5.10 Type Checking

type()

Returns an object’s exact type. Good for debugging/inspection, but generally NOT recommended for validation logic (see isinstance() below), since it does an exact match and ignores subclassing.

>>> type(5) == int
True

isinstance()

What Is It?

Checks whether an object IS an instance of a type, INCLUDING subclasses.

Why Is It Preferred?

It’s preferred over type() for validation because it correctly handles inheritance — e.g. bool is a subclass of int, so isinstance(True, int) is True even though type(True) == int is False.

>>> isinstance(5, int)
True
>>> isinstance(True, int)    # bool IS-A int (subclass)
True
>>> type(True) == int        # exact type match fails here
False

id()

Returns an object’s unique identity (see 5.2 Python Object Model) — used far more often for understanding reference/aliasing behavior than for everyday type checking.

>>> id(5)
11755816

Quick Interview Answer

“type() returns an object’s exact type and does an exact match, which breaks for subclasses. isinstance() checks whether an object IS an instance of a type INCLUDING subclasses, which is why it’s the preferred choice for validation — isinstance(True, int) is True since bool subclasses int, even though type(True) == int is False. id() is a different tool entirely — it returns identity, used for understanding references, not for type checks.”

Common Mistakes

• Using type(x) == SomeClass for validation when a subclass instance should also pass — isinstance() handles that correctly, type() does not.
• Forgetting that bool is a subclass of int, so an isinstance(x, int) check unexpectedly also accepts True/False.
• Reaching for id() when a simple == or isinstance() check was actually what was needed.