in, not in

Overview

'in' and 'not in' test whether a value is a member of a sequence or collection. They work on strings, lists, tuples, sets, and dicts (key check). Set membership is O(1) while list membership is O(n).

Syntax

python

# List membership — O(n)
3 in [1, 2, 3, 4]       # True
5 not in [1, 2, 3]      # True

# String membership
"ell" in "hello"         # True  (substring check)
"xyz" not in "hello"     # True

# Set membership — O(1)
5 in {1, 3, 5, 7}        # True

# Dict membership (checks keys)
"name" in {"name": "Alice"}  # True
"Alice" in {"name": "Alice"} # False (checks keys)

Common Pitfalls

Using 'in' on a list is O(n) — for repeated lookups on large collections, convert to a set first for O(1) performance.
'in' on a dict checks keys only — use 'in dict.values()' to check values (still O(n)).
Interview tip: 'in' on a string checks for substrings, not character-by-character — 'ab' in 'abc' is True.

Real-World Example

Fast lookup using a set for allowed permissions

example

python

ALLOWED_ROLES = {"admin", "editor", "moderator"}

def check_access(user_role: str, resource: str) -> bool:
    if user_role not in ALLOWED_ROLES:
        print(f"Role '{user_role}' is not permitted")
        return False
    print(f"Access granted to {resource}")
    return True

check_access("admin", "/dashboard")    # Access granted
check_access("viewer", "/dashboard")  # Role 'viewer' is not permitted

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