如何从词典列表中修改词典的值

Question

I am declaring a method named add_to_cart (db, itemid, quantity). Whenever the method is called, it will look into the database for the session data. Session data contains a list of dictionaries. The purpose of this method is to create a new entry (dictionary) to the list or update the value of existing entry. Dictionary has the following keys: id, quantity

So far I have developed the following code. First of all after fetching the data from the database, I am matching the itemid with the dictionary key: 'id'. If the itemid does not match with any of the values of the dictionaries, then it will append a new dictionary to that list.

def add_to_cart(db, itemid, quantity):
    # ......
    row = cursor.fetchone()
    if row is not None:
        cart = json.loads(row['data'])
        for dic in cart:
            if str(dic.get("id")) == str(itemid):
                dic['quantity'] = int(dic['quantity']) + quantity
                data = json.dumps(cart)
                # update the 'data' to the database
                break
     else:
         if counter == len(cart):
              item = {
                      'id': itemid,
                      'quantity': quantity
                     }
              cart.append(item)
              data = json.dumps(cart)  
              # update the 'data' to the database
              break

Let the initial cart is like:

[{'id': '40', 'quantity': '2'}, {'id': '41', 'quantity': '5'}]

When I add 1 more of the item 40 to the cart, this should be like:

[{'id': '40', 'quantity': '3'}, {'id': '41', 'quantity': '5'}]

but I am getting :

[{'id': '40', 'quantity': '2'}, {'id': '41', 'quantity': '5'}, {'id': '40', 'quantity': '1'}]

Answer 1

You are adding a new dictionary to the list when you do cart.append(item) , hence the list
[{'id': '40', 'quantity': '2'}, {'id': '41', 'quantity': '5'}]

ends up being

[{'id': '40', 'quantity': '2'}, {'id': '41', 'quantity': '5'}, {'id': '40', 'quantity': '1'}]

But you want to find the matching id in that list of dictionaries, and add to the quantity for that dictionary.

So the code will look like as below:

li = [{'id': '40', 'quantity': '2'}, {'id': '41', 'quantity': '5'}]

def add_elem(li, id, to_add):

    #Iterate over the dictionaries
    for item in li:
        #If the id is found
        if str(id) in item.values():
            #Increment the quantity
            item['quantity'] = str(int(item['quantity']) + to_add)

    #Return the updated list
    return li

print(add_elem(li, 40, 1))

The output will be

[{'id': '40', 'quantity': '3'}, {'id': '41', 'quantity': '5'}]

Answer 2

The problem seems to be that you are going simply adding a new dict to the list (cart), through append . You need to go through the list, find the dict with the itemid you need, then add to the quantity .

Try this -

for dict in cart:
     if dict[itemid] == itemid:
        dict['quantity'] += str(quantity)
        break

item = {
        'id': itemid,
        'quantity': quantity
       }
cart.append(item)