python - 为什么不能在 for 循环中使用类型提示？

Question

在 for 语句中注释目标是无效的:

>>> for i: str in test_string:
  File "<stdin>", line 1
    for i: str in test_string:
         ^
SyntaxError: invalid syntax

我想知道此时拒绝注释的原因。

Answer 1

冒号 : 历史上在 Python 中有多种含义。例如，它表示 block 的开始(with a as b:)、切片订阅(a[:])和字典对({a: b}).

这使得在某些位置引入 : 类型提示变得不可能或至少是模棱两可的。具体来说，for 或 with 等 block header 与 : 表示 header 结束的含义存在冲突。虽然可以 定义明确的语法，但要权衡 future 维护和扩展语法的需要。到目前为止，这被认为不值得。

PEP 526 - Rejected/Postponed Proposals

Allow annotations in with and for statement: This was rejected because in for it would make it hard to spot the actual iterable, and in with it would confuse the CPython's LL(1) parser.

---

目前，您可以使用前置类型提示，或内联类型注释:

a: str
for a in my_iterable:
    ...

for b in my_iterable:  # type: str
    ...

PEP 526 – Where annotations aren't allowed

Only single assignment targets and single right hand side values are allowed. In addition, one cannot annotate variables used in a for or with statement; they can be annotated ahead of time, in a similar manner to tuple unpacking:[...]