Searching dict within a list

Question

If I have the following list of items for a shopping store:

shop_list = [{'item': 'apple', 'amount': 10, 'cost': 5}, 
             {'item': 'banana', 'amount': 12, 'cost': 6}, 
             {'item': 'strawberry', 'amount': 8, 'cost': 9}]

So I have several dicts within a list. I want to find out how to get the item dict knowing the item. For example:

def x(item)
    #do something to get the dict
    print dict
x('apple')          #print {'item': 'apple', 'amount': 10, 'cost': 5}
x('banana')          #print {'item': 'banana', 'amount': 12, 'cost': 6}

What's the shortest, most efficient way to do this?

Answer 1

If you intend to lookup entries by their 'item' , then you should consider having a dict which keys are the 'item' instead of a list of dict .

shop_list = {
    'apple': {'amount': 10, 'cost': 5},
    'banana': {'amount': 12, 'cost': 6},
    'strawberry': {'amount': 8, 'cost': 9}
}

shop_list['banana'] # {'amount': 10, 'cost': 5}

In particular, this makes the lookup O(1) instead of the O(n) required for traversing the list .

If you cannot update the code that generated the original shop_list , then you can transform the already existing data with a dict-comprehension.

formatted_shop_list = {product['item']: product for product in shop_list}

Answer 2

def x(shop_list, item): # remove first parameter if you want to use global variable
    for i in shop_list:
        if i['item'] == item:
            return i

Then, you can call this function as:

>>> todays_shop_list = [{'item': 'apple', 'amount': 10, 'cost': 5}, 
...                     {'item': 'banana', 'amount': 12, 'cost': 6}, 
...                     {'item': 'strawberry', 'amount': 8, 'cost': 9}]

>>> x(todays_shop_list, 'apple')
{'item': 'apple', 'amount': 10, 'cost': 5}

Answer 3

You can try iterating through the list and just extract the dict that matches your shop item:

def shop_item(shop_list, item):
    return next((x for x in shop_list if x['item'] == item), None)
    # or next(filter(lambda x: x['item'] == item, shop_list), None)

Which works as follows:

>>> shop_list = [{'item': 'apple', 'amount': 10, 'cost': 5},
...              {'item': 'banana', 'amount': 12, 'cost': 6},
...              {'item': 'strawberry', 'amount': 8, 'cost': 9}]
>>> shop_item(shop_list, 'apple')
{'item': 'apple', 'amount': 10, 'cost': 5}
>>> shop_item(shop_list, 'grape')
None

The above uses next() with a generator expression to iterate though the list until the condition is met, and returns None if item is not found.

Answer 4

You can try this:

def x(item):
    return [elements for elements in shop_list if elements['item'] == item]

x('apple')          #print {'item': 'apple', 'amount': 10, 'cost': 5}
x('banana')          #print {'item': 'banana', 'amount': 12, 'cost': 6}

This will return the list item if found

[{'amount': 12, 'cost': 6, 'item': 'banana'}]

and if the result is not found an empty list will be returned.