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Python: type-hinting a classmethod that returns an instance of the class, for a class that is inherited

For an example of how to apply Brian’s answer: with a TypeVar: from typing import TypeVar AnyA = TypeVar(“AnyA”, bound=”A”) class A: def __init__(self): print(“A”) self.hello = “hello” @classmethod def bobo(cls: type[AnyA]) -> AnyA: return cls() class B(A): def __init__(self): print(“B”) super().__init__() self.world = “world” reveal_type(B.bobo()) # B with Self: from typing import Self class … Read more

Type annotation for classmethod returning instance

The trick is to explicitly add an annotation to the cls parameter, in combination with TypeVar, for generics, and Type, to represent a class rather than the instance itself, like so: from typing import TypeVar, Type # Create a generic variable that can be ‘Parent’, or any subclass. T = TypeVar(‘T’, bound=’Parent’) class Parent: def … Read more

How to annotate function that takes a tuple of variable length? (variadic tuple type annotation)

We can annotate variable-length homogeneous tuples using the … literal (aka Ellipsis) like this: def process_tuple(t: Tuple[str, …]): … or for Python3.9+ def process_tuple(t: tuple[str, …]): … After that, the errors should go away. From the docs: To specify a variable-length tuple of homogeneous type, use literal ellipsis, e.g. Tuple[int, …]. A plain Tuple is … Read more

typed python: using the classes’ own type inside class definition [duplicate]

The name Foo isn’t bound yet, because the class itself has not yet been defined at the time that you try to use the name (remember: function arguments are evaluated at function definition time, not at function call time). From Python 3.7+ you can postpone evaluation of annotations by adding this import at the top … Read more

mypy, type hint: Union[float, int] -> is there a Number type?

Use float only, as int is implied in that type: def my_func(number: float): PEP 484 Type Hints specifically states that: Rather than requiring that users write import numbers and then use numbers.Float etc., this PEP proposes a straightforward shortcut that is almost as effective: when an argument is annotated as having type float, an argument … Read more

A way to subclass NamedTuple for purposes of typechecking

The way named tuples are constructed make inheritance from typing.NamedTuple classes as yet not possible. You’d have to write your own metaclass to extend the typing.NamedTupleMeta class to make subclassing work, and even then the class generated by collections.namedtuple() is just not built to extend. Instead, you want to use the new dataclasses module to … Read more

Can I omit Optional if I set default to None?

No. Omitting Optional was previously allowed, but has since been removed. A past version of this PEP allowed type checkers to assume an optional type when the default value is None […] This is no longer the recommended behavior. Type checkers should move towards requiring the optional type to be made explicit. Some tools may … Read more

Specify length of Sequence or List with Python typing module

You can’t. A list is a mutable, variable length structure. If you need a fixed-length structure, use a tuple instead: Tuple[float, float, float, float, float, float, float, float, float, float] Or better still, use a named tuple, which has both indices and named attributes: class BunchOfFloats(NamedTuple): foo: float bar: float baz: float spam: float ham: … Read more

Exclude type in Python typing annotation

Python type hinting can’t exclude types. You can’t exclude Nones, strs or any another type. The only thing you can use to try to emulate None exclusion is to use Union and write every type you are actually using in the dictionary.

How to use type hints in python 3.6?

Type hints are entirely meant to be ignored by the Python runtime, and are checked only by 3rd party tools like mypy and Pycharm’s integrated checker. There are also a variety of lesser known 3rd party tools that do typechecking at either compile time or runtime using type annotations, but most people use mypy or … Read more