Python: Strings

Python Strings

python
strings
The file system is organized into a hierarchical structure, starting with the root directory
Author

Isaac Boixaderas

Published

Sunday, June 1, 2025

Modified

Sunday, November 16, 2025

1 Overview

πŸ“˜ Strings in Python

In Python, strings are used for representing textual data. A string is a sequence of characters enclosed in either single quotes (’’) or double quotes (β€œβ€). The Python language provides various built-in methods and functionalities to work with strings efficiently. Numbers tell computers how much or how many. Strings tell computers what β€” names, words, sentences, or any text. In other words, strings are how computers understand and manipulate tex

2 Strings

  • With strings, a computer can:

    • Say hello to someone.

    • Store a name.

    • Send a message.

    • Print a receipt.

3 Strings in Python

  • Sequence of characters enclosed in single, double or triple quotes.
string1 = 'Hello, World!'

string2 = "Hello, World!"

string3 = '''Multi-line
string 
example'''

string4 = """Multi-line 2
string 
example"""
  • Take care when mixing quotes!
failed_string = 'You're the best!'
SyntaxError: invalid syntax 
failed_string = "and then she said: "I love this Python course!""
SyntaxError: invalid syntax
  • Properly mixing them:
successful_string = "You're the best!"
successful_string = 'she said: "I love this Python course!"'

4 Escape sequences

  • Include special characters in your strings by using backslash (\)

  • Common escape sequences: \', \", \\, \n, \t.

# Using a single quote inside a string enclosed in single quotes
print('You\'re the best!')
You’re the best! 
# Using a double quote inside a string enclosed in double quotes
print("she said: \"I love this Python course!\"")
she said: β€œI love this Python course!” 
# Use a double backslash to show a single backslash
print('C:\\Users\\Documents')
C: 
# Use \n to enter a new line
print('Hello,\nWorld!')
Hello,
World! 
# Use \t to add a tab
print('Hello,\tWorld!')
Hello, World!

5 Raw strings

  • Create string literals without processing escape sequences
  • Add r or R before the string.
  • Useful when working with paths, regular expressions, etc.
print(r'this string is \n not split.')
this string is not split.

–>

# Without a raw string
file_path = 'C:\\Users\\username\\Documents\\file.txt'
print(file_path)
# With a raw string
file_path = r'C:\Users\username\Documents\file.txt'
print(file_path)
C:.txt
C:.txt

–>

6 Single or double quotes?

  • No defined rules.

  • Some opinions:

    • Be consistent throughout your code, do not mix both types of quotes randomly, as this makes the code a bit more complex to read.

    • Do not use triple quotes if the string is not multi-line, as this makes the code a bit more complex to read.

7 Indexing

  • Access specific string characters by index.

String indexing in python

String indexing in python
From: https://www.codespeedy.com/

8 Positive indexing

  • Use [index number] after the string, indicating the index to access.
  • Zero-based.
greeting = 'Hello, World!'

print(greeting[0])  # Access the 1st character
print(greeting[2])  # Access the 3rd character
print(greeting[11]) # Access 12th character
H
l
d

9 Negative indexing

  • For accessing the elements of a string β€œbackwards”.

  • Use -1 for accessing the last element, -2 for the second to last, etc.

greeting = 'Hello, World!'

print(greeting[-1])  # Access the last character
print(greeting[-2])  # Access the second to last character
print(greeting[-12]) # Access the 12th character starting from the end
!
d
e
  • We get an error if we specify an index outside the range.
greeting = 'Hello, World!'

print(greeting[12])
β€˜!’ 
greeting[13]
IndexError: string index out of range 
greeting[-15]
IndexError: string index out of range

10 Slicing

  • Extract a substring from a string.
  • Specify: start index, end index and step (optional).
    • substring = original_string[start:end:step]
  • Start is included.
  • End is not included.
  • Step defaults to 1 if not provided.
  • Start and end default to 0 if not provided.
greeting = 'Hello, World!'

# Extract the substring from index 0 to 4 (inclusive)
print(greeting[0:5])
Hello 
# Extract the first four characters
print(greeting[:4])
Hell 
# Extract the last three characters
print(greeting[-3:])
ld! 
greeting = 'Hello, World!'

# Extract characters with a step of 2
print(greeting[::2])
Hlo ol! 
# Extract the substring from index 1 to 6 with a step of 2
print(greeting[1:6:2])
el, 
# Trick for reversing a string
print(greeting[::-1])
!dlroW ,olleH

11 Concatenation

  • With + operator:
full_name = 'John' + ' ' + 'Doe'
print(full_name)
John Doe

12 Repetition

  • With * operator:
laugh = 'Ha' * 3
print(laugh)

laugh = 'Ha' * 10
print(laugh)
HaHaHa
HaHaHaHaHaHaHaHaHaHa

13 Comparison

  • Standard comparison operators: ==, <, >=, etc.
  • Comparison performed based on the Unicode code point (ordinal) value of each character. It is similar to the way words are sorted in a dictionary.
print('a' < 'b')
print('a' < 'A')
print('a' < 'aa')
print('abc' == 'abc')
True
False
True
True

14 Methods

  • Python provides a variety of functions, string methods, and keywords to make string manipulation easier.

  • Built-in functions:

    • len(): Function that returns the length of the string.
greeting = 'Hello, World!'

print(len(greeting))
13
  • String methods:
    • upper(): Converts the string to uppercase.
    • lower(): Converts the string to lowercase.
greeting = 'Hello, World!'

print(greeting.lower())
hello, world! 
print(greeting.upper())
HELLO, WORLD!
  • find(): Returns the index of the first occurrence of a substring.
greeting = 'Hello, World!'

print(greeting.find('Wo'))
7
  • replace(): Replaces a substring with another substring.
greeting = 'Hello, World!'

print(greeting.replace('World', 'Universe'))
Hello, Universe!
  • Trick to remove part of a string:
greeting = 'Hello, World!'

print(greeting.replace(', World', ''))
Hello!
  • Keywords:
    • in: Checks if a specified substring is present within a given string. It returns True if the substring is found, and False otherwise.
    • not in: Checks if a substring is not present in a given string.
greeting = 'Hello, World!'

print('Hello' in greeting)
True 
print('Hello' not in greeting)
False

15 Formatting

  • Process of creating a new string by combining text with variables.
  • Several methods:
    • %-formatting.
    • .format() method.
    • f-strings.

15.1 %-formatting

  • Old-style formatting.
  • Use % inside the string with element specifier.
  • Use % after the string for adding the variables.
  • See specifiers at: https://docs.python.org/3/library/stdtypes.html#old-string-formatting
name = 'John'
age = 30
formatted = 'My name is %s, and I am %i years old.' % (name, age)

print(formatted)
My name is John, and I am 30 years old.

15.2 format() method

  • New-style formatting.
  • String method.
  • Use curly braces {} in text as placeholders for variables.
  • Specify variables as format() arguments.
name = 'John'
age = 30
formatted = 'My name is {}, and I am {} years old.'.format(name, age)

print(formatted)
My name is John, and I am 30 years old.

15.3 F-strings

  • Most recent.
  • Recommended.
  • Add f before the string.
  • Add variables inside text enclosed with curly braces {}.
name = 'John'
age = 30
formatted = f'My name is {name}, and I am {age} years old.'

print(formatted)
My name is John, and I am 30 years old.
  • F-strings allow nice options for formatting.
  • For example, rounding to a specific number of decimals by including :.Xf
print(f'One divided by six is {1 / 6}')
print(f'One divided by six rounded {1 / 6:.0f}')
print(f'One divided by six with 1 decimal {1 / 6:.1f}')
print(f'One divided by six with 2 decimals is {1 / 6:.2f}')
One divided by six is 0.16666666666666666
One divided by six rounded 0
One divided by six with 1 decimal 0.2
One divided by six with 2 decimals is 0.17
  • Or displaying a large integer number separating the thousands with commas to make it more readable:
big_number = 1000000

print(f'Big number difficult to visualize: {big_number}')
print(f'Big number easy to visualize: {big_number:,}')
Big number difficult to visualize: 1000000
Big number easy to visualize: 1,000,000
  • You can find some more formatting examples here: https://www.pythonmorsels.com/string-formatting/

16 Immutability

  • In Python, strings are immutable.
  • Original string is never modified.
  • For changing a string, we have to create a new one.
  • It is forbidden to change strings using indexing:
original_string = 'Hello, World!'

# Strings are immutable,
# trying to change 'H' for 'J' results in an error
original_string[0] = 'J'
TypeError: β€˜str’ object does not support item assignment
  • We should create a new string variable with the desired changes.
  • For example, using slicing and concatenation:
original_string = 'Hello, World!'

# Changing 'H' for 'J'
new_string = 'J' + original_string[1:]

print(new_string)
Jello, World!
  • Another common option is to use the replace() method:
modified_string = original_string.replace('World', 'Python')

print(original_string)  # has not changed
print(modified_string)
Hello, World!
Hello, Python!
  • If the value returned by a method is not assigned to a variable, the result is lost:
original_string = 'Hello, World!'
# The resulting string is not assigned to a variable nor printed
original_string.replace('World', 'Python')

print(original_string)
Hello, World!

17 Unicode

  • Standard for representing characters from all languages.
  • Python strings support Unicode by default.
  • UTF-8 is the most common Unicode encoding for Python strings.
japanese_greeting = "こんにけは"  # Japanese
korean_greeting = "μ•ˆλ…•ν•˜μ„Έμš”"  # Korean
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