Class _ScalarAttributeImpl

lib/sqlalchemy/orm/attributes.py:1210–1336

represents a scalar value-holding InstrumentedAttribute.

class _ScalarAttributeImpl(_AttributeImpl):
    """represents a scalar value-holding InstrumentedAttribute."""

    default_accepts_scalar_loader = True
    uses_objects = False
    supports_population = True
    collection = False
    dynamic = False

    __slots__ = (
        "_default_scalar_value",
        "_replace_token",
        "_append_token",
        "_remove_token",
    )

    def __init__(self, *arg, default_scalar_value=None, **kw):
        super().__init__(*arg, **kw)
        self._default_scalar_value = default_scalar_value
        self._replace_token = self._append_token = AttributeEventToken(
            self, OP_REPLACE
        )
        self._remove_token = AttributeEventToken(self, OP_REMOVE)

    def _default_value(
        self, state: InstanceState[Any], dict_: _InstanceDict
    ) -> Any:
        """Produce an empty value for an uninitialized scalar attribute."""

        assert self.key not in dict_, (
            "_default_value should only be invoked for an "
            "uninitialized or expired attribute"
        )
        value = self._default_scalar_value
        for fn in self.dispatch.init_scalar:
            ret = fn(state, value, dict_)
            if ret is not ATTR_EMPTY:
                value = ret

        return value

    def delete(self, state: InstanceState[Any], dict_: _InstanceDict) -> None:
        if self.dispatch._active_history:
            old = self.get(state, dict_, PASSIVE_RETURN_NO_VALUE)
        else:
            old = dict_.get(self.key, NO_VALUE)

        if self.dispatch.remove:
            self.fire_remove_event(state, dict_, old, self._remove_token)
        state._modified_event(dict_, self, old)

        existing = dict_.pop(self.key, NO_VALUE)
        if (
            existing is NO_VALUE
            and old is NO_VALUE
            and not state.expired
            and self.key not in state.expired_attributes
        ):