Conditional logic

Determine if a specific item exists in a list, set, or other iterable before proceeding.

User input validation: checking if entered option is allowed; Dictionary handling: verifying a key exists before access; Loop filtering: testing each element against a target value

Common in Python scripts, data processing, web backends, automation scripts.

if item in container:

if ... in ...:

item: any hashable or comparable object; container: iterable (list, set, tuple, dict keys, etc.)

• else clause
• break
• continue
• list comprehension
• filter function

• Using `any()` with generator expression
• using `dict.get(key)` is not None
• using `in` with sets for O(1) lookup.

Confusing `in` with `==` for equality; using `in` on strings for substring vs element; forgetting that `in` on dict checks keys not values.

`not in` for negation; `any()` for more complex conditions; `all()` for ensuring all items present.

Coming from languages like C/Java where you need explicit loops or library functions; may forget Python's `in` works on many types.

• membership testing
• conditional execution
• containment checks

Works with any iterable; for strings checks substring; for dict checks keys; performance O(n) for list, O(1) for set/dict.

Makes intent explicit and readable; avoids manual iteration.

Think 'if apple in basket'.

Prefer sets or dict keys for O(1) membership tests when checking many items; using 'in' on lists is O(n).

Use a set for faster membership tests when checking many items.