256 lines
10 KiB
Python
256 lines
10 KiB
Python
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"""
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This file contains a variety of plugins for refining how mypy infers types of
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expressions involving Enums.
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Currently, this file focuses on providing better inference for expressions like
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'SomeEnum.FOO.name' and 'SomeEnum.FOO.value'. Note that the type of both expressions
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will vary depending on exactly which instance of SomeEnum we're looking at.
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Note that this file does *not* contain all special-cased logic related to enums:
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we actually bake some of it directly in to the semantic analysis layer (see
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semanal_enum.py).
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"""
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from typing import Iterable, Optional, Sequence, TypeVar, cast
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from typing_extensions import Final
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import mypy.plugin # To avoid circular imports.
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from mypy.types import Type, Instance, LiteralType, CallableType, ProperType, get_proper_type
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from mypy.typeops import make_simplified_union
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from mypy.nodes import TypeInfo
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from mypy.subtypes import is_equivalent
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from mypy.semanal_enum import ENUM_BASES
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ENUM_NAME_ACCESS: Final = {f"{prefix}.name" for prefix in ENUM_BASES} | {
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f"{prefix}._name_" for prefix in ENUM_BASES
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}
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ENUM_VALUE_ACCESS: Final = {f"{prefix}.value" for prefix in ENUM_BASES} | {
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f"{prefix}._value_" for prefix in ENUM_BASES
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}
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def enum_name_callback(ctx: 'mypy.plugin.AttributeContext') -> Type:
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"""This plugin refines the 'name' attribute in enums to act as if
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they were declared to be final.
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For example, the expression 'MyEnum.FOO.name' normally is inferred
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to be of type 'str'.
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This plugin will instead make the inferred type be a 'str' where the
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last known value is 'Literal["FOO"]'. This means it would be legal to
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use 'MyEnum.FOO.name' in contexts that expect a Literal type, just like
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any other Final variable or attribute.
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This plugin assumes that the provided context is an attribute access
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matching one of the strings found in 'ENUM_NAME_ACCESS'.
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"""
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enum_field_name = _extract_underlying_field_name(ctx.type)
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if enum_field_name is None:
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return ctx.default_attr_type
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else:
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str_type = ctx.api.named_generic_type('builtins.str', [])
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literal_type = LiteralType(enum_field_name, fallback=str_type)
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return str_type.copy_modified(last_known_value=literal_type)
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_T = TypeVar('_T')
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def _first(it: Iterable[_T]) -> Optional[_T]:
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"""Return the first value from any iterable.
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Returns ``None`` if the iterable is empty.
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"""
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for val in it:
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return val
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return None
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def _infer_value_type_with_auto_fallback(
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ctx: 'mypy.plugin.AttributeContext',
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proper_type: Optional[ProperType]) -> Optional[Type]:
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"""Figure out the type of an enum value accounting for `auto()`.
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This method is a no-op for a `None` proper_type and also in the case where
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the type is not "enum.auto"
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"""
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if proper_type is None:
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return None
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if not (isinstance(proper_type, Instance) and
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proper_type.type.fullname == 'enum.auto'):
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return proper_type
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assert isinstance(ctx.type, Instance), 'An incorrect ctx.type was passed.'
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info = ctx.type.type
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# Find the first _generate_next_value_ on the mro. We need to know
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# if it is `Enum` because `Enum` types say that the return-value of
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# `_generate_next_value_` is `Any`. In reality the default `auto()`
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# returns an `int` (presumably the `Any` in typeshed is to make it
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# easier to subclass and change the returned type).
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type_with_gnv = _first(
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ti for ti in info.mro if ti.names.get('_generate_next_value_'))
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if type_with_gnv is None:
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return ctx.default_attr_type
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stnode = type_with_gnv.names['_generate_next_value_']
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# This should be a `CallableType`
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node_type = get_proper_type(stnode.type)
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if isinstance(node_type, CallableType):
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if type_with_gnv.fullname == 'enum.Enum':
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int_type = ctx.api.named_generic_type('builtins.int', [])
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return int_type
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return get_proper_type(node_type.ret_type)
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return ctx.default_attr_type
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def _implements_new(info: TypeInfo) -> bool:
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"""Check whether __new__ comes from enum.Enum or was implemented in a
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subclass. In the latter case, we must infer Any as long as mypy can't infer
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the type of _value_ from assignments in __new__.
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"""
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type_with_new = _first(
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ti
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for ti in info.mro
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if ti.names.get('__new__') and not ti.fullname.startswith('builtins.')
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)
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if type_with_new is None:
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return False
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return type_with_new.fullname not in ('enum.Enum', 'enum.IntEnum', 'enum.StrEnum')
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def enum_value_callback(ctx: 'mypy.plugin.AttributeContext') -> Type:
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"""This plugin refines the 'value' attribute in enums to refer to
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the original underlying value. For example, suppose we have the
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following:
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class SomeEnum:
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FOO = A()
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BAR = B()
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By default, mypy will infer that 'SomeEnum.FOO.value' and
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'SomeEnum.BAR.value' both are of type 'Any'. This plugin refines
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this inference so that mypy understands the expressions are
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actually of types 'A' and 'B' respectively. This better reflects
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the actual runtime behavior.
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This plugin works simply by looking up the original value assigned
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to the enum. For example, when this plugin sees 'SomeEnum.BAR.value',
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it will look up whatever type 'BAR' had in the SomeEnum TypeInfo and
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use that as the inferred type of the overall expression.
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This plugin assumes that the provided context is an attribute access
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matching one of the strings found in 'ENUM_VALUE_ACCESS'.
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"""
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enum_field_name = _extract_underlying_field_name(ctx.type)
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if enum_field_name is None:
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# We do not know the enum field name (perhaps it was passed to a
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# function and we only know that it _is_ a member). All is not lost
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# however, if we can prove that the all of the enum members have the
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# same value-type, then it doesn't matter which member was passed in.
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# The value-type is still known.
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if isinstance(ctx.type, Instance):
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info = ctx.type.type
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# As long as mypy doesn't understand attribute creation in __new__,
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# there is no way to predict the value type if the enum class has a
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# custom implementation
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if _implements_new(info):
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return ctx.default_attr_type
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stnodes = (info.get(name) for name in info.names)
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# Enums _can_ have methods and instance attributes.
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# Omit methods and attributes created by assigning to self.*
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# for our value inference.
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node_types = (
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get_proper_type(n.type) if n else None
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for n in stnodes
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if n is None or not n.implicit)
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proper_types = list(
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_infer_value_type_with_auto_fallback(ctx, t)
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for t in node_types
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if t is None or not isinstance(t, CallableType))
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underlying_type = _first(proper_types)
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if underlying_type is None:
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return ctx.default_attr_type
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# At first we try to predict future `value` type if all other items
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# have the same type. For example, `int`.
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# If this is the case, we simply return this type.
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# See https://github.com/python/mypy/pull/9443
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all_same_value_type = all(
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proper_type is not None and proper_type == underlying_type
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for proper_type in proper_types)
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if all_same_value_type:
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if underlying_type is not None:
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return underlying_type
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# But, after we started treating all `Enum` values as `Final`,
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# we start to infer types in
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# `item = 1` as `Literal[1]`, not just `int`.
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# So, for example types in this `Enum` will all be different:
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#
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# class Ordering(IntEnum):
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# one = 1
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# two = 2
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# three = 3
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#
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# We will infer three `Literal` types here.
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# They are not the same, but they are equivalent.
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# So, we unify them to make sure `.value` prediction still works.
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# Result will be `Literal[1] | Literal[2] | Literal[3]` for this case.
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all_equivalent_types = all(
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proper_type is not None and is_equivalent(proper_type, underlying_type)
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for proper_type in proper_types)
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if all_equivalent_types:
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return make_simplified_union(cast(Sequence[Type], proper_types))
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return ctx.default_attr_type
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assert isinstance(ctx.type, Instance)
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info = ctx.type.type
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# As long as mypy doesn't understand attribute creation in __new__,
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# there is no way to predict the value type if the enum class has a
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# custom implementation
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if _implements_new(info):
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return ctx.default_attr_type
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stnode = info.get(enum_field_name)
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if stnode is None:
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return ctx.default_attr_type
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underlying_type = _infer_value_type_with_auto_fallback(
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ctx, get_proper_type(stnode.type))
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if underlying_type is None:
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return ctx.default_attr_type
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return underlying_type
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def _extract_underlying_field_name(typ: Type) -> Optional[str]:
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"""If the given type corresponds to some Enum instance, returns the
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original name of that enum. For example, if we receive in the type
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corresponding to 'SomeEnum.FOO', we return the string "SomeEnum.Foo".
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This helper takes advantage of the fact that Enum instances are valid
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to use inside Literal[...] types. An expression like 'SomeEnum.FOO' is
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actually represented by an Instance type with a Literal enum fallback.
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We can examine this Literal fallback to retrieve the string.
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"""
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typ = get_proper_type(typ)
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if not isinstance(typ, Instance):
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return None
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if not typ.type.is_enum:
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return None
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underlying_literal = typ.last_known_value
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if underlying_literal is None:
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return None
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# The checks above have verified this LiteralType is representing an enum value,
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# which means the 'value' field is guaranteed to be the name of the enum field
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# as a string.
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assert isinstance(underlying_literal.value, str)
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return underlying_literal.value
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