Strings in Python

Introduction to Strings¶

Textual data in Python is managed using str objects, also known as strings.

String Creation¶

Strings can be created in several ways:

Single quotes: 'allows embedded "double" quotes'
Double quotes: "allows embedded 'single' quotes'
- Use case for double quotes: When the string contains single quotes, such as “don’t”:

my_string = "don't worry"
print(my_string)  # Output: don't worry

don't worry

What happens without double quotes:

my_string = 'don't'  # SyntaxError: invalid syntax

  Cell In[2], line 1
    my_string = 'don't'  # SyntaxError: invalid syntax
                      ^
SyntaxError: unterminated string literal (detected at line 1)

Key Features of Strings¶

Strings are immutable: Once created, they cannot be changed.
Indexing returns a string of length 1:

my_string = "hello"
print(my_string[0])  # Output: 'h'

Strings support slicing and sequence operations:
- Example of slicing:

my_string = "hello"
print(my_string[1:4])  # Output: 'ell'

ell

Slicing options:
- my_string[start:end]: Extract characters from start to end (not inclusive).
- my_string[:end]: Extract from the start to end.
- my_string[start:]: Extract from start to the end.
- my_string[:]: Extract the entire string.
- my_string[start:stop:step]: Extract characters from start to end (not inclusive), incrementing every step
- my_string[::2]: Extract every second character from the entire string, starting from index 0.
Sequence operations:
1. Concatenation (+):

print("hello" + " world")  # Output: 'hello world'

hello world

 2. **Repetition** (`*`):

print("ha" * 3)  # Output: 'hahaha'

hahaha

 3. **Membership Testing** (`in`):

print("ell" in "hello")  # Output: True

True

Essential String Methods¶

Strings implement common sequence operations and additional methods for text manipulation.

1. str.capitalize(): Capitalizes the first character and lowers others.

print("hello world".capitalize())  # Output: 'Hello world'

Hello world

2. str.casefold(): Case-insensitive string comparison.

print("ß".casefold())  # Output: 'ss'

ss

3. str.center(width, fillchar=' '): Centers the string.

print("hello".center(10, "*"))  # Output: '**hello***'

**hello***

4. str.count(sub): Counts occurrences of a substring.

print("banana".count("a"))  # Output: 3

5. str.endswith(suffix): Checks if the string ends with suffix.

print("hello.py".endswith(".py"))  # Output: True

True

6. str.expandtabs(tabsize): Replaces tabs with spaces.

print("01\t012".expandtabs(4))  # Output: '01  012'

01  012

7. str.find(sub): Returns the lowest index of sub or -1.

print("hello world".find("world"))  # Output: 6

8. str.join(iterable): Joins elements of an iterable with a separator.

print(", ".join(["apple", "banana"]))  # Output: 'apple, banana'

apple, banana

9. str.split(sep): Splits the string by sep. Here we are splitting by “,”

Note that split method will create a list of strings.

print("1,2,3".split(","))  # Output: ['1', '2', '3']

['1', '2', '3']

10. str.title(): Returns a title-cased string.

print("hello world".title()) # Output: 'Hello World'

Hello World

11. str.zfill(width): Pads the string with zeros.

print("42".zfill(5)) # Output: '00042'

12. str.replace(old, new, count): Replaces occurrences of old with new.

print("banana".replace("a", "o", 2)) # Output: 'bonona'

bonona

Case Conversion¶

str.upper(): Converts all characters to uppercase.
str.lower(): Converts all characters to lowercase.
str.swapcase(): Swaps the case of all characters.
str.istitle(): Checks if the string is title-cased.

String Testing Methods¶

str.isalpha(): Checks if all characters are alphabetic.
str.isdigit(): Checks if all characters are digits.
str.isalnum(): Checks if all characters are alphanumeric.
str.isspace(): Checks if all characters are whitespace.

Cleaning User Input¶

To ensure consistent and clean input:

Use .strip() to remove spaces.
Apply .title() for capitalization.

Example:¶

name = input("What's your name? ").strip().title()
print(f"Hello, {name}")

Hello,