Python Strings A to Z Mastery

In Python, strings are immutable — you cannot change them in place.

So:

s = "hello"
s[0] = "H"   # ❌ TypeError: 'str' object does not support item assignment

✅ Correct ways to make "Hello":

s = "hello"

# Method 1: Concatenate
s = "H" + s[1:]
print(s)  # Hello

# Method 2: Using str.replace (only first character)
s = s.replace("h", "H", 1)
print(s)  # Hello

# Method 3: Convert to list, modify, then join
lst = list("hello")
lst[0] = "H"
s = "".join(lst)
print(s)  # Hello

🔹 Reasons why Python strings are immutable

1. Memory efficiency & interning
   • Python often reuses the same string object internally (string interning).
   • Example: a = "hello" b = "hello" print(a is b) # True → both point to same object
   • If strings were mutable, changing a would accidentally change b. Immutability prevents this.
2. Hashing & Dictionary keys
   • Strings are commonly used as keys in dicts and elements in sets.
   • Hash-based collections require objects to be immutable (hash must stay stable).
   • Example: d = {"hello": 123} # If "hello" could mutate into "hellx", its hash would change → dict lookup breaks.
3. Thread-safety
   • Multiple threads can safely share string objects since they can’t be changed.
   • No need for locks or synchronization just for reading strings.
4. Predictability & simplicity
   • If strings were mutable, debugging would be harder because one function might unexpectedly change a string that another function relies on.
   • Immutability makes behavior more predictable and aligns with Python’s philosophy: “Simple is better than complex.”
5. Encourages functional style
   • You build new strings from old ones instead of altering in place, which leads to cleaner, bug-resistant code.

👉 That said, if you really need a mutable sequence of characters, Python provides:

• list of characters: list("hello") → ['h', 'e', 'l', 'l', 'o']
• bytearray (mutable version of bytes).

⚡ Quick analogy:
Think of strings like numbers — 5 is immutable. If you do x = 5; x += 1, Python gives you a new 6. Same with "hello" → "Hello".