Dictionary in Python

• Dictionary is a data structure in python where data is stored as a key:value pair.
• Python dictionary is a ordered data structure since Python 3.7 i.e it will remember the order of insertion.
• Python dictionaries are mutable, we can change the value of a given key at any point of time.
• Dictionaries do not allow duplicate keys, since it uses hashing concept to store keys.
• Getting a value using key is a constant time operation O(1)

Dictionary Methods

• Given most of the commonly used in built methods in python. Note: The given methods are performed on the below dictionary

dictionary = dict() #declaring a dictionary

1. Inserting Key:Value pairs

   • we don't have any exclisve function to do the job but we have a specific sintax to do it, as shown below,

    dictionary['name'] = 'milk'
    dictionary['color'] = 'white'
    dictionary['source']  = 'cow'
    print(dictionary) 
    # output  :  { 'name' : 'milk', 'color' : 'white', 'source' : 'cow' }
   

2. Accessing a Value using Key

   • There are two ways you can access a value using key
   • In python, there is a method .get() which takes a key as a argument, used to get a value as shown
   • .get() will return None , if key is not found in a dictionary

       value = dictionary.get('name')
       print(value)
       #output: 'milk'
       value = dictionary.get('unknown_key')
       print(value)
       #output : None
   

• The other way to access a value is using square brackets, the major difference between two methods is that it will through error if key is not found in dictionary.
  

      name = dictionary['name']
      print(name)
      #output: 'milk'
      name = dictionary['unknown_key]
      #output : KeyError: 'unknown_key'
  

Methods used while looping a Dictionary

1. .items() :

   • This method generates dict_items which are tuples of key,value pairs. we can use tuple unpacking technique to access the key and values as shown.

       items = dictionary.items()
       for key,value in items:
               print(key , value)
       #output: name milk
                color white
                source cow
   

2. .keys():

   • This method will return dict_keys which are list of keys in a dictionary. we can use loops to access all the keys.

       keys = dictionary.keys()
       for key in keys:
           print(key)
       #output: name
                color
                source
   

3. .values():

   • This method will return dict_values which are list of values in a dictionary. we can use loops to access all the values.

       keys = dictionary.values()
       for value in values:
           print(value)
       #output: 'milk'
                'white'
                 'cow'
   

4. in:

   • It is a keyword which can be used to find weather the key is present or not
   • it will return a boolean value as shown

       boolean = 'name' in dictionary
       print(boolean)
       #output: True
       boolean = 'unknown_key' in dictionary
       print(boolean)
       #output: False
   

5. .update():

   • This will take another dictionary as argument and addes it to the old one

       new_dict = { 'food' : 'grass' }
       dictionary.update(new_dict)
       print(dictionary)
       #output: {'name': 'milk', 'color': 'white', 'source': 'cow', 'food': 'grass'}
   

6. .copy():

   • This method copies the entire dictionary to the new one

        new_dict = dictionary.copy()
        print(new_dict)
        #output: {'name': 'milk', 'color': 'white', 'source': 'cow', 'food': 'grass'}
   

7. Update using Square Brackets

   • We can append a key value pair in to a dictionary or we can update a value using the key as shown.

       dictionary['source'] = 'buffelo'
       dictionary['year'] = '2015'
       print(dictionary)
       #output: {'name': 'milk', 'color': 'white', 'source': 'buffelo', 'food': 'grass'      'year': '2015'}
   

8. .pop('key'):

   • This method will take key as a argument and will remove the key value pair from dictionary as shown
   • It will return a value of the key that was poped
   • This will through KeyError if key is not found

       print(dictionary.pop('food'))
       #output: 'grass'
       print(dictionary.pop('unknown_key'))
       #output: unknown_key : 'unknown_key'
   

9. .popitem():

   • This method will pop the last item that was inserted and will return a tuple of key and value
   • This will return KeyError if dictionary is empty

       print(dictionary.popitem())
       #output:('year', '2015')
   

10. .clear():

    • This method will clear the dictionary and returns None

        dictionary.clear() 
        print(dictionary)
        #output: {}
    

11. defaultdict:

    • This function can be imported from collections module in python
    • this will take a customized function reference as input, when ever tha key is not present in dictionary it will exicute the custom function.

        from collections import defaultdict
    
        def default_fun():
            print('key is not present')
    
        dct = defaultdict(default_fun)
        print(dct['unknown_key'])
        #output: key is not present
    

References:

• Python Docs
• w3 Schools