dataclasses.py

"""Plugin that provides support for dataclasses."""

from typing import Dict, List, Set, Tuple, Optional
from typing_extensions import Final

from mypy.nodes import (
    ARG_OPT, ARG_NAMED, ARG_NAMED_OPT, ARG_POS, ARG_STAR, ARG_STAR2, MDEF,
    Argument, AssignmentStmt, CallExpr,  TypeAlias,  Context, Expression, JsonDict,
    NameExpr, RefExpr, SymbolTableNode, TempNode, TypeInfo, Var, TypeVarExpr,
    PlaceholderNode
)
from mypy.plugin import ClassDefContext, SemanticAnalyzerPluginInterface
from mypy.plugins.common import (
    add_method, _get_decorator_bool_argument, deserialize_and_fixup_type, add_attribute_to_class,
)
from mypy.typeops import map_type_from_supertype
from mypy.types import (
    Type, Instance, NoneType, TypeVarType, CallableType, TupleType, LiteralType,
    get_proper_type, AnyType, TypeOfAny,
)
from mypy.server.trigger import make_wildcard_trigger
from mypy.state import state

# The set of decorators that generate dataclasses.
dataclass_makers: Final = {
    'dataclass',
    'dataclasses.dataclass',
}
# The set of functions that generate dataclass fields.
field_makers: Final = {
    'dataclasses.field',
}


SELF_TVAR_NAME: Final = "_DT"


class DataclassAttribute:
    def __init__(
            self,
            name: str,
            is_in_init: bool,
            is_init_var: bool,
            has_default: bool,
            line: int,
            column: int,
            type: Optional[Type],
            info: TypeInfo,
            kw_only: bool,
    ) -> None:
        self.name = name
        self.is_in_init = is_in_init
        self.is_init_var = is_init_var
        self.has_default = has_default
        self.line = line
        self.column = column
        self.type = type
        self.info = info
        self.kw_only = kw_only

    def to_argument(self) -> Argument:
        arg_kind = ARG_POS
        if self.kw_only and self.has_default:
            arg_kind = ARG_NAMED_OPT
        elif self.kw_only and not self.has_default:
            arg_kind = ARG_NAMED
        elif not self.kw_only and self.has_default:
            arg_kind = ARG_OPT
        return Argument(
            variable=self.to_var(),
            type_annotation=self.type,
            initializer=None,
            kind=arg_kind,
        )

    def to_var(self) -> Var:
        return Var(self.name, self.type)

    def serialize(self) -> JsonDict:
        assert self.type
        return {
            'name': self.name,
            'is_in_init': self.is_in_init,
            'is_init_var': self.is_init_var,
            'has_default': self.has_default,
            'line': self.line,
            'column': self.column,
            'type': self.type.serialize(),
            'kw_only': self.kw_only,
        }

    @classmethod
    def deserialize(
        cls, info: TypeInfo, data: JsonDict, api: SemanticAnalyzerPluginInterface
    ) -> 'DataclassAttribute':
        data = data.copy()
        if data.get('kw_only') is None:
            data['kw_only'] = False
        typ = deserialize_and_fixup_type(data.pop('type'), api)
        return cls(type=typ, info=info, **data)

    def expand_typevar_from_subtype(self, sub_type: TypeInfo) -> None:
        """Expands type vars in the context of a subtype when an attribute is inherited
        from a generic super type."""
        if self.type is not None:
            self.type = map_type_from_supertype(self.type, sub_type, self.info)


class DataclassTransformer:
    """Implement the behavior of @dataclass.

    Note that this may be executed multiple times on the same class, so
    everything here must be idempotent.

    This runs after the main semantic analysis pass, so you can assume that
    there are no placeholders.
    """

    def __init__(self, ctx: ClassDefContext) -> None:
        self._ctx = ctx

    def transform(self) -> bool:
        """Apply all the necessary transformations to the underlying
        dataclass so as to ensure it is fully type checked according
        to the rules in PEP 557.
        """
        ctx = self._ctx
        info = self._ctx.cls.info
        attributes = self.collect_attributes()
        if attributes is None:
            # Some definitions are not ready. We need another pass.
            return False
        for attr in attributes:
            if attr.type is None:
                return False
        decorator_arguments = {
            'init': _get_decorator_bool_argument(self._ctx, 'init', True),
            'eq': _get_decorator_bool_argument(self._ctx, 'eq', True),
            'order': _get_decorator_bool_argument(self._ctx, 'order', False),
            'frozen': _get_decorator_bool_argument(self._ctx, 'frozen', False),
            'slots': _get_decorator_bool_argument(self._ctx, 'slots', False),
            'match_args': _get_decorator_bool_argument(self._ctx, 'match_args', True),
        }
        py_version = self._ctx.api.options.python_version

        # If there are no attributes, it may be that the semantic analyzer has not
        # processed them yet. In order to work around this, we can simply skip generating
        # __init__ if there are no attributes, because if the user truly did not define any,
        # then the object default __init__ with an empty signature will be present anyway.
        if (decorator_arguments['init'] and
                ('__init__' not in info.names or info.names['__init__'].plugin_generated) and
                attributes):

            args = [attr.to_argument() for attr in attributes if attr.is_in_init
                    and not self._is_kw_only_type(attr.type)]

            if info.fallback_to_any:
                # Make positional args optional since we don't know their order.
                # This will at least allow us to typecheck them if they are called
                # as kwargs
                for arg in args:
                    if arg.kind == ARG_POS:
                        arg.kind = ARG_OPT

                nameless_var = Var('')
                args = [Argument(nameless_var, AnyType(TypeOfAny.explicit), None, ARG_STAR),
                        *args,
                        Argument(nameless_var, AnyType(TypeOfAny.explicit), None, ARG_STAR2),
                        ]

            add_method(
                ctx,
                '__init__',
                args=args,
                return_type=NoneType(),
            )

        if (decorator_arguments['eq'] and info.get('__eq__') is None or
                decorator_arguments['order']):
            # Type variable for self types in generated methods.
            obj_type = ctx.api.named_type('builtins.object')
            self_tvar_expr = TypeVarExpr(SELF_TVAR_NAME, info.fullname + '.' + SELF_TVAR_NAME,
                                         [], obj_type)
            info.names[SELF_TVAR_NAME] = SymbolTableNode(MDEF, self_tvar_expr)

        # Add <, >, <=, >=, but only if the class has an eq method.
        if decorator_arguments['order']:
            if not decorator_arguments['eq']:
                ctx.api.fail('eq must be True if order is True', ctx.cls)

            for method_name in ['__lt__', '__gt__', '__le__', '__ge__']:
                # Like for __eq__ and __ne__, we want "other" to match
                # the self type.
                obj_type = ctx.api.named_type('builtins.object')
                order_tvar_def = TypeVarType(SELF_TVAR_NAME, info.fullname + '.' + SELF_TVAR_NAME,
                                            -1, [], obj_type)
                order_return_type = ctx.api.named_type('builtins.bool')
                order_args = [
                    Argument(Var('other', order_tvar_def), order_tvar_def, None, ARG_POS)
                ]

                existing_method = info.get(method_name)
                if existing_method is not None and not existing_method.plugin_generated:
                    assert existing_method.node
                    ctx.api.fail(
                        f'You may not have a custom {method_name} method when order=True',
                        existing_method.node,
                    )

                add_method(
                    ctx,
                    method_name,
                    args=order_args,
                    return_type=order_return_type,
                    self_type=order_tvar_def,
                    tvar_def=order_tvar_def,
                )

        if decorator_arguments['frozen']:
            self._propertize_callables(attributes, settable=False)
            self._freeze(attributes)
        else:
            self._propertize_callables(attributes)

        if decorator_arguments['slots']:
            self.add_slots(info, attributes, correct_version=py_version >= (3, 10))

        self.reset_init_only_vars(info, attributes)

        if (decorator_arguments['match_args'] and
                ('__match_args__' not in info.names or
                 info.names['__match_args__'].plugin_generated) and
                attributes):
            str_type = ctx.api.named_type("builtins.str")
            literals: List[Type] = [LiteralType(attr.name, str_type)
                        for attr in attributes if attr.is_in_init]
            match_args_type = TupleType(literals, ctx.api.named_type("builtins.tuple"))
            add_attribute_to_class(ctx.api, ctx.cls, "__match_args__", match_args_type)

        self._add_dataclass_fields_magic_attribute()

        info.metadata['dataclass'] = {
            'attributes': [attr.serialize() for attr in attributes],
            'frozen': decorator_arguments['frozen'],
        }

        return True

    def add_slots(self,
                  info: TypeInfo,
                  attributes: List[DataclassAttribute],
                  *,
                  correct_version: bool) -> None:
        if not correct_version:
            # This means that version is lower than `3.10`,
            # it is just a non-existent argument for `dataclass` function.
            self._ctx.api.fail(
                'Keyword argument "slots" for "dataclass" '
                'is only valid in Python 3.10 and higher',
                self._ctx.reason,
            )
            return

        generated_slots = {attr.name for attr in attributes}
        if ((info.slots is not None and info.slots != generated_slots)
                or info.names.get('__slots__')):
            # This means we have a slots conflict.
            # Class explicitly specifies a different `__slots__` field.
            # And `@dataclass(slots=True)` is used.
            # In runtime this raises a type error.
            self._ctx.api.fail(
                '"{}" both defines "__slots__" and is used with "slots=True"'.format(
                    self._ctx.cls.name,
                ),
                self._ctx.cls,
            )
            return

        info.slots = generated_slots

    def reset_init_only_vars(self, info: TypeInfo, attributes: List[DataclassAttribute]) -> None:
        """Remove init-only vars from the class and reset init var declarations."""
        for attr in attributes:
            if attr.is_init_var:
                if attr.name in info.names:
                    del info.names[attr.name]
                else:
                    # Nodes of superclass InitVars not used in __init__ cannot be reached.
                    assert attr.is_init_var
                for stmt in info.defn.defs.body:
                    if isinstance(stmt, AssignmentStmt) and stmt.unanalyzed_type:
                        lvalue = stmt.lvalues[0]
                        if isinstance(lvalue, NameExpr) and lvalue.name == attr.name:
                            # Reset node so that another semantic analysis pass will
                            # recreate a symbol node for this attribute.
                            lvalue.node = None

    def collect_attributes(self) -> Optional[List[DataclassAttribute]]:
        """Collect all attributes declared in the dataclass and its parents.

        All assignments of the form

          a: SomeType
          b: SomeOtherType = ...

        are collected.

        Return None if some dataclass base class hasn't been processed
        yet and thus we'll need to ask for another pass.
        """
        # First, collect attributes belonging to the current class.
        ctx = self._ctx
        cls = self._ctx.cls
        attrs: List[DataclassAttribute] = []
        known_attrs: Set[str] = set()
        kw_only = _get_decorator_bool_argument(ctx, 'kw_only', False)
        for stmt in cls.defs.body:
            # Any assignment that doesn't use the new type declaration
            # syntax can be ignored out of hand.
            if not (isinstance(stmt, AssignmentStmt) and stmt.new_syntax):
                continue

            # a: int, b: str = 1, 'foo' is not supported syntax so we
            # don't have to worry about it.
            lhs = stmt.lvalues[0]
            if not isinstance(lhs, NameExpr):
                continue

            sym = cls.info.names.get(lhs.name)
            if sym is None:
                # There was probably a semantic analysis error.
                continue

            node = sym.node
            assert not isinstance(node, PlaceholderNode)

            if isinstance(node, TypeAlias):
                ctx.api.fail(
                    (
                        'Type aliases inside dataclass definitions '
                        'are not supported at runtime'
                    ),
                    node
                )
                # Skip processing this node. This doesn't match the runtime behaviour,
                # but the only alternative would be to modify the SymbolTable,
                # and it's a little hairy to do that in a plugin.
                continue

            assert isinstance(node, Var)

            # x: ClassVar[int] is ignored by dataclasses.
            if node.is_classvar:
                continue

            # x: InitVar[int] is turned into x: int and is removed from the class.
            is_init_var = False
            node_type = get_proper_type(node.type)
            if (isinstance(node_type, Instance) and
                    node_type.type.fullname == 'dataclasses.InitVar'):
                is_init_var = True
                node.type = node_type.args[0]

            if self._is_kw_only_type(node_type):
                kw_only = True

            has_field_call, field_args = _collect_field_args(stmt.rvalue, ctx)

            is_in_init_param = field_args.get('init')
            if is_in_init_param is None:
                is_in_init = True
            else:
                is_in_init = bool(ctx.api.parse_bool(is_in_init_param))

            has_default = False
            # Ensure that something like x: int = field() is rejected
            # after an attribute with a default.
            if has_field_call:
                has_default = 'default' in field_args or 'default_factory' in field_args

            # All other assignments are already type checked.
            elif not isinstance(stmt.rvalue, TempNode):
                has_default = True

            if not has_default:
                # Make all non-default attributes implicit because they are de-facto set
                # on self in the generated __init__(), not in the class body.
                sym.implicit = True

            is_kw_only = kw_only
            # Use the kw_only field arg if it is provided. Otherwise use the
            # kw_only value from the decorator parameter.
            field_kw_only_param = field_args.get('kw_only')
            if field_kw_only_param is not None:
                is_kw_only = bool(ctx.api.parse_bool(field_kw_only_param))

            known_attrs.add(lhs.name)
            attrs.append(DataclassAttribute(
                name=lhs.name,
                is_in_init=is_in_init,
                is_init_var=is_init_var,
                has_default=has_default,
                line=stmt.line,
                column=stmt.column,
                type=sym.type,
                info=cls.info,
                kw_only=is_kw_only,
            ))

        # Next, collect attributes belonging to any class in the MRO
        # as long as those attributes weren't already collected.  This
        # makes it possible to overwrite attributes in subclasses.
        # copy() because we potentially modify all_attrs below and if this code requires debugging
        # we'll have unmodified attrs laying around.
        all_attrs = attrs.copy()
        for info in cls.info.mro[1:-1]:
            if 'dataclass_tag' in info.metadata and 'dataclass' not in info.metadata:
                # We haven't processed the base class yet. Need another pass.
                return None
            if 'dataclass' not in info.metadata:
                continue

            super_attrs = []
            # Each class depends on the set of attributes in its dataclass ancestors.
            ctx.api.add_plugin_dependency(make_wildcard_trigger(info.fullname))

            for data in info.metadata["dataclass"]["attributes"]:
                name: str = data["name"]
                if name not in known_attrs:
                    attr = DataclassAttribute.deserialize(info, data, ctx.api)
                    # TODO: We shouldn't be performing type operations during the main
                    #       semantic analysis pass, since some TypeInfo attributes might
                    #       still be in flux. This should be performed in a later phase.
                    with state.strict_optional_set(ctx.api.options.strict_optional):
                        attr.expand_typevar_from_subtype(ctx.cls.info)
                    known_attrs.add(name)
                    super_attrs.append(attr)
                elif all_attrs:
                    # How early in the attribute list an attribute appears is determined by the
                    # reverse MRO, not simply MRO.
                    # See https://docs.python.org/3/library/dataclasses.html#inheritance for
                    # details.
                    for attr in all_attrs:
                        if attr.name == name:
                            all_attrs.remove(attr)
                            super_attrs.append(attr)
                            break
            all_attrs = super_attrs + all_attrs
            all_attrs.sort(key=lambda a: a.kw_only)

        # Ensure that arguments without a default don't follow
        # arguments that have a default.
        found_default = False
        # Ensure that the KW_ONLY sentinel is only provided once
        found_kw_sentinel = False
        for attr in all_attrs:
            # If we find any attribute that is_in_init, not kw_only, and that
            # doesn't have a default after one that does have one,
            # then that's an error.
            if found_default and attr.is_in_init and not attr.has_default and not attr.kw_only:
                # If the issue comes from merging different classes, report it
                # at the class definition point.
                context = (Context(line=attr.line, column=attr.column) if attr in attrs
                           else ctx.cls)
                ctx.api.fail(
                    'Attributes without a default cannot follow attributes with one',
                    context,
                )

            found_default = found_default or (attr.has_default and attr.is_in_init)
            if found_kw_sentinel and self._is_kw_only_type(attr.type):
                context = (Context(line=attr.line, column=attr.column) if attr in attrs
                           else ctx.cls)
                ctx.api.fail(
                    'There may not be more than one field with the KW_ONLY type',
                    context,
                )
            found_kw_sentinel = found_kw_sentinel or self._is_kw_only_type(attr.type)

        return all_attrs

    def _freeze(self, attributes: List[DataclassAttribute]) -> None:
        """Converts all attributes to @property methods in order to
        emulate frozen classes.
        """
        info = self._ctx.cls.info
        for attr in attributes:
            sym_node = info.names.get(attr.name)
            if sym_node is not None:
                var = sym_node.node
                assert isinstance(var, Var)
                var.is_property = True
            else:
                var = attr.to_var()
                var.info = info
                var.is_property = True
                var._fullname = info.fullname + '.' + var.name
                info.names[var.name] = SymbolTableNode(MDEF, var)

    def _propertize_callables(self,
                              attributes: List[DataclassAttribute],
                              settable: bool = True) -> None:
        """Converts all attributes with callable types to @property methods.

        This avoids the typechecker getting confused and thinking that
        `my_dataclass_instance.callable_attr(foo)` is going to receive a
        `self` argument (it is not).

        """
        info = self._ctx.cls.info
        for attr in attributes:
            if isinstance(get_proper_type(attr.type), CallableType):
                var = attr.to_var()
                var.info = info
                var.is_property = True
                var.is_settable_property = settable
                var._fullname = info.fullname + '.' + var.name
                info.names[var.name] = SymbolTableNode(MDEF, var)

    def _is_kw_only_type(self, node: Optional[Type]) -> bool:
        """Checks if the type of the node is the KW_ONLY sentinel value."""
        if node is None:
            return False
        node_type = get_proper_type(node)
        if not isinstance(node_type, Instance):
            return False
        return node_type.type.fullname == 'dataclasses.KW_ONLY'

    def _add_dataclass_fields_magic_attribute(self) -> None:
        attr_name = '__dataclass_fields__'
        any_type = AnyType(TypeOfAny.explicit)
        field_type = self._ctx.api.named_type_or_none('dataclasses.Field', [any_type]) or any_type
        attr_type = self._ctx.api.named_type('builtins.dict', [
            self._ctx.api.named_type('builtins.str'),
            field_type,
        ])
        var = Var(name=attr_name, type=attr_type)
        var.info = self._ctx.cls.info
        var._fullname = self._ctx.cls.info.fullname + '.' + attr_name
        self._ctx.cls.info.names[attr_name] = SymbolTableNode(
            kind=MDEF,
            node=var,
            plugin_generated=True,
        )


def dataclass_tag_callback(ctx: ClassDefContext) -> None:
    """Record that we have a dataclass in the main semantic analysis pass.

    The later pass implemented by DataclassTransformer will use this
    to detect dataclasses in base classes.
    """
    # The value is ignored, only the existence matters.
    ctx.cls.info.metadata['dataclass_tag'] = {}


def dataclass_class_maker_callback(ctx: ClassDefContext) -> bool:
    """Hooks into the class typechecking process to add support for dataclasses.
    """
    transformer = DataclassTransformer(ctx)
    return transformer.transform()


def _collect_field_args(expr: Expression,
                        ctx: ClassDefContext) -> Tuple[bool, Dict[str, Expression]]:
    """Returns a tuple where the first value represents whether or not
    the expression is a call to dataclass.field and the second is a
    dictionary of the keyword arguments that field() was called with.
    """
    if (
            isinstance(expr, CallExpr) and
            isinstance(expr.callee, RefExpr) and
            expr.callee.fullname in field_makers
    ):
        # field() only takes keyword arguments.
        args = {}
        for name, arg, kind in zip(expr.arg_names, expr.args, expr.arg_kinds):
            if not kind.is_named():
                if kind.is_named(star=True):
                    # This means that `field` is used with `**` unpacking,
                    # the best we can do for now is not to fail.
                    # TODO: we can infer what's inside `**` and try to collect it.
                    message = 'Unpacking **kwargs in "field()" is not supported'
                else:
                    message = '"field()" does not accept positional arguments'
                ctx.api.fail(message, expr)
                return True, {}
            assert name is not None
            args[name] = arg
        return True, args
    return False, {}