python - 注释不存在的 dict 字段不应该产生错误吗？

Question

有人能解释一下为什么在注释不存在的字典字段时没有错误吗？

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 dict 字段也不会产生任何结果。

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。引用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 ，将表明这些值没有被存储。大概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.