重用数据类类型提示

Question

我正在尝试在我的函数签名中重用来自数据类的类型提示 - 也就是说，无需再次键入签名。

解决这个问题的最佳方法是什么？

from dataclasses import dataclass
from typing import Set, Tuple, Type

@dataclass
class MyDataClass:
    force: Set[Tuple[str, float, bool]]

# I've had to write the same type annotation in the dataclass and the
# function signature - yuck
def do_something(force: Set[Tuple[str, float, bool]]):
    print(force)

# I want to do something like this, where I reference the type annotation from
# the dataclass. But, doing it this way, pycharm thinks `force` is type `Any`
def do_something_2(force: Type["MyDataClass.force"]):
    print(force)

Answer 1

What would be the best way of going about this?

PEP 484 为这种情况给出了一个明确的选择

Type aliases

Type aliases are defined by simple variable assignments: (...) Type aliases may be as complex as type hints in annotations -- anything that is acceptable as a type hint is acceptable in a type alias:

应用于您的示例，这相当于（Mypy 确认这是正确的）

from dataclasses import dataclass

Your_Type = set[tuple[str, float, bool]]


@dataclass
class MyDataClass:
    force: Your_Type


def do_something(force: Your_Type):
    print(force)

以上是使用 Python 3.9 以后编写的 Generic Alias Type. The syntax is more concise and modern since typing.Set and typing.Tuple 已被弃用。

现在，根据 Python Data Model 完全理解这一点比看起来要复杂得多：

3.1. Objects, values and types

Every object has an identity, a type and a value.

你第一次尝试使用 Type 会得到一个惊人的结果

>>> type(MyDataClass.force)

AttributeError: type object 'MyDataClass' has no attribute 'force'

这是因为内置函数 type returns 是一个类型（它本身就是一个对象）但是 MyDataClass 是“一个 Class”（一个声明）并且“Class 属性”force 在 Class 上，而不是在 type() 查找它的 class 的类型对象上。仔细注意数据模型的区别：

Classes

These objects normally act as factories for new instances of themselves

Class Instances

Instances of arbitrary classes

如果您改为检查实例的类型，您将得到以下结果

>>> init_values: set = {(True, "the_str", 1.2)}

>>> a_var = MyDataClass(init_values)

>>> type(a_var)
<class '__main__.MyDataClass'>

>>> type(a_var.force)
<class 'set'>

现在让我们通过将 type() 应用于前面提到的 __anotations__ on the Class declaration object (here we see the Generic Alias type 来恢复 force 上的类型对象（不是类型提示）。（这里我们确实检查了 class 属性 force 上的类型对象）。

>>> type(MyDataClass.__annotations__['force'])
<class 'typing._GenericAlias'>

或者我们可以检查 Class 实例上的注释，并恢复我们习惯看到的类型提示。

>>> init_values: set = {(True, "the_str", 1.2)}
>>> a_var = MyDataClass(init_values)
>>> a_var.__annotations__

{'force': set[tuple[str, float, bool]]}

I've had to write the same type annotation in the dataclass and the function signature -

对于元组注释往往会变成长文字，这证明为简洁起见创建目的变量是合理的。但一般来说，显式签名更具描述性，这是大多数 API 的目标。

The typing Module

Fundamental building blocks:

Tuple, used by listing the element types, for example Tuple[int, int, str]. The empty tuple can be typed as Tuple[()]. Arbitrary-length homogeneous tuples can be expressed using one type and ellipsis, for example Tuple[int, ...]. (The ... here are part of the syntax, a literal ellipsis.)

重用数据类类型提示

Reusing Dataclass Type Hints

python

python-3.x

alias

mypy

python-typing