Strings in Python

Introduction to Strings

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

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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
    

  • What happens without double quotes:

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

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Key Features of Strings

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

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

3. Strings support slicing and sequence operations:
   • Example of slicing:

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

     2. Repetition (*):

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

     3. Membership Testing (in):

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

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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'

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

   print("ß".casefold())  # Output: 'ss'

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

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

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

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

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']

10. str.title(): Returns a title-cased string. python print("hello world".title()) # Output: 'Hello World'

11. str.zfill(width): Pads the string with zeros. python print("42".zfill(5)) # Output: '00042'

12. str.replace(old, new, count): Replaces occurrences of old with new. python print("banana".replace("a", "o", 2)) # Output: 'bonona'

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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.

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String Testing Methods

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

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Cleaning User Input

To ensure consistent and clean input:

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

Example:

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

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Online quiz : Python Strings

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