String

Overview

Strings in Python are immutable sequences of Unicode characters. Every string method returns a new string. Mastering built-in string methods eliminates the need for manual character parsing in most real-world tasks.

Syntax

python

s = "Hello, World!"

s.upper()          # "HELLO, WORLD!"
s.lower()          # "hello, world!"
s.strip()          # removes leading/trailing whitespace
s.replace("World", "Python")  # "Hello, Python!"
s.split(", ")      # ["Hello", "World!"]
s.startswith("He") # True
s.find("World")    # 7 (index)
len(s)             # 13

Common Pitfalls

Strings are immutable — s[0] = 'H' raises TypeError. Use replace() or convert to list.
s.find() returns -1 if not found; s.index() raises ValueError — choose based on whether absence is expected.
Interview tip: Know the difference between join and split: ', '.join(['a','b','c']) → 'a, b, c'.

Real-World Example

Clean and normalize user input from a form submission

example

python

def normalize_email(email: str) -> str:
    return email.strip().lower().replace(" ", "")

raw = "  Alice@EXAMPLE.COM  "
clean = normalize_email(raw)
print(clean)  # "alice@example.com"

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