Python String Methods

Strings are everywhere: names, emails, logs, files, messages, and API responses.

  Important concept:
  Strings are **immutable**. That means methods return a **new** string, they do not change the original string.
  
  ## Case conversion
  
  ```python
  text = "Hello World"
  
  print(text.upper())
  print(text.lower())
  print(text.capitalize())
  print(text.title())
  print(text.swapcase())
  ```
  
  Expected output:
  
  ```
  HELLO WORLD
  hello world
  Hello world
  Hello World
  hELLO wORLD
  ```
  
  ## Finding and counting
  
  ```python
  text = "Python is awesome. Python is easy."
  
  print(text.find("Python"))
  print(text.find("Python", 10))
  print(text.find("Java"))
  
  print(text.count("Python"))
  ```
  
  Expected output:
  
  ```
  0
  19
  -1
  2
  ```
  
  ## Replacing
  
  ```python
  text = "I love Python"
  new_text = text.replace("love", "enjoy")
  
  print(text)
  print(new_text)
  ```
  
  Expected output:
  
  ```
  I love Python
  I enjoy Python
  ```
  
  Replace only first N occurrences:
  
  ```python
  text = "ha ha ha"
  print(text.replace("ha", "he", 2))
  ```
  
  Expected output:
  
  ```
  he he ha
  ```
  
  ## Splitting and joining (very practical)
  
  ```python
  csv = "apple,banana,orange"
  fruits = csv.split(",")
  print(fruits)
  
  cities = ["Miami", "Austin", "Denver"]
  text = ", ".join(cities)
  print(text)
  ```
  
  Expected output:
  
  ```
  ['apple', 'banana', 'orange']
  Miami, Austin, Denver
  ```
  
  ## Trimming spaces
  
  ```python
  text = "   hello   "
  print(text.strip())
  print(text.lstrip())
  print(text.rstrip())
  ```
  
  Expected output:
  
  ```
  hello
  hello   
     hello
  ```
  
  ## Checking content (validation)
  
  ```python
  print("hello".isalpha())
  print("123".isdigit())
  print("hello123".isalnum())
  print("HELLO".isupper())
  print("hello".islower())
  ```
  
  Expected output:
  
  ```
  True
  True
  True
  True
  True
  ```
  
  ## Real pattern: clean user input
  
  ```python
  raw = "   SARAH.JOHNSON@EXAMPLE.COM   "
  clean = raw.strip().lower()
  
  print(clean)
  ```
  
  Expected output:
  
  ```
  sarah.johnson@example.com
  ```
  
  ## Graph: string cleaning pipeline
  
  ```mermaid
  flowchart LR
    A[Raw input] --> B[strip spaces]
    B --> C[lower/upper]
    C --> D[replace/split]
    D --> E[Clean output]
  ```
  
  You now have a strong foundation of practical string methods and patterns used in real applications.