Type hints and chained assignment and multiple assignments

问题

I guess these two questions are related, so I'll post them together:

1.- Is it possible to put type hint in chained assignments?

These two attempts failed:

>>> def foo(a:int):
...     b: int = c:int = a
  File "<stdin>", line 2
    b: int = c:int = a
              ^
SyntaxError: invalid syntax
>>> def foo(a:int):
...     b = c:int = a
  File "<stdin>", line 2
    b = c:int = a
         ^
SyntaxError: invalid syntax

2.- Is it possible to put type hint in multiple assignments?

These were my attempts:

>>> from typing import Tuple
>>> def bar(a: Tuple[int]):
...     b: int, c:int = a
  File "<stdin>", line 2
    b: int, c:int = a
          ^
SyntaxError: invalid syntax
>>> def bar(a: Tuple[int]):
...     b, c:Tuple[int] = a
... 
  File "<stdin>", line 2
SyntaxError: only single target (not tuple) can be annotated
>>> def bar(a: Tuple[int]):
...     b, c:int = a
... 
  File "<stdin>", line 2
SyntaxError: only single target (not tuple) can be annotated

I am aware that in both cases the type is inferred from the type hint of a, but I have a long variable list (in the __init__ of a class) and I want to be extra-explicit.

I am using Python 3.6.8.

回答1:

1. As explicitly stated in PEP 526, section "Rejected/postponed proposals", annotations in chained assignments are not supported. Quoting the PEP:

   This has problems of ambiguity and readability similar to tuple unpacking, for example in:
   x: int = y = 1
z = w: int = 1
   it is ambiguous, what should the types of y and z be? Also the second line is difficult to parse.

2. For unpacking, per the same PEP, you should place bare annotations for your variables before the assignment. Example from the PEP:

   # Tuple unpacking with variable annotation syntax
   header: str
   kind: int
   body: Optional[List[str]]
   header, kind, body = message
   

来源：https://stackoverflow.com/questions/56611983/type-hints-and-chained-assignment-and-multiple-assignments