[ Python 常見問題 ] How should a python type hint require that a value has a given attribute?

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Question
Let's say I have a simple function like this:

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1. def handle(d: People) -> None:  
2.   print(f"{d.name}/{d.age}")  

I would like type hinting People to check if the given object has attributes "name" (str) and "age" (int).

HowTo
This is exactly what Protocols are for. In short, Protocols let you use structural instead of nominal subtyping. With nominal subtyping, type A is a subtype of B if A explicitly inherits or extends B. With structural subtyping, type A is a subtype of B if it has the same method and attribute "signatures" as B (with some restrictions).

For example:

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1. from typing import Protocol  
2.   
3.   
4. class People(Protocol):  
5.   name: str  
6.   age: int  
7.   
8.   
9. class P1:  
10.   def __init__(self) -> None:  
11.     self.name = 'John'  
12.     self.age = 1  
13.   
14.   
15. class P2:  
16.   def __init__(self) -> None:  
17.     self.name = 'Ken'  
18.     self.age = 56  
19.     self.extra_attr = 'test'  
20.   
21.   
22. class P3:  
23.   def __init__(self) -> None:  
24.     self.name = 'Mary'  
25.   
26.   
27. class P4:  
28.   def __init__(self) -> None:  
29.     self.age = 99  
30.   
31.   
32. def handle(d: People) -> None:  
33.   print(f"{d.name}/{d.age}")  
34.   
35. handle(P1())  
36. handle(P2())  
37. # "P3" is missing following "People" protocol member: age  
38. handle(P3())  
39. # "P4" is missing following "Device" protocol member: name  
40. handle(P4())  

You can leverage mypy to do the typing check:

# mypy test.py
test.py:40: error: Argument 1 to "handle" has incompatible type "P3"; expected "People"
test.py:40: note: "P3" is missing following "People" protocol member:
test.py:40: note: age
test.py:42: error: Argument 1 to "handle" has incompatible type "P4"; expected "People"
test.py:42: note: "P4" is missing following "People" protocol member:
test.py:42: note: name
Found 2 errors in 1 file (checked 1 source file)

You can also find slightly more complex examples of using Protocols in typeshed, the repository of type hints for the Python standard library.

Though, I suppose this all matters only if you actually intend on using static analysis in your code. If not, you could maybe do something simpler and just define a custom type alias to Any, document what that alias is "supposed" to mean, and use that alias instead of a full-fledged protocol. That alias would be almost completely useless for the purposes of static analysis/autocompletion tools/etc, but humans generally have no issues reading comments.

Supplement
* Is there a way to make flake8 check for type hints in the source