identity_equality

Identity vs Equality

== compares values; is compares object identity (same memory address).

Files

File	Description
example.py	is vs ==, hashability, custom equality

---

Descriptive Example

Scenario

Two lists with identical contents are equal but not the same object. None checks must always use is.

a = [1, 2, 3]
b = [1, 2, 3]
c = a

print(a == b)   # True  — same values
print(a is b)   # False — different objects in memory
print(a is c)   # True  — c is an alias for a

# Always use `is` for None
value = None
if value is None:
    print("safe None check")

Hashable vs unhashable

hash((1, 2, 3))     # OK — tuple of ints is hashable
hash([1, 2, 3])     # TypeError — lists are mutable, unhashable

# Can't use list as dict key
{[1, 2]: "value"}   # TypeError

---

Interview Q&A

Q1: When should you use is vs ==?
A: Use == for value comparison. Use is only for identity checks with singletons: None, True, False. Never use is for strings or numbers (except None).

Q2: Why can't lists be dict keys?
A: Dict keys must be hashable (immutable + consistent hash). Lists are mutable — their hash would change if contents change.

Q3: If two objects are equal, must they have the same hash?
A: Yes — hash contract: a == b implies hash(a) == hash(b). Violating this breaks dicts and sets.

Q4: What happens to __hash__ when you define __eq__?
A: Python sets __hash__ = None (unhashable) unless you explicitly define __hash__. Mutable objects with custom equality should stay unhashable.

Q5: Are small integers cached in CPython?
A: Integers -5 to 256 are interned — a is b may be True for equal small ints. Never rely on this; always use == for value comparison.

Q6: What is id(obj)?
A: Returns the object's identity (memory address in CPython). a is b is equivalent to id(a) == id(b).

---

Run

python3 example.py