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?
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:
collections.namedtuple
got a new defaults
keyword so this is no longer an advantage)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
>>> 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
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.
typing.NamedTuple
is changed from a type (class
) to a function (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
The metaclass acrobatics are gone, now it's just a simple factory function which calls collections.namedtuple
and then sets __annotations__
on the returned type. Multiple inheritance using NamedTuple
is now disallowed (it did not work properly in the first place).