String searching / membership test

Determine if a string contains a specific substring to branch logic accordingly.

Validating user input for a keyword; filtering log lines that contain a term; parsing configuration values.

General Python scripting, data processing, web scraping, automation.

if substring in s:

if ... in ...:

substring: str literal or variable; s: str variable or expression

• string methods like .find()
• .index()
• slicing
• equality checks
• loops

• using s.find('sub') != -1
• using re.search
• using 'sub' in s.lower() for case-insensitive

confusing 'in' with equality; forgetting quotes; using 'in' on non-string iterables inadvertently

if s.startswith('sub'): checks prefix; if s.endswith('sub'): checks suffix; if 'sub' not in s: negation

Coming from languages like C where you'd use strstr or indexOf; may forget Python's 'in' operator works on strings.

• if 'sub' not in s
• str.find
• str.index
• re.search

Case-sensitive; works on any iterable; empty substring always returns True; performance O(n).

Enables conditional logic based on substring presence, allowing filtering, validation, or triggering actions based on text content.

Like checking if a key fits a lock before turning it.

The 'in' operator works on any iterable, not just strings, and short‑circuits on the first match; for strings it is case‑sensitive and an empty substring always returns True.

re.search(pattern, string) for regex matching or str.find(substring) to locate the index