Common Sequence Operations

Strings are sequences, which means they support common sequence operations that work with other sequence types like lists and tuples. These operations include:

• Membership: Check if a character or substring exists
• Length: Get the number of characters
• Minimum and Maximum: Find the smallest and largest characters (by ASCII/Unicode value)
• Counting: Count the number of occurrences of a character or substring
• Searching: Find the index of a character or substring
• Indexing: Access characters by their position
• Slicing: Extract portions of the string
• Concatenation: Combine strings
• Repetition: Repeat strings

Indexing, Slicing, and Immutability

Strings support indexing and slicing just like lists and tuples. Characters can be accessed using positive indices (starting at 0) or negative indices (counting from the end), and strings can be sliced using the same [start:stop:step] syntax. However, strings are immutable: their contents cannot be changed after creation.

1
text = "Hello"
2

3
print(text[0])  # H
4
print(text[-1])  # o
5
print(text[1:4])  # ell
6
print(text[::2])  # Hlo
7

8
text[0] = "h"  # TypeError: 'str' object does not support item assignment

Note

Since there is no separate character type in Python, indexing a string produces another string of length 1, not a character type.

Warning

Since strings are immutable, characters cannot be modified, added, or deleted. Slicing creates a new string, leaving the original unchanged.

String Concatenation

Strings can be concatenated using the + operator or by placing them adjacent to each other:

1
# Using + operator
2
greeting = "Hello" + " " + "World"  # "Hello World"
3

4
# Adjacent strings (implicit concatenation)
5
text = "ABC" "DEF"  # "ABCDEF"
6
text = "Hello" " " "World"  # "Hello World"

Tip

Adjacent string literals are automatically concatenated by Python, which is useful for breaking long strings across multiple lines without using +.

Escape Characters

Escape characters allow the inclusion of special characters in strings that would otherwise be difficult or impossible to represent:

1
text = "Hello\nWorld"  # Prints on two lines
2
path = "C:\\Users\\Name"  # Escaped backslashes
3
quote = "He said \"Hello\""  # Escaped quotes

Raw Strings

Raw strings (prefixed with r or R) treat backslashes as literal characters, which is useful for file paths, regular expressions, and other cases where escape sequences should not be interpreted:

1
# Regular string
2
text = "Hey\nHow\tare\ryou?"  # Escape sequences are interpreted
3

4
# Raw string
5
raw_text = r"Hey\nHow\tare\ryou?"  # Escape sequences are literal
6
print(raw_text)  # Prints: Hey\nHow\tare\ryou?
7

8
# Useful for file paths
9
path = r"C:\Users\Name\Documents"  # No need to escape backslashes

Looping, Membership, and Operators

Strings support the same iteration and operator behaviors as lists and tuples: you can loop through strings using for or while loops, check membership with in and not in, and use identity operators is and is not. The assignment operator works the same way, but since strings are immutable, assigning a string to multiple variables is safe.

1
text = "Hello"
2

3
# Looping
4
for char in text:
5
    print(char)
6

7
for i in range(len(text)):
8
    print(i, ":", text[i])
9

10
i = 0
11
while i < len(text):
12
    print(text[i])
13
    i += 1
14

15
# Membership (works with substrings too)
16
print("H" in text)  # True
17
print("ell" in text)  # True
18
print("xyz" not in text)  # True

Notable String Methods

Strings have many useful methods. Here are some of the most commonly used:

1
text = "Hello World"
2

3
print(text.count("l"))  # 3
4
print(text.find("World"))  # 6
5
print(text.find("Python"))  # -1
6
print(text.index("World"))  # 6
7
print(text.startswith("Hello"))  # True
8
print(text.endswith("World"))  # True

1
text = "hello WORLD"
2

3
print(text.lower())  # "hello world"
4
print(text.upper())  # "HELLO WORLD"
5
print(text.capitalize())  # "Hello world"
6
print(text.title())  # "Hello World"

1
text = "apple,banana,cherry"
2
fruits = text.split(",")  # ['apple', 'banana', 'cherry']
3

4
words = "Hello World Python"
5
word_list = words.split()  # ['Hello', 'World', 'Python']
6

7
# Joining
8
separator = ", "
9
result = separator.join(fruits)  # "apple, banana, cherry"

String Formatting

Python provides several ways to format strings with variables.

1
txt = "My name is {}, and I am {} years old"
2
print(txt.format("Ali", 25))  # "My name is Ali, and I am 25 years old"
3

4
txt = "Your total is {}, it becomes {} after discount"
5
print(txt.format(50, 42))  # "Your total is 50, it becomes 42 after discount"

1
txt = "I have {0} apples and {1} oranges. Total: {0} + {1} = {2}"
2
print(txt.format(5, 3, 8))  # "I have 5 apples and 3 oranges. Total: 5 + 3 = 8"
3

4
txt = "You have ordered {0} items, {0} items, {0} ITEMS, don't be a Karen!"
5
print(txt.format(50))  # "You have ordered 50 items, 50 items, 50 ITEMS, don't be a Karen!"

1
txt = "My name is {name}, and I am {age} years old"
2
print(txt.format(name="Ali", age=25))  # "My name is Ali, and I am 25 years old"
3

4
txt = "I have ordered {items} items, you charged me {total}, and the total is {discount}"
5
print(txt.format(total=50, discount=42, items=10))  # "I have ordered 10 items, you charged me 50, and the total is 42"

1
name = "Ali"
2
age = 25
3

4
txt = f"My name is {name}, and I am {age} years old"
5
print(txt)  # "My name is Ali, and I am 25 years old"
6

7
# Can include expressions
8
x = 10
9
y = 20
10
result = f"The sum of {x} and {y} is {x + y}"  # "The sum of 10 and 20 is 30"

String as a Sequence

Strings are sequences, so let’s see all the sequence operations in one example:

1
text = "Hello"
2

3
print(text)  # Hello
4
print(len(text))  # 5
5
print(min(text))  # H (by ASCII value)
6
print(max(text))  # o (by ASCII value)
7

8
print("e" in text)  # True
9
print("z" in text)  # False
10

11
print(text.count("l"))  # 2
12
print(text.find("l"))  # 2
13

14
print(text[1::2])  # el
15
print(text * 2)  # HelloHello
16
print(text + " World")  # Hello World

Why Are Strings Immutable?

There are several important reasons why strings are immutable in Python:

1. Performance: Immutable strings allow Python to allocate fixed space at creation time, knowing the string will never change. This enables optimizations like string interning.

2. Elemental Nature: Strings are considered as “elemental” as numbers. Just as the number 1 will always be 1, a string like "Hello" should always be "Hello". We don’t change values, we create new ones.

3. Hashability: Since strings are immutable, they can be used as dictionary keys and set elements.

Exercises

1
text = input()
2
for char in text:
3
    print(char)

1
text = input()
2
vowels = "aeiouAEIOU"
3
count = 0
4
for char in text:
5
    if char in vowels:
6
        count += 1
7
print(count)

1
text = input()
2
reversed_text = ""
3
for i in range(len(text) - 1, -1, -1):
4
    reversed_text += text[i]
5
print(reversed_text)

1
text = input().lower()
2
reversed_text = ""
3
for i in range(len(text) - 1, -1, -1):
4
    reversed_text += text[i]
5
if text == reversed_text:
6
    print("Yes")
7
else:
8
    print("No")

1
text = input().strip()
2
if text == "":
3
    print(0)
4
else:
5
    words = text.split()
6
    print(len(words))

1
text = input()
2
old = input()
3
new = input()
4
result = ""
5
i = 0
6
while i < len(text):
7
    if text[i:i+len(old)] == old:
8
        result += new
9
        i += len(old)
10
    else:
11
        result += text[i]
12
        i += 1
13
print(result)

1
text = input()
2
prefix = input()
3
if len(prefix) > len(text):
4
    print("No")
5
else:
6
    match = True
7
    for i in range(len(prefix)):
8
        if text[i] != prefix[i]:
9
            match = False
10
            break
11
    if match:
12
        print("Yes")
13
    else:
14
        print("No")

1
text = input()
2
result = ""
3
for char in text:
4
    if 'a' <= char <= 'z':
5
        result += chr(ord(char) - ('a' - 'A'))
6
    else:
7
        result += char
8
print(result)

1
text = input()
2
result = ""
3
for char in text:
4
    if char != " ":
5
        result += char
6
print(result)

1
n = int(input())
2
strings = []
3
for i in range(n):
4
    strings.append(input())
5
separator = input()
6
result = ""
7
for i in range(len(strings)):
8
    result += strings[i]
9
    if i < len(strings) - 1:
10
        result += separator
11
print(result)

1
name = input()
2
age = input()
3
result = f"My name is {name} and I am {age} years old"
4
print(result)

1
text = input()
2
seen = []
3
for char in text:
4
    if char not in seen:
5
        count = text.count(char)
6
        print(f"{char} {count}")
7
        seen.append(char)