def _get_field()

in library/dataclasses.py [0:0]

• 31 lines of code
• 33 McCabe index (conditional complexity)

def _get_field(cls, a_name, a_type):
    # Return a Field object for this field name and type.  ClassVars
    # and InitVars are also returned, but marked as such (see
    # f._field_type).

    # If the default value isn't derived from Field, then it's only a
    # normal default value.  Convert it to a Field().
    default = getattr(cls, a_name, MISSING)
    if isinstance(default, Field):
        f = default
    else:
        # TODO(T42989996): Uncomment when member descriptors are implemented
        # if isinstance(default, types.MemberDescriptorType):
        #     # This is a field in __slots__, so it has no default value.
        #     default = MISSING
        f = field(default=default)

    # Only at this point do we know the name and the type.  Set them.
    f.name = a_name
    f.type = a_type

    # Assume it's a normal field until proven otherwise.  We're next
    # going to decide if it's a ClassVar or InitVar, everything else
    # is just a normal field.
    f._field_type = _FIELD

    # In addition to checking for actual types here, also check for
    # string annotations.  get_type_hints() won't always work for us
    # (see https://github.com/python/typing/issues/508 for example),
    # plus it's expensive and would require an eval for every stirng
    # annotation.  So, make a best effort to see if this is a ClassVar
    # or InitVar using regex's and checking that the thing referenced
    # is actually of the correct type.

    # For the complete discussion, see https://bugs.python.org/issue33453

    # If typing has not been imported, then it's impossible for any
    # annotation to be a ClassVar.  So, only look for ClassVar if
    # typing has been imported by any module (not necessarily cls's
    # module).
    typing = sys.modules.get('typing')
    if typing:
        if (_is_classvar(a_type, typing)
            or (isinstance(f.type, str)
                and _is_type(f.type, cls, typing, typing.ClassVar,
                             _is_classvar))):
            f._field_type = _FIELD_CLASSVAR

    # If the type is InitVar, or if it's a matching string annotation,
    # then it's an InitVar.
    if f._field_type is _FIELD:
        # The module we're checking against is the module we're
        # currently in (dataclasses.py).
        dataclasses = sys.modules[__name__]
        if (_is_initvar(a_type, dataclasses)
            or (isinstance(f.type, str)
                and _is_type(f.type, cls, dataclasses, dataclasses.InitVar,
                             _is_initvar))):
            f._field_type = _FIELD_INITVAR

    # Validations for individual fields.  This is delayed until now,
    # instead of in the Field() constructor, since only here do we
    # know the field name, which allows for better error reporting.

    # Special restrictions for ClassVar and InitVar.
    if f._field_type in (_FIELD_CLASSVAR, _FIELD_INITVAR):
        if f.default_factory is not MISSING:
            raise TypeError(f'field {f.name} cannot have a '
                            'default factory')
        # Should I check for other field settings? default_factory
        # seems the most serious to check for.  Maybe add others.  For
        # example, how about init=False (or really,
        # init=<not-the-default-init-value>)?  It makes no sense for
        # ClassVar and InitVar to specify init=<anything>.

    # For real fields, disallow mutable defaults for known types.
    if f._field_type is _FIELD and isinstance(f.default, (list, dict, set)):
        raise ValueError(f'mutable default {type(f.default)} for field '
                         f'{f.name} is not allowed: use default_factory')

    return f