Invalid Named Tuple

What it does

Checks for invalidly defined NamedTuple classes.

Why is this bad?

An invalidly defined NamedTuple class may lead to the type checker drawing incorrect conclusions. It may also lead to TypeErrors or AttributeErrors at runtime.

Examples

A class definition cannot combine NamedTuple with other base classes in multiple inheritance; doing so raises a TypeError at runtime. The sole exception to this rule is Generic[], which can be used alongside NamedTuple in a class's bases list.

>>> from typing import NamedTuple
>>> class Foo(NamedTuple, object): ...
TypeError: can only inherit from a NamedTuple type and Generic

Further, NamedTuple field names cannot start with an underscore:

>>> from typing import NamedTuple
>>> class Foo(NamedTuple):
...     _bar: int
ValueError: Field names cannot start with an underscore: '_bar'

NamedTuple classes also have certain synthesized attributes (like _asdict, _make, _replace, etc.) that cannot be overwritten. Attempting to assign to these attributes without a type annotation will raise an AttributeError at runtime.

>>> from typing import NamedTuple
>>> class Foo(NamedTuple):
...     x: int
...     _asdict = 42
AttributeError: Cannot overwrite NamedTuple attribute _asdict