String Modification (immutable str, list+join)

Modifying strings in Python means producing a new str—slicing, replace, list-of-chars mutation then join, or bytearray for bytes. True in-place character edits do not exist for str.

Why this shows up in the real world

Immutable configs are updated by cloning and swapping references. Game grids stored as strings of tiles are rebuilt each tick from rules.

Core idea (explained for students)

Common pattern: chars = list(s); chars[i] = ch; out = ''.join(chars). For many replacements, consider building a list of segments and joining once.

Try this in Python

def swap_edges(s: str) -> str:
    if len(s) < 2:
        return s
    return s[-1] + s[1:-1] + s[0]


print(swap_edges("abcde"))

Common mistakes

• s[i] = x syntax error—strings are immutable.
• Chained replace order changing unintended substrings.

Key takeaways

• For frequent updates, list or array('u') historically; often io.StringIO or list buffers for builders.
• Snapshot before destructive transforms for easier tests.