Strings

String Methods

split, join, strip, replace, find

Interview: Essential string manipulation

Last Updated: June 12, 2026 • 8 min read

Python strings have a rich set of built-in methods for searching, transforming, and analyzing text. Since strings are immutable, all methods return new strings rather than modifying in place.

Searching Methods

find(sub) / rfind(sub) — returns index or -1 if not found
index(sub) — like find() but raises ValueError if not found
count(sub) — count non-overlapping occurrences
startswith(prefix) / endswith(suffix) — boolean checks

Transformation Methods

strip() / lstrip() / rstrip() — remove whitespace (or specified chars)
replace(old, new, count) — replace substrings
split(sep) / rsplit(sep, maxsplit) — split into list
join(iterable) — join list into string
lower() / upper() / title() / swapcase() — case transformation
casefold() — aggressive lowercase for case-insensitive comparison

Testing Methods

isdigit() / isalpha() / isalnum() — character type checks
isspace() / isidentifier() — whitespace and identifier checks
isupper() / islower() / istitle() — case checks

Formatting Methods

center(width, fillchar) — center-align with padding
ljust(width) / rjust(width) — left/right align
zfill(width) — pad with zeros
encode(encoding) — convert to bytes

Use Cases

Parsing CSV, log files, and structured text data

Cleaning user input (strip whitespace, normalize case)

Building formatted output for reports and CLI tools

Text analysis and natural language preprocessing

Common Mistakes

Using split() without a separator argument when you need split on specific character

Forgetting that split() returns strings — need to convert to int/float manually

Using lower() instead of casefold() for case-insensitive Unicode comparison

Not using partition() when you need exactly 3 parts (before, separator, after)