How to add a dataclass field without annotating the type?

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'

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.