces here
                                             due to the commas

you probably wanted

Cart['Milk']=[qnty, total, gst , offer]

But that's still a strange concept, because someone reading the code and accessing the array would need to know in which order the items appear. He might easily confuse quantity with total.

We don't use arrays to store different kind of data. We normally use arrays for the same kind of data. Semantically and type similar, not only type similar.

Why? Well, with an array you can do things like

sum(array)

which is really cool if we sum up prices or lenghts or energy - as long as they have the same unit. But in your case, what is the meaning if you calculate the sum of

sum([qnty, total, gst, offer])

Right: it's not even possible, because these things in the array have different units: no unit, currency, percentage and something.

b) you want to update the quantity. But you don't want to update the total? That way, things will get cheaper with every item added to the cart. The total should not be a stored value, it should be a calculated value.

Create a class that does this for you, like

class CartEntry:
    def __init__(self, quantity, gst, offer, price):
        self.quantity = quantity
        self.gst = gst
        self.offer = offer
        self.price = price

    @property
    def total(self):
        return self.quantity * self.price  # TODO: consider special offers 

c) you have duplicate code. Do you recognize that there are lines very similar to each other?

Cart['Milk']   =[qnty],[total],[gst],[offer]
Cart['Butter'] =[qnty],[total],[gst],[offer]
Cart['Cookies']=[qnty],[total],[gst],[offer]

If you work at a large shop, you will not finish programming ever if you go on like that. If you need a mapping from an integer to a string, use a dictionary like

names = {1:"Milk", 2:"Butter", 3:"Cookies"}

(and don't code that, but get it from a file or database instead)

That way you can do it all in one line:

Cart[names[x]] = [qnty],[total],[gst],[offer]

d) Why store items in the cart by name? They seem to have an ID already. Just use the ID. There's no need for the dictionary key to be human readable.

How does all this help you? You get IDE support and it prevents bugs:

Answer 4

The method you are using creates tuples.

Try this.

Cart[ "milk" ] = [ [qnty],[total],[gst],[offer] ]

Then you can access and modify items at will.

print( Cart[ "milk" ][ 0 ] )
Cart[ "milk" ][ 0 ] = [ 45 ]
print( Cart[ "milk" ][ 0 ] )

It would be better if you created dictionaries instead of lists like this.

Cart[ "milk" ] = dict( Quantity = qnty, Total = total, GST = gst, Offers = offer )

Then you could access data like so...

print( Cart[ "milk" ][ "Total" ] )
Cart[ "milk" ][ "Total" ] = 66
print( Cart[ "milk" ][ "Total" ] )