Python Dictionary

Accessing Values in Dictionary:

To access dictionary elements, you can use the familiar square brackets along with the key to obtain its value. Following is a simple example −

#!/usr/bin/pythondict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}print "dict['Name']: ", dict['Name']print "dict['Age']: ", dict['Age']

When the above code is executed, it produces the following result −

dict['Name']:  Zaradict['Age']:  7

If we attempt to access a data item with a key, which is not part of the dictionary, we get an error as follows −

#!/usr/bin/pythondict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}print "dict['Alice']: ", dict['Alice']

When the above code is executed, it produces the following result −

dict['Alice']:Traceback (most recent call last):   File "test.py", line 4, in 
   
          print "dict['Alice']: ", dict['Alice'];KeyError: 'Alice'

Updating Dictionary

You can update a dictionary by adding a new entry or a key-value pair, modifying an existing entry, or deleting an existing entry as shown below in the simple example −

#!/usr/bin/pythondict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}dict['Age'] = 8; # update existing entrydict['School'] = "DPS School"; # Add new entryprint "dict['Age']: ", dict['Age']print "dict['School']: ", dict['School']

When the above code is executed, it produces the following result −

dict['Age']:  8dict['School']:  DPS School

Delete Dictionary Elements

You can either remove individual dictionary elements or clear the entire contents of a dictionary. You can also delete entire dictionary in a single operation.

To explicitly remove an entire dictionary, just use the del statement. Following is a simple example −

#!/usr/bin/pythondict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}del dict['Name']; # remove entry with key 'Name'dict.clear();     # remove all entries in dictdel dict ;        # delete entire dictionaryprint "dict['Age']: ", dict['Age']print "dict['School']: ", dict['School']

This produces the following result. Note that an exception is raised because after del dict dictionary does not exist any more −

dict['Age']:Traceback (most recent call last):  File "test.py", line 8, in 
   
        print "dict['Age']: ", dict['Age'];TypeError: 'type' object is unsubscriptable

Note: del() method is discussed in subsequent section.

Properties of Dictionary Keys

Dictionary values have no restrictions. They can be any arbitrary Python object, either standard objects or user-defined objects. However, same is not true for the keys.

There are two important points to remember about dictionary keys −

(a) More than one entry per key not allowed. Which means no duplicate key is allowed. When duplicate keys encountered during assignment, the last assignment wins. For example −

#!/usr/bin/pythondict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'}print "dict['Name']: ", dict['Name']

When the above code is executed, it produces the following result −

dict['Name']:  Manni

(b) Keys must be immutable. Which means you can use strings, numbers or tuples as dictionary keys but something like ['key'] is not allowed. Following is a simple example:

#!/usr/bin/pythondict = {['Name']: 'Zara', 'Age': 7}print "dict['Name']: ", dict['Name']

When the above code is executed, it produces the following result −

Traceback (most recent call last):   File "test.py", line 3, in 
   
          dict = {['Name']: 'Zara', 'Age': 7};TypeError: list objects are unhashable

Built-in Dictionary Functions & Methods −

Python includes the following dictionary functions −

SN	Function with Description
1	Compares elements of both dict.
2	Gives the total length of the dictionary. This would be equal to the number of items in the dictionary.
3	Produces a printable string representation of a dictionary
4	Returns the type of the passed variable. If passed variable is dictionary, then it would return a dictionary type.

Python includes following dictionary methods −

SN	Methods with Description
1	Removes all elements of dictionary dict
2	Returns a shallow copy of dictionary dict
3	Create a new dictionary with keys from seq and values set to value.
4	For key key, returns value or default if key not in dictionary
5	Returns true if key in dictionary dict, false otherwise
6	Returns a list of dict's (key, value) tuple pairs
7	Returns list of dictionary dict's keys
8	Similar to get(), but will set dict[key]=default if key is not already in dict
9	Adds dictionary dict2's key-values pairs to dict
10	Returns list of dictionary dict's values

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