python - What's the difference between namedtuple and NamedTuple?

Question

The typing module documentation says that the two code snippets below are equivalent.

from typing import NamedTuple

class Employee(NamedTuple):
    name: str
    id: int

and

from collections import namedtuple

Employee = namedtuple('Employee', ['name', 'id'])

Are they the exact same thing or, if not, what are the differences between the two implementations?

Answer 1

The type generated by subclassing typing.NamedTuple is equivalent to a collections.namedtuple, but with __annotations__, _field_types and _field_defaults attributes added. The generated code will behave the same, for all practical purposes, since nothing in Python currently acts on those typing related attributes (your IDE might use them, though).

As a developer, using the typing module for your namedtuples allows a more natural declarative interface:

• You can easily specify default values for the fields (edit: in Python 3.7, collections.namedtuple got a new defaults keyword so this is no longer an advantage)
• You don't need to repeat the type name twice ("Employee")
• You can customize the type directly (e.g. adding a docstring or some methods)

As before, your class will be a subclass of tuple, and instances will be instances of tuple as usual. Interestingly, your class will not be a subclass of NamedTuple. If you want to know why, read on for more info about the implementation detail.

from typing import NamedTuple

class Employee(NamedTuple):
    name: str
    id: int

Behaviour in Python <= 3.8

>>> issubclass(Employee, NamedTuple)
False
>>> isinstance(Employee(name='guido', id=1), NamedTuple)
False

typing.NamedTuple is a class, it uses metaclasses and a custom __new__ to handle the annotations, and then it delegates to collections.namedtuple, anyway, to build and return the type. As you may have guessed from the lowercased name convention, collections.namedtuple is not a type/class - it's a factory function. It works by building up a string of Python source code, and then calling exec on this string. The generated constructor is plucked out of a namespace and included in a 3-argument invocation of the metaclass type to build and return your class. This explains the weird inheritance breakage seen above, NamedTuple uses a metaclass in order to use a different metaclass to instantiate the class object.

Behaviour in Python >= 3.9

typing.NamedTuple is changed from a class to a def.

>>> issubclass(Employee, NamedTuple)
TypeError: issubclass() arg 2 must be a class or tuple of classes
>>> isinstance(Employee(name="guido", id=1), NamedTuple)
TypeError: isinstance() arg 2 must be a type or tuple of types

Multiple inheritance using NamedTuple is now disallowed (it did not work properly in the first place).

See bpo40185 / GH-19371 for the change.