Python - 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'}

You can create dictionary in the following way as well:

dict1 = { 'abc': 456 }; dict2 = { 'abc': 123, 98.6: 37 };

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 use the familiar square brackets along with the key to obtain its value:

Example:

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

This will produce 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:

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

This will produce following result:

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

Updating Dictionary:

You 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:

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'];

This will produce following result:

dict['Age']: 8 dict['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:

Example:

#!/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'];

This will produce following result. Note an exception raised, this is because afterdel dict dictionary does not exist any more:

dict['Age']: Traceback (most recent call last): File "test.py", line 8, in <module> 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.

Example:

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

This will produce 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.

Example:

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

This will produce following result. Note an exception raised:

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

Built-in Dictionary Functions & Methods:

Python includes following dictionary functions

SN	Function with Description
1	cmp(dict1, dict2) Compares elements of both dict.
2	len(dict) Gives the total length of the dictionary. This would be equal to the number of items in the dictionary.
3	str(dict) Produces a printable string representation of a dictionary
4	type(variable) 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	dict.clear() Removes all elements of dictionary dict
2	dict.copy() Returns a shallow copy of dictionary dict
2	dict.fromkeys() Create a new dictionary with keys from seq and values set to value.
3	dict.get(key, default=None) For key key, returns value or default if key not in dictionary
4	dict.has_key(key) Returns true if key in dictionary dict, false otherwise
5	dict.items() Returns a list of dict's (key, value) tuple pairs
6	dict.keys() Returns list of dictionary dict's keys
7	dict.setdefault(key, default=None) Similar to get(), but will set dict[key]=default if key is not already in dict
8	dict.update(dict2) Adds dictionary dict2's key-values pairs to dict
9	dict.values() Returns list of dictionary dict2's values