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Programming in Python

Introduction to Strings

Explore the fundamentals of Python strings by understanding how to create, access, and manipulate them. Learn about string indexing, slicing, immutability, concatenation, and replication to build a strong foundation in text handling.

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What is a string?

A Python string is a collection of Unicode characters. Python strings can be enclosed in single, double, or triple quotes, as shown below:

  • 'BlindSpot'
  • "BlindSpot"
  • '''BlindSpot'''
  • """Blindspot"""
Python 3.8
# simple strings
msg1 = 'Educative'
print(msg1)

If there are characters like ', ", or \ within a string, they can be retained in two ways:

  • Escape them by preceding them with a \.
  • Prepend the string with r, therefore, indicating that it is a raw string.
Python 3.8
msg1 = 'He said, \'Let Us Python.\''
print(msg1)  # prints He said, 'Let Us Python.'
msg2 = r"He said, 'Let Us Python.'"
print(msg2)  # prints He said, 'Let Us Python.'
file1 = 'C:\\temp\\newfile'
print(file1)
file2 = r'C:\temp\newfile' # raw string - prepend r
print(file2)

Multiline strings can be created in three ways:

  • All but the last line ends with \
  • Enclosed within """some msg""" or '''some msg'''
  • Enclosed statements or messages within small brackets, like this:
    ('one msg'
 'another msg')

Let’s try these ways in the code widget below:

Python 3.8
str1 = "Hello \
World"
str2 = '''Hello 
World'''
# strings are concatenated properly. ( ) necessary
str3 = ('Hello '
        'World')
print(str1)
print(str2)
print(str3)

Accessing string elements

String elements can be accessed using an index value starting with 0. A negative index value is allowed. The last character is considered to be at index -1. Positive and negative indices are shown in the figure below.

Layout of a string in memory
Layout of a string in memory

Here are examples of positive and negative indexing:

Python 3.8
msg = 'Hello'
print(msg[0])  # yields H
print(msg[4])  # yields o
print(msg[-0]) # yields H, -0 is same as 0
print(msg[-1]) # yields o
print(msg[-2]) # yields l
print(msg[-5]) # yields H

A substring can be sliced out of a string using:

  • s[start : end]: It extracts a substring from start to end - 1.
  • s[start :]: It extracts a substring from start to end.
  • s[: end]: It extracts a substring from start to end - 1.
  • s[-start :]: It extracts a substring from -start (included) to end.
  • s[: -end]: It extracts a substring from beginning to -end - 1.

Using too large an index reports an error, but using too large an index while slicing is handled elegantly.

Python 3.8
s = 'Bamboozled'
# extract mboozled
print(s[2:10])
print(s[2:])
print(s[-8:])
# extract Bamboo
print(s[0:6])
print(s[:6])
print(s[-10:-4])
print(s[:-4])
# reverse Bamboozled
print(s[::-1])
print(s[0:10:1])
print(s[0:10:2])
print(s[0:10:3])
print(s[0:10:4])
msg = 'Rafting'
print(msg[3:100]) # prints elements from 't' up to end of string
print(msg[100])   # error since 100th element doesn't exist

String properties

  • Python strings are immutable, and they cannot be changed.
s = 'Hello'
s[0] = 'M' # rejected, attempt to mutate 
s = 'Bye'  # s is a variable, it can change
  • Strings can be concatenated using +.
Python 3.8
msg1 = 'Hello ' 
msg2 = 'World'
msg3 = msg1 + msg2
print(msg3)
s = 'Bamboozled'
s = s + 'Hype!'
print(s)
s = s[:6] + 'Monger' + s[-1]
print(s)
  • Strings can be replicated during printing. Replication occurs when multiplying a string (or concatenated string) by a number using the * operator.
Python 3.8
print('-' * 50) # prints 50 dashes
  • Determining whether one string is part of another can be done using in, as shown below.
Python 3.8
print('e' in 'Hello')  # prints True
print('z' in 'Hello')  # print False
print('lo' in 'Hello') # prints True