Indexing and Slicing

Indexing and Slicing is another important part of Python programming as it helps in managing different objects involved in functions. Learn more here.

Paramer Description
The object that you want to extract a “sub-object” from
The index of that you want the sub-object to start from (keep in mind that Python is zero-indexed,
meaning that the first item of has an index of ). If omitted, defaults to .
obj 0 0

    The (non-inclusive) index of            that you want the sub-object to end at. If omitted, defaults to     .

obj len(obj)
Allows you to select only every item. If omitted, defaults to .
step 1

Indexing and Slicing: Basic Slicing

For any iterable (for eg. a string, list, etc), Python allows you to slice and return a substring or sublist of its data.

Format for slicing:



start is the first index of the slice. Defaults to 0 (the index of the first element)

stop one past the last index of the slice. Defaults to len(iterable)

step is the step size (better explained by the examples below)

a = "abcdef"
a # "abcdef"
# Same as a[:] or a[::] since it uses the defaults for all three indices
a[-1] # "f"
a[:] # "abcdef"
a[::] # "abcdef"
a[3:] # "def" (from index 3, to end(defaults to size of iterable))
a[:4] # "abcd" (from beginning(default 0) to position 4 (excluded))
a[2:4] # "cd" (from position 2, to position 4 (excluded))

In addition, any of the above can be used with the step size defined:

a[::2] # "ace" (every 2nd element)
a[1:4:2] # "bd" (from index 1, to index 4 (excluded), every 2nd element)

Indices can be negative, in which case they’re computed from the end of the sequence

a[:-1] # "abcde" (from index 0 (default), to the second last element (last element - 1))
a[:-2] # "abcd" (from index 0 (default), to the third last element (last element -2))
a[-1:] # "f" (from the last element to the end (default len())

Step sizes can also be negative, in which case slice will iterate through the list in reverse order:

a[3:1:-1] # "dc" (from index 2 to None (default), in reverse order)

This construct is useful for reversing an iterable

a[::-1] # “fedcba” (from last element (default len()-1), to first, in reverse order(-1))

Notice that for negative steps the default end_index is None (see )

a[5:None:-1] # "fedcba" (this is equivalent to a[::-1])
a[5:0:-1] # "fedcb" (from the last element (index 5) to second element (index 1)

Reversing an object

You can use slices to very easily reverse a str, list, or tuple (or basically any collection object that implements slicing with the step parameter). Here is an example of reversing a string, although this applies equally to the other types listed above:

s = 'reverse me!'
s[::-1] # '!em esrever'

Let’s quickly look at the syntax. [::-1] means that the slice should be from the beginning until the end of the string (because start and end are omitted) and a step of -1 means that it should move through the string in reverse.

Indexing and Slicing: Slice assignment

Another neat feature using slices is slice assignment. Python allows you to assign new slices to replace old slices of a list in a single operation.

This means that if you have a list, you can replace multiple members in a single assignment:

lst = [1, 2, 3]
lst[1:3] = [4, 5]
print(lst) # Out: [1, 4, 5]

The assignment shouldn’t match in size as well, so if you wanted to replace an old slice with a new slice that is different in size, you could:

lst = [1, 2, 3, 4, 5]
lst[1:4] = [6]
print(lst) # Out: [1, 6, 5]

It’s also possible to use the known slicing syntax to do things like replacing the entire list:

lst = [1, 2, 3]
lst[:] = [4, 5, 6]
print(lst) # Out: [4, 5, 6]

Or just the last two members:

lst = [1, 2, 3]
lst[-2:] = [4, 5, 6]
print(lst) # Out: [1, 4, 5, 6]

Making a shallow copy of an array

A quick way to make a copy of an array (as opposed to assigning a variable with another reference to the original array) is:


Let’s examine the syntax. [:] means that start, end, and slice are all omitted. They default to 0, len(arr), and 1, respectively, meaning that subarray that we are requesting will have all of the elements of arr from the beginning until the very end.

In practice, this looks something like:

arr = ['a', 'b', 'c']
copy = arr[:]
print(arr) # ['a', 'b', 'c', 'd']
print(copy) # ['a', 'b', 'c']

As you can see, arr.append(‘d’) added d to arr, but copy remained unchanged!

Note that this makes a shallow copy, and is identical to arr.copy().

Indexing and Slicing: Indexing custom classes: getitem, setitem and delitem

class MultiIndexingList:
def init(self, value):
self.value = value
def repr(self):
return repr(self.value)
def getitem(self, item):
if isinstance(item, (int, slice)):
return self.class(self.value[item])
return [self.value[i] for i in item]
def setitem(self, item, value):
if isinstance(item, int):
self.value[item] = value
elif isinstance(item, slice):
raise ValueError('Cannot interpret slice with multiindexing')
for i in item:
if isinstance(i, slice):
raise ValueError('Cannot interpret slice with multiindexing')
self.value[i] = value
def delitem(self, item):
if isinstance(item, int):
del self.value[item]
elif isinstance(item, slice):
del self.value[item]
if any(isinstance(elem, slice) for elem in item):
raise ValueError('Cannot interpret slice with multiindexing')
item = sorted(item, reverse=True)
for elem in item:
del self.value[elem]
This allows slicing and indexing for element access:
a = MultiIndexingList([1,2,3,4,5,6,7,8])

Out: [1, 2, 3, 4, 5, 6, 7, 8]

Out: [2, 6, 3, 7, 2] a[4, 1, 5:, 2, ::2]
Out: [5, 2, [6, 7, 8], 3, [1, 3, 5, 7]]
4|1-|----50:---|2-|-----::2----- <-- indicated which element came from which index

While setting and deleting elements only allows for comma separated integer indexing (no slicing):

a[4] = 1000
Out: [1, 2, 3, 4, 1000, 6, 7, 8] a[2,6,1] = 100
Out: [1, 100, 100, 4, 1000, 6, 100, 8] del a[5]
Out: [1, 100, 100, 4, 1000, 100, 8] del a[4,2,5]
Out: [1, 100, 4, 8]

Indexing and Slicing: Basic Indexing

Python lists are 0-based i.e. the first element in the list can be accessed by the index 0

arr = ['a', 'b', 'c', 'd']

You can access the second element in the list by index 1, third element by index 2 and so on:


You can also use negative indices to access elements from the end of the list. eg. index -1 will give you the last element of the list and index -2 will give you the second-to-last element of the list:

If you try to access an index which is not present in the list, an IndexError will be raised:
print arr[6]
Traceback (most recent call last):
File "", line 1, in
IndexError: list index out of range


Must Read Python Interview Questions

200+ Python Tutorials With Coding Examples

Python Language Basics TutorialPython String Representations of Class Instances
Python For Beginners TutorialPython Debugging Tutorial
Python Data Types TutorialReading and Writing CSV File Using Python
Python Indentation TutorialWriting to CSV in Python from String/List
Python Comments and Documentation TutorialPython Dynamic Code Execution Tutorial
Python Date And Time TutorialPython Code Distributing using Pyinstaller
Python Date Formatting TutorialPython Data Visualization Tutorial
Python Enum TutorialPython Interpreter Tutorial
Python Set TutorialPython Args and Kwargs
Python Mathematical Operators TutorialPython Garbage Collection Tutorial
Python Bitwise Operators TutorialPython Pickle Data Serialisation
Python Bolean Operators TutorialPython Binary Data Tutorial
Python Operator Precedance TutorialPython Idioms Tutorial
Python Variable Scope And Binding TutorialPython Data Serialization Tutorial
Python Conditionals TutorialPython Multiprocessing Tutorial
Python Comparisons TutorialPython Multithreading Tutorial
Python Loops TutorialPython Processes and Threads
Python Arrays TutorialPython Concurrency Tutorial
Python Multidimensional Arrays TutorialPython Parallel Computation Tutorial
Python List TutorialPython Sockets Module Tutorial
Python List Comprehensions TutorialPython Websockets Tutorial
Python List Slicing TutorialSockets Encryption Decryption in Python
Python Grouby() TutorialPython Networking Tutorial
Python Linked Lists TutorialPython http Server Tutorial
Linked List Node TutorialPython Flask Tutorial
Python Filter TutorialIntroduction to Rabbitmq using Amqpstorm Python
Python Heapq TutorialPython Descriptor Tutorial
Python Tuple TutorialPython Tempflile Tutorial
Python Basic Input And Output TutorialInput Subset and Output External Data Files using Pandas in Python
Python Files And Folders I/O TutorialUnzipping Files in Python Tutorial
Python os.path TutorialWorking with Zip Archives in Python
Python Iterables And Iterators Tutorialgzip in Python Tutorial
Python Functions TutorialStack in Python Tutorial
Defining Functions With List Arguments In PythonWorking with Global Interpreter Lock (GIL)
Functional Programming In PythonPython Deployment Tutorial
Partial Functions In PythonPython Logging Tutorial
Decorators Function In PythonPython Server Sent Events Tutorial
Python Classes TutorialPython Web Server Gateway Interface (WSGI)
Python Metaclasses TutorialPython Alternatives to Switch Statement
Python String Formatting TutorialPython Packing and Unpacking Tutorial
Python String Methods TutorialAccessing Python Sourcecode and Bytecode
Using Loops Within Functions In PythonPython Mixins Tutorial
Python Importing Modules TutorialPython Attribute Access Tutorial
Difference Betweeb Module And Package In PythonPython Arcpy Tutorial
Python Math Module TutorialPython Abstract Base Class Tutorial
Python Complex Math TutorialPython Plugin and Extension Classes
Python Collections Module TutorialPython Immutable Datatypes Tutorial
Python Operator Module TutorialPython Incompatibilities Moving from Python 2 to Python 3
Python JSON Module TutorialPython 2to3 Tool Tutorial
Python Sqlite3 Module TutorialNon-Official Python implementations
Python os Module TutorialPython Abstract Syntax Tree
Python Locale Module TutorialPython Unicode and Bytes
Python Itertools Module TutorialPython Serial Communication (pyserial)
Python Asyncio Module TutorialNeo4j and Cypher using Py2Neo
Python Random Module TutorialBasic Curses with Python
Python Functools Module TutorialTemplates in Python
Python dis Module TutorialPython Pillow
Python Base64 Module TutorialPython CLI subcommands with precise help output
Python Queue Module TutorialPython Database Access
Python Deque Module TutorialConnecting Python to SQL Server
Python Webbrowser Module TutorialPython and Excel
Python tkinter TutorialPython Turtle Graphics
Python pyautogui Module TutorialPython Persistence
Python Indexing And Slicing TutorialPython Design Patterns
Python Plotting With Matplotlib TutorialPython hashlib
Python Graph Tool TutorialCreating a Windows Service Using Python
Python Generators TutorialMutable vs Immutable (and Hashable) in Python
Python Reduce TutorialPython configparser
Python Map Function TutorialPython Optical Character Recognition
Python Exponentiation TutorialPython Virtual Environments
Python Searching TutorialPython Virtual Environment – virtualenv
Sorting Minimum And Maximum In PythonPython Virtual environment with virtualenvwrapper
Python Print Function TutorialCreate virtual environment with virtualenvwrapper in windows
Python Regular Expressions Regex TutorialPython sys Tutorial
Copying Data In Python TutorialChemPy – Python package
Python Context Managers (“with” Statement) TutorialPython pygame
Python Name Special Variable TutorialPython pyglet
Checking Path Existence And Permissions In PythonWorking with Audio in Python
Creating Python Packages TutorialPython pyaudio
Usage of pip Module In Python TutorialPython shelve
Python PyPi Package Manager TutorialIoT Programming with Python and Raspberry PI
Parsing Command Line Arguments In Pythonkivy – Cross-platform Python Framework for NUI Development
Python Subprocess Library TutorialPandas Transform
Python TutorialPython vs. JavaScript
Python Recursion TutorialCall Python from C#
Python Type Hints TutorialPython Writing Extensions
Python Exceptions TutorialPython Lex-Yacc
Raise Custom Exceptions In PythonPython Unit Testing
Python Commonwealth Exceptions TutorialPython py.test
Python urllib TutorialPython Profiling
Web Scraping With Python TutorialPython Speed of Program
Python HTML Parsing TutorialPython Performance Optimization
Manipulating XML In PythonPython Security and Cryptography
Python Requests Post TutorialSecure Shell Connection in Python
Python Distribution TutorialPython Anti Patterns
Python Property Objects TutorialPython Common Pitfalls
Python Overloading TutorialPython Hidden Features
Python Polymorphism TutorialPython For Machine Learning
Python Method Overriding TutorialPython Interview Questions And Answers For Experienced
Python User Defined Methods TutorialPython Coding Interview Questions And Answers
Python Programming Tutorials With Examples

Other Python Tutorials

Leave a Comment