Python Dictionary

• Python dictionary is used to store key-value pairs.

• you can define dictionary in curly bracket or by calling dict object and can assign key values to it

>>>items = dict() # or you can define like items = {}
items = {
    'name': 'milk powder',
    'qty':1,
    'price': 200.0,
    'description': 'some descriptions',
    'related_items': [
        #... add all related items
    ]
}

• Dictionaries doesn’t allow duplicate keys which means if you add a duplicate key then the last duplicate value will be present in the dictionary

items = {
    'name': 'milk powder',
    'qty':1,
    'price': 200.0,
    'price': 400.0,
    'description': 'some descriptions',
    'related_items': [
        #... add all related items
    ]
}

so the output of above dictionary will be

>>> items
{'name': 'milk powder', 
 'qty': 1, 
 'price': 400.0, 
 'description': 'some descriptions', 
 'related_items': []
 }

As you can see the duplicate key price value is updated with the last duplicate value.

Access Dictionary items:

you can access dictionary items with the square bracket and key name. For example, if you want to access the names of our items then you can do it by following

>>> items['name']
'milk powder'

Dictionary is Changeable:

Dictionary is changeable which means you can change value dictionary. For example, Suppose you want to change the name of items to some other name then you can do it by following

>>> items['name'] = 'Sugar pack of 1 KG'
>>> items
{'name': 'Sugar pack of 1 KG', 'qty': 1, 'price': 400.0, 'description': 'some descriptions', 'related_items': []}

Calculate the length of dictionary:

you can use calculate the length of the dictionary by using len function

>>> print(len(items))
5

Sort dictionary by key:

>>> items = dict(sorted(items.items()))
>>> items
{'description': 'some descriptions', 'name': 'milk powder', 'price': 400.0, 'qty': 1, 'related_items': []}

Sort dictionary by values:

here few things to note

1. If the dictionary value is numeric then you can simply write the following code. It will sort dictionary items by ascending order by value

>>> items = dict(sorted(items.items(), key=lambda x:x[1]))

if you want to sort descending order by value

>>> items = dict(sorted(items.items(), key=lambda x:x[1], reverse=True))

2. If the dictionary is a string then you have to write additional comparator, if you just write with lambda function then you will get some error as shown

>>> items = dict(sorted(items.items(), key=lambda x:x[1]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'float' and 'str'

Please note: dictionary value must be of the same type if you want to sort it.

Now take an example you want to sort the following dictionary

user_items_mapping={
    'abc':4,
    'xyz':44,
    'pqr':33
}

>>> dict(sorted(user_items_mapping.items(), key=lambda x:x[1]))
{'abc': 4, 'pqr': 33, 'xyz': 44}

If you want to sort in descending order then add reverse=True

>>> dict(sorted(user_items_mapping.items(), key=lambda x:x[1], reverse=True))
{'xyz': 44, 'pqr': 33, 'abc': 4}

Now solve one Leetcode question to understand the whole python dictionary concept practically

class Solution:
    def frequencySort(self, nums: List[int]) -> List[int]:
        items = {}
        
        # Count frequency of elements
        for ele in nums:
            if ele in items:
                items[ele] += 1
            else:
                items[ele] = 1
        
        # Sort elements in decending by key
        items = dict(sorted(items.items(), reverse=True)) # sort by a
        # Now sort elements in ascending order by frequency which means value
        items = dict(sorted(items.items(), key=lambda x: x[1]))

        # Now add final dictionary to result array and return
        res = []
        for key in items:
            res += [key]*items[key]
        return res