Python tutorial #5 | Dictionary

A dictionary is mutable and is another container type that can store any number of Python objects, including other container types. Dictionaries consist of pairs (called items) of keys and their corresponding values.

Python dictionaries are also known as associative arrays or hash tables. The general syntax of a dictionary is as follows:

dict = {'Alice': '2341', 'Beth': '9102', 'Cecil': '3258'}

Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. An empty dictionary without any items is written with just two curly braces, like this: {}.

Keys are unique within a dictionary while values may not be. The values of a dictionary can be of any type, but the keys must be of an immutable data type such as strings, numbers, or tuples.

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:

dict = {'Name': 'pranav', 'Age': 19, 'Class': 'First'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])

Output:

dict['Name']: pranav
dict['Age']: 19

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

Code segment with missing key:

dict = {'Name': 'pranav', 'Age': 19, 'Class': 'First'}
print ("dict['Alice']: ", dict['Alice'])

Output with error message:

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


Updating Dictionary:

We can update a dictionary by adding a new entry or item (i.e., a key-value pair), modifying an existing entry, or deleting an existing entry as shown below in the simple example:

dict = {'Name': 'pranav', 'Age': 19, 'Class': 'First'};

dict['Age'] = 20; # update existing entry
dict['School'] = "RAIT"; # Add new entry


print "dict['Age']: ", dict['Age'];
print "dict['School']: ", dict['School'];


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

dict['Age']: 20
dict['School']: RAIT

Delete Dictionary Elements:

We 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:

dict = {'Name': 'pranav', 'Age': 19, 'Class': 'First'};

del dict['Name']; # remove entry with key 'Name'
dict.clear();     # remove all entries in dict
del dict ;        # delete entire dictionary

print "dict['Age']: ", dict['Age'];
print "dict['School']: ", dict['School'];



Output with error

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