For common and simple use cases, no other functionality is required. There are, however, some dataclass features that require additional per-field information. To satisfy this need for additional information, you can replace the default field value with a call to the provided field() function. For example:
As shown above, the MISSING value is a sentinel object used to detect if some parameters are provided by the user. This sentinel is used because None is a valid value for some parameters with a distinct meaning. No code should directly use the MISSING value.
The parameters to field() are:
default: If provided, this will be the default value for this field. This is needed because the field() call itself replaces the normal position of the default value.
default_factory: If provided, it must be a zero-argument callable that will be called when a default value is needed for this field. Among other purposes, this can be used to specify fields with mutable default values, as discussed below. It is an error to specify both default and default_factory.
init: If true (the default), this field is included as a parameter to the generated __init__() method.
repr: If true (the default), this field is included in the string returned by the generated __repr__() method.
hash: This can be a bool or None. If true, this field is included in the generated __hash__() method. If false, this field is excluded from the generated __hash__(). If None (the default), use the value of compare: this would normally be the expected behavior, since a field should be included in the hash if it’s used for comparisons. Setting this value to anything other than None is discouraged. One possible reason to set hash=False but compare=True would be if a field is expensive to compute a hash value for, that field is needed for equality testing, and there are other fields that contribute to the type’s hash value. Even if a field is excluded from the hash, it will still be used for comparisons.
compare: If true (the default), this field is included in the generated equality and comparison methods (__eq__(), __gt__(), et al.).
metadata: This can be a mapping or None. None is treated as an empty dict. This value is wrapped in MappingProxyType() to make it read-only, and exposed on the Field object. It is not used at all by Data Classes, and is provided as a third-party extension mechanism. Multiple third-parties can each have their own key, to use as a namespace in the metadata.
kw_only: If true, this field will be marked as keyword-only. This is used when the generated __init__() method’s parameters are computed. Keyword-only fields are also not included in __match_args__.
Added in version 3.10.
doc: optional docstring for this field.
Added in version 3.14.
If the default value of a field is specified by a call to field(), then the class attribute for this field will be replaced by the specified default value. If default is not provided, then the class attribute will be deleted. The intent is that after the @dataclass decorator runs, the class attributes will all contain the default values for the fields, just as if the default value itself were specified. For example, after:
@dataclassclass C: x: int y: int = field(repr=False) z: int = field(repr=False, default=10) t: int = 20
The class attribute C.z will be 10, the class attribute C.t will be 20, and the class attributes C.x and C.y will not be set.