Introduction to Dictionaries 

The data type dictionary falls under mapping. It is a mapping between a set of keys and a set of values. The key-value pair is called an item. A key is separated from its value by a colon (:) and consecutive items are separated by commas. Items in dictionaries are unordered, so we may not get back the data in the same order in which we had entered the data initially in the dictionary. 

Creating a Dictionary 

To create a dictionary, the items entered are separated by commas and enclosed in curly braces. Each item is a key value pair, separated through colon (:). The keys in the dictionary must be unique and should be of any immutable data type like number, string or tuple. The values can be repeated and can be of any data type. Let us take few examples of creating dictionaries. 

#dict1 is an empty Dictionary created 

#curly braces are used for dictionary

 >>> dict1 = {} 

>>> dict1

 {}

 #dict2 is an empty dictionary created using

 #built-in function

 >>> dict2 = dict()

 >>> dict2 {} 

#dict3 is the dictionary that maps names 

#of the students to respective marks in #percentage

 >>> dict3 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85} 

>>> dict3 

{'Mohan': 95, 'Ram': 89, 'Suhel': 92, 'Sangeeta': 85 

Accessing Items in a Dictionary 

We have already seen that the items of a sequence (string, list and tuple) are accessed using a technique called indexing. The items of a dictionary are accessed via the keys rather than via their relative positions or indices. Each key serves as the index and maps to a value. The following example shows how a dictionary returns the value corresponding to the given key: >>> dict3 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85} >>> dict3['Ram'] 89 >>> dict3['Sangeeta'] 85 #the key does not exist >>> dict3['Shyam'] KeyError: 'Shyam' In the above examples the key 'Ram' always maps to the value 89 and key 'Sangeeta' always maps to the value 85. So, the order of items does not matter. If the key is not present in the dictionary we get KeyError. 

Adding a new item 

We can add a new item to the dictionary as shown in the following example: >>> dict1 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85} >>> dict1['Meena'] = 78 >>> dict1 {'Mohan': 95, 'Ram': 89, 'Suhel': 92,'Sangeeta': 85, 'Meena': 78} 

Modifying an Existing Item 

The existing dictionary can be modified by just overwriting the key-value pair. Example to modify a given item in the dictionary: 

>>> dict1 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85} 

#Marks of Suhel changed to 93.5

 >>> dict1['Suhel'] = 93.5

 >>> dict1 {'Mohan': 95, 'Ram': 89, 'Suhel': 93.5, 'Sangeeta': 85}

 Dictionary Operations

 Except membership, other operations like concatenation, repetition and slicing are not supported by dictionaries. Let us learn membership operation on dictionary data type in Python programming.

 Membership

 The membership operator ‘in’ checks if the key is present in the dictionary and returns True, else it returns False. 

>>> dict1 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85} 

>>> 'Suhel' in dict1

 True The not inoperator returns True if the key is not present in the dictionary, else it returns False. 

>>> dict1 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85} 

>>> 'Suhel' not in dict1 

False