注释不存在的 dict 字段不应该产生错误吗？

Question

谁能解释一下为什么在注释不存在的字典字段时没有错误？

dict_1 = {}
dict_1['a']: 'aa' #used colon by mistake instead of assign, code passes without any error on python3.7.2

print(__annotations__) # prints empty dict {}
dict_1['a'] # as expected KeyError: 'a'

编辑：在测试了更多案例后，我发现注释现有的字典字段也会默默地产生任何结果。

dict_2 = {'a': 'b'}
dict_2['a']: 'c' # no error here so I would expect to get new annotation
print(__annotations__) # produces empty dict {}

Answer 1

您可以注释任何有效的分配目标。参考 Annotating Expressions 来自 PEP 536：

The target of the annotation can be any valid single assignment target, at least syntactically (it is up to the type checker what to do with this):
class Cls:
    pass

c = Cls()
c.x: int = 0  # Annotates c.x with int.
c.y: int      # Annotates c.y with int.

d = {}
d['a']: int = 0  # Annotates d['a'] with int.
d['b']: int      # Annotates d['b'] with int.
Note that even a parenthesized name is considered an expression, not a simple name:
(x): int      # Annotates x with int, (x) treated as expression by compiler.
(y): int = 0  # Same situation here.

documentation for annotated assignment statements 表示这些值未存储。大概是由静态类型检查工具来存储它们。

For expressions as assignment targets, the annotations are evaluated if in class or module scope, but not stored.

注释不存在的 dict 字段不应该产生错误吗？

Shouldn't annotating nonexisting dict field produce error?

python

python-3.x

python-3.7