not in

Absence, spelled the way you say it: `if key not in cache:`.

Membership operatorPython 1.0+Live demo
Common call
if key not in cache:
Returns
bool
Replaces
preferred over `not key in cache` — same result, better reading
Watch out
same dict-keys and O(n)-list rules as in
itemitem — The candidate.type: Any · required not in containercontainer — Where to look.type: iterable · required
→ bool

Demo

Live evaluation
Try:
Inputs
itemAnyto find
containerlistcomma-separated items
Output
'z' not in ['a', 'b', 'c']
True

The exact inverse of in — True for absence. Python treats "not in" as a single operator token, which is why it reads so naturally.

Operands

NameTypeRequiredDescription
itemAnyyesThe candidate.
containeriterableyesWhere to look.

Return value

bool — True when item is NOT a member of container. Exactly `not (item in container)`, as one readable operator.

Common patterns

Initialize-if-missing
The check-then-create dict pattern.
if key not in groups:
    groups[key] = []
groups[key].append(item)
Filtering out exclusions
Comprehension with a blocklist.
kept = [x for x in items if x not in BANNED]

Examples

1. Absent element
"z" not in ["a", "b"]
Returns
True
2. Missing key
"k" not in {}
Returns
True
3. Substring form
"xyz" not in "hello"
Returns
True

Pitfalls

1. `not x in y` reads wrong
Identical semantics, but the split spelling invites misreading — and linters flag it.
Awkward
if not key in d:
works; flagged by style checkers
Fix
if key not in d:
idiomatic

When to use

Use it
  • Absence checks and guards
  • Exclusion filters
Reach for something else
  • Absence-then-store on dicts → dict.setdefault covers both steps
  • Big-list exclusion sets → convert to set first

Notes

Complexity
same as in: O(1) sets/dicts, O(n) lists
Return
bool
CPython impl
CONTAINS_OP with invert flag — one opcode
Memory
No allocation
Thread-safe
Yes for stable containers

FAQ

Yes — grammatically a single comparison operator, compiled to one opcode. It is not `not` applied afterwards, though the result is identical.

History

1.0
Core operator from the beginning.