misc/proper_plugin.py

from mypy.plugin import Plugin, FunctionContext
from mypy.types import (
    Type, Instance, CallableType, UnionType, get_proper_type, ProperType,
    get_proper_types, TupleType, NoneTyp, AnyType
)
from mypy.nodes import TypeInfo
from mypy.subtypes import is_proper_subtype

from typing_extensions import Type as typing_Type
from typing import Optional, Callable


class ProperTypePlugin(Plugin):
    """
    A plugin to ensure that every type is expanded before doing any special-casing.

    This solves the problem that we have hundreds of call sites like:

        if isinstance(typ, UnionType):
            ...  # special-case union

    But after introducing a new type TypeAliasType (and removing immediate expansion)
    all these became dangerous because typ may be e.g. an alias to union.
    """
    def get_function_hook(self, fullname: str
                          ) -> Optional[Callable[[FunctionContext], Type]]:
        if fullname == 'builtins.isinstance':
            return isinstance_proper_hook
        if fullname == 'mypy.types.get_proper_type':
            return proper_type_hook
        if fullname == 'mypy.types.get_proper_types':
            return proper_types_hook
        return None


def isinstance_proper_hook(ctx: FunctionContext) -> Type:
    right = get_proper_type(ctx.arg_types[1][0])
    for arg in ctx.arg_types[0]:
        if (is_improper_type(arg) or
                isinstance(get_proper_type(arg), AnyType) and is_dangerous_target(right)):
            if is_special_target(right):
                return ctx.default_return_type
            ctx.api.fail('Never apply isinstance() to unexpanded types;'
                         ' use mypy.types.get_proper_type() first', ctx.context)
    return ctx.default_return_type


def is_special_target(right: ProperType) -> bool:
    """Whitelist some special cases for use in isinstance() with improper types."""
    if isinstance(right, CallableType) and right.is_type_obj():
        if right.type_object().fullname() == 'builtins.tuple':
            # Used with Union[Type, Tuple[Type, ...]].
            return True
        if right.type_object().fullname() in ('mypy.types.Type',
                                              'mypy.types.ProperType',
                                              'mypy.types.TypeAliasType'):
            # Special case: things like assert isinstance(typ, ProperType) are always OK.
            return True
        if right.type_object().fullname() in ('mypy.types.UnboundType',
                                              'mypy.types.TypeVarType',
                                              'mypy.types.RawExpressionType',
                                              'mypy.types.EllipsisType',
                                              'mypy.types.StarType',
                                              'mypy.types.TypeList',
                                              'mypy.types.CallableArgument',
                                              'mypy.types.PartialType',
                                              'mypy.types.ErasedType'):
            # Special case: these are not valid targets for a type alias and thus safe.
            # TODO: introduce a SyntheticType base to simplify this?
            return True
    elif isinstance(right, TupleType):
        return all(is_special_target(t) for t in get_proper_types(right.items))
    return False


def is_improper_type(typ: Type) -> bool:
    """Is this a type that is not a subtype of ProperType?"""
    typ = get_proper_type(typ)
    if isinstance(typ, Instance):
        info = typ.type
        return info.has_base('mypy.types.Type') and not info.has_base('mypy.types.ProperType')
    if isinstance(typ, UnionType):
        return any(is_improper_type(t) for t in typ.items)
    return False


def is_dangerous_target(typ: ProperType) -> bool:
    """Is this a dangerous target (right argument) for an isinstance() check?"""
    if isinstance(typ, TupleType):
        return any(is_dangerous_target(get_proper_type(t)) for t in typ.items)
    if isinstance(typ, CallableType) and typ.is_type_obj():
        return typ.type_object().has_base('mypy.types.Type')
    return False


def proper_type_hook(ctx: FunctionContext) -> Type:
    """Check if this get_proper_type() call is not redundant."""
    arg_types = ctx.arg_types[0]
    if arg_types:
        arg_type = get_proper_type(arg_types[0])
        proper_type = get_proper_type_instance(ctx)
        if is_proper_subtype(arg_type, UnionType.make_union([NoneTyp(), proper_type])):
            # Minimize amount of spurious errors from overload machinery.
            # TODO: call the hook on the overload as a whole?
            if isinstance(arg_type, (UnionType, Instance)):
                ctx.api.fail('Redundant call to get_proper_type()', ctx.context)
    return ctx.default_return_type


def proper_types_hook(ctx: FunctionContext) -> Type:
    """Check if this get_proper_types() call is not redundant."""
    arg_types = ctx.arg_types[0]
    if arg_types:
        arg_type = arg_types[0]
        proper_type = get_proper_type_instance(ctx)
        item_type = UnionType.make_union([NoneTyp(), proper_type])
        ok_type = ctx.api.named_generic_type('typing.Iterable', [item_type])
        if is_proper_subtype(arg_type, ok_type):
            ctx.api.fail('Redundant call to get_proper_types()', ctx.context)
    return ctx.default_return_type


def get_proper_type_instance(ctx: FunctionContext) -> Instance:
    types = ctx.api.modules['mypy.types']  # type: ignore
    proper_type_info = types.names['ProperType']
    assert isinstance(proper_type_info.node, TypeInfo)
    return Instance(proper_type_info.node, [])


def plugin(version: str) -> typing_Type[ProperTypePlugin]:
    return ProperTypePlugin