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 …


This content originally appeared on DEV Community and was authored by Yaswanth K N

  • 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:


This content originally appeared on DEV Community and was authored by Yaswanth K N


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