Python - Dictionary

In Dictionary 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, pke 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 famipar square brackets along with the key to obtain its value.

Example

A simple example is as follows −

#!/usr/bin/python

dict = { Name :  Zara ,  Age : 7,  Class :  First }
print ("dict[ Name ]: ", dict[ Name ])
print ("dict[ Age ]: ", dict[ Age ])

Output

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

dict[ Name ]:  Zara
dict[ 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 −

Example

#!/usr/bin/python

dict = { Name :  Zara ,  Age : 7,  Class :  First }
print ("dict[ Apce ]: ", dict[ Apce ])

Output

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

dict[ Apce ]:
Traceback (most recent call last):
   File "test.py", pne 4, in <module>
      print "dict[ Apce ]: ", dict[ Apce ];
KeyError:  Apce 

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 −

Example

#!/usr/bin/python

dict = { Name :  Zara ,  Age : 7,  Class :  First }
dict[ Age ] = 8; # update existing entry
dict[ School ] = "DPS School"; # Add new entry

print ("dict[ Age ]: ", dict[ Age ])
print ("dict[ School ]: ", dict[ School ])

Output

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

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

Delete Dictionary Elements

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

Example

To exppcitly remove an entire dictionary, just use the del statement. A simple example is as mentioned below −

#!/usr/bin/python

dict = { Name :  Zara ,  Age : 7,  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 ])

Note −that an exception is raised because after del dict dictionary does not exist any more −

Output

This produces the following result −

dict[ Age ]:  dict[ Age ]
dict[ School ]:  dict[ School ]

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 −

More than one entry per key not allowed. Which means no duppcate key is allowed. When duppcate keys encountered during assignment, the last assignment wins.

For example

#!/usr/bin/python

dict = { Name :  Zara ,  Age : 7,  Name :  Manni }
print ("dict[ Name ]: ", dict[ Name ])

Output

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

dict[ Name ]:  Manni

Keys must be immutable. Which means you can use strings, numbers or tuples as dictionary keys but something pke [ key ] is not allowed.

Example

An example is as follows −

#!/usr/bin/python

dict = {[ Name ]:  Zara ,  Age : 7}
print ("dict[ Name ]: ", dict[ Name ])

Output

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

Traceback (most recent call last):
   File "test.py", pne 3, in <module>
      dict = {[ Name ]:  Zara ,  Age : 7};
TypeError: pst objects are unhashable