How to add a dataclass field without annotating th

问题:

When there is a field in a dataclass for which the type can be anything, how can you omit the annotation?

@dataclass
class Favs:
    fav_number: int = 80085
    fav_duck = object()
    fav_word: str = 'potato'

It seems the code above doesn't actually create a field for fav_duck. It just makes that a plain old class attribute.

>>> Favs()
Favs(fav_number=80085, fav_word='potato')
>>> print(*Favs.__dataclass_fields__)
fav_number fav_word
>>> Favs.fav_duck
<object at 0x7fffea519850>

回答1:

The dataclass decorator examines the class to find fields, by looking for names in __annotations__. It is the presence of annotation which makes the field, so, you do need an annotation.

You can, however, use a generic one:

@dataclass
class Favs:
    fav_number: int = 80085
    fav_duck: 'typing.Any' = object()
    fav_word: str = 'potato'

回答2:

According to PEP 557 which defines the meaning of data classes,

The dataclass decorator examines the class to find fields. A field is defined as any variable identified in __annotations__. That is, a variable that has a type annotation.

Which is to say that the premise of this question (e.g. "How can I use dataclass with a field that has no type annotation) must be rejected. The term 'field' in the context of dataclass necessitates that the attribute has a type annotation by definition.

Note that using a generic type annotation like typing.Any is not the same as having an unannotated attribute, since the attribute will appear in __annotations__.

Finally, the helper function make_dataclass will automatically use typing.Any for the type annotation in cases when only an attribute name is supplied, and this is also mentioned in the PEP with an example.