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Python - Dictionary
Neha Kumawat
a year ago
Table of Contents
- Introduction
- Accessing Values in Dictionary
- Updating Dictionary
- Delete Dictionary Elements
- Properties of Dictionary Keys
Introduction
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, like this: {}.
Accessing Values in Dictionary
To access dictionary elements, you can use the familiar square brackets along with the key to obtaining its value. Following is a simple example −
#!/usr/bin/python
dict = {'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']: 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']
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/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']
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 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/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 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, the 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/python
dict = {'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/python
dict = {['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
I hope you enjoyed reading this article and finally, you came to know about Python - Dictionary. For more such blogs/courses on data science, machine learning, artificial intelligence and emerging new technologies do visit us at InsideAIML.
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