New Typing Features in Python 3.13: ReadOnly, @warnings.deprecated, TypeIs, is_protocol, get_protocol_members, Default TypeVars, NoDefault and Performance Improvements

Python 3.13 introduces seven new typing features that enhance code reliability and developer productivity.

The ReadOnly type can be used inside TypedDict to mark specific fields as immutable, causing type‑checkers to flag assignments to those fields.

from typing import TypedDict, ReadOnly

class Leader(TypedDict):
    name: ReadOnly[str]
    age: int

author: Leader = {'name': 'Yang Zhou', 'age': 30}
author['age'] = 31  # ok
author['name'] = 'Yang'  # Type check error

A simpler syntax also allows ReadOnly directly in the TypedDict definition.

from typing import TypedDict, ReadOnly

Leader = TypedDict('Leader', {'name': ReadOnly[str], 'age': int})
author: Leader = {'name': 'Yang Zhou', 'age': 30}
author['age'] = 31
author['name'] = 'Tim'  # Type check error

The new @warnings.deprecated decorator marks functions or classes as deprecated; IDEs such as PyCharm will display a strikethrough warning.

Python 3.13 adds the TypeIs protocol for more precise type narrowing, allowing functions to return True when an argument conforms to a narrower type.

The is_protocol function quickly checks whether an object is a Protocol type.

from typing import is_protocol, Protocol

class PersonProto(Protocol):
    name: str
    age: int

print(is_protocol(PersonProto))  # True
print(is_protocol(int))         # False

The get_protocol_members function returns a frozenset of member names defined in a protocol.

from typing import Protocol, get_protocol_members

class PersonProto(Protocol):
    name: str
    age: int

print(get_protocol_members(PersonProto))  # frozenset({'age', 'name'})

Type variables ( TypeVar), ParamSpec, and TypeVarTuple now support default types via the default= argument, and the has_default() method can query this.

from typing import TypeVar
T = TypeVar('T', default=int)
print(T.has_default())  # True
S = TypeVar('S')
print(S.has_default())  # False

The new NoDefault sentinel indicates that a type variable has no default value.

from typing import TypeVar, NoDefault
T = TypeVar('T')
print(T.__default__ is NoDefault)  # True
S = TypeVar('S', default=None)
print(S.__default__ is NoDefault)  # False

Performance improvements include removing the deprecated typing.io and typing.re namespaces and the keyword‑argument method for creating TypedDict, reducing import time of the typing module by about one‑third.