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+# ext/index.py
+# Copyright (C) 2005-2022 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: https://www.opensource.org/licenses/mit-license.php
+
+"""Define attributes on ORM-mapped classes that have "index" attributes for
+columns with :class:`_types.Indexable` types.
+
+"index" means the attribute is associated with an element of an
+:class:`_types.Indexable` column with the predefined index to access it.
+The :class:`_types.Indexable` types include types such as
+:class:`_types.ARRAY`, :class:`_types.JSON` and
+:class:`_postgresql.HSTORE`.
+
+
+
+The :mod:`~sqlalchemy.ext.indexable` extension provides
+:class:`_schema.Column`-like interface for any element of an
+:class:`_types.Indexable` typed column. In simple cases, it can be
+treated as a :class:`_schema.Column` - mapped attribute.
+
+
+.. versionadded:: 1.1
+
+Synopsis
+========
+
+Given ``Person`` as a model with a primary key and JSON data field.
+While this field may have any number of elements encoded within it,
+we would like to refer to the element called ``name`` individually
+as a dedicated attribute which behaves like a standalone column::
+
+    from sqlalchemy import Column, JSON, Integer
+    from sqlalchemy.ext.declarative import declarative_base
+    from sqlalchemy.ext.indexable import index_property
+
+    Base = declarative_base()
+
+    class Person(Base):
+        __tablename__ = 'person'
+
+        id = Column(Integer, primary_key=True)
+        data = Column(JSON)
+
+        name = index_property('data', 'name')
+
+
+Above, the ``name`` attribute now behaves like a mapped column.   We
+can compose a new ``Person`` and set the value of ``name``::
+
+    >>> person = Person(name='Alchemist')
+
+The value is now accessible::
+
+    >>> person.name
+    'Alchemist'
+
+Behind the scenes, the JSON field was initialized to a new blank dictionary
+and the field was set::
+
+    >>> person.data
+    {"name": "Alchemist'}
+
+The field is mutable in place::
+
+    >>> person.name = 'Renamed'
+    >>> person.name
+    'Renamed'
+    >>> person.data
+    {'name': 'Renamed'}
+
+When using :class:`.index_property`, the change that we make to the indexable
+structure is also automatically tracked as history; we no longer need
+to use :class:`~.mutable.MutableDict` in order to track this change
+for the unit of work.
+
+Deletions work normally as well::
+
+    >>> del person.name
+    >>> person.data
+    {}
+
+Above, deletion of ``person.name`` deletes the value from the dictionary,
+but not the dictionary itself.
+
+A missing key will produce ``AttributeError``::
+
+    >>> person = Person()
+    >>> person.name
+    ...
+    AttributeError: 'name'
+
+Unless you set a default value::
+
+    >>> class Person(Base):
+    >>>     __tablename__ = 'person'
+    >>>
+    >>>     id = Column(Integer, primary_key=True)
+    >>>     data = Column(JSON)
+    >>>
+    >>>     name = index_property('data', 'name', default=None)  # See default
+
+    >>> person = Person()
+    >>> print(person.name)
+    None
+
+
+The attributes are also accessible at the class level.
+Below, we illustrate ``Person.name`` used to generate
+an indexed SQL criteria::
+
+    >>> from sqlalchemy.orm import Session
+    >>> session = Session()
+    >>> query = session.query(Person).filter(Person.name == 'Alchemist')
+
+The above query is equivalent to::
+
+    >>> query = session.query(Person).filter(Person.data['name'] == 'Alchemist')
+
+Multiple :class:`.index_property` objects can be chained to produce
+multiple levels of indexing::
+
+    from sqlalchemy import Column, JSON, Integer
+    from sqlalchemy.ext.declarative import declarative_base
+    from sqlalchemy.ext.indexable import index_property
+
+    Base = declarative_base()
+
+    class Person(Base):
+        __tablename__ = 'person'
+
+        id = Column(Integer, primary_key=True)
+        data = Column(JSON)
+
+        birthday = index_property('data', 'birthday')
+        year = index_property('birthday', 'year')
+        month = index_property('birthday', 'month')
+        day = index_property('birthday', 'day')
+
+Above, a query such as::
+
+    q = session.query(Person).filter(Person.year == '1980')
+
+On a PostgreSQL backend, the above query will render as::
+
+    SELECT person.id, person.data
+    FROM person
+    WHERE person.data -> %(data_1)s -> %(param_1)s = %(param_2)s
+
+Default Values
+==============
+
+:class:`.index_property` includes special behaviors for when the indexed
+data structure does not exist, and a set operation is called:
+
+* For an :class:`.index_property` that is given an integer index value,
+  the default data structure will be a Python list of ``None`` values,
+  at least as long as the index value; the value is then set at its
+  place in the list.  This means for an index value of zero, the list
+  will be initialized to ``[None]`` before setting the given value,
+  and for an index value of five, the list will be initialized to
+  ``[None, None, None, None, None]`` before setting the fifth element
+  to the given value.   Note that an existing list is **not** extended
+  in place to receive a value.
+
+* for an :class:`.index_property` that is given any other kind of index
+  value (e.g. strings usually), a Python dictionary is used as the
+  default data structure.
+
+* The default data structure can be set to any Python callable using the
+  :paramref:`.index_property.datatype` parameter, overriding the previous
+  rules.
+
+
+Subclassing
+===========
+
+:class:`.index_property` can be subclassed, in particular for the common
+use case of providing coercion of values or SQL expressions as they are
+accessed.  Below is a common recipe for use with a PostgreSQL JSON type,
+where we want to also include automatic casting plus ``astext()``::
+
+    class pg_json_property(index_property):
+        def __init__(self, attr_name, index, cast_type):
+            super(pg_json_property, self).__init__(attr_name, index)
+            self.cast_type = cast_type
+
+        def expr(self, model):
+            expr = super(pg_json_property, self).expr(model)
+            return expr.astext.cast(self.cast_type)
+
+The above subclass can be used with the PostgreSQL-specific
+version of :class:`_postgresql.JSON`::
+
+    from sqlalchemy import Column, Integer
+    from sqlalchemy.ext.declarative import declarative_base
+    from sqlalchemy.dialects.postgresql import JSON
+
+    Base = declarative_base()
+
+    class Person(Base):
+        __tablename__ = 'person'
+
+        id = Column(Integer, primary_key=True)
+        data = Column(JSON)
+
+        age = pg_json_property('data', 'age', Integer)
+
+The ``age`` attribute at the instance level works as before; however
+when rendering SQL, PostgreSQL's ``->>`` operator will be used
+for indexed access, instead of the usual index operator of ``->``::
+
+    >>> query = session.query(Person).filter(Person.age < 20)
+
+The above query will render::
+
+    SELECT person.id, person.data
+    FROM person
+    WHERE CAST(person.data ->> %(data_1)s AS INTEGER) < %(param_1)s
+
+"""  # noqa
+from __future__ import absolute_import
+
+from .. import inspect
+from .. import util
+from ..ext.hybrid import hybrid_property
+from ..orm.attributes import flag_modified
+
+
+__all__ = ["index_property"]
+
+
+class index_property(hybrid_property):  # noqa
+    """A property generator. The generated property describes an object
+    attribute that corresponds to an :class:`_types.Indexable`
+    column.
+
+    .. versionadded:: 1.1
+
+    .. seealso::
+
+        :mod:`sqlalchemy.ext.indexable`
+
+    """
+
+    _NO_DEFAULT_ARGUMENT = object()
+
+    def __init__(
+        self,
+        attr_name,
+        index,
+        default=_NO_DEFAULT_ARGUMENT,
+        datatype=None,
+        mutable=True,
+        onebased=True,
+    ):
+        """Create a new :class:`.index_property`.
+
+        :param attr_name:
+            An attribute name of an `Indexable` typed column, or other
+            attribute that returns an indexable structure.
+        :param index:
+            The index to be used for getting and setting this value.  This
+            should be the Python-side index value for integers.
+        :param default:
+            A value which will be returned instead of `AttributeError`
+            when there is not a value at given index.
+        :param datatype: default datatype to use when the field is empty.
+            By default, this is derived from the type of index used; a
+            Python list for an integer index, or a Python dictionary for
+            any other style of index.   For a list, the list will be
+            initialized to a list of None values that is at least
+            ``index`` elements long.
+        :param mutable: if False, writes and deletes to the attribute will
+            be disallowed.
+        :param onebased: assume the SQL representation of this value is
+            one-based; that is, the first index in SQL is 1, not zero.
+        """
+
+        if mutable:
+            super(index_property, self).__init__(
+                self.fget, self.fset, self.fdel, self.expr
+            )
+        else:
+            super(index_property, self).__init__(
+                self.fget, None, None, self.expr
+            )
+        self.attr_name = attr_name
+        self.index = index
+        self.default = default
+        is_numeric = isinstance(index, int)
+        onebased = is_numeric and onebased
+
+        if datatype is not None:
+            self.datatype = datatype
+        else:
+            if is_numeric:
+                self.datatype = lambda: [None for x in range(index + 1)]
+            else:
+                self.datatype = dict
+        self.onebased = onebased
+
+    def _fget_default(self, err=None):
+        if self.default == self._NO_DEFAULT_ARGUMENT:
+            util.raise_(AttributeError(self.attr_name), replace_context=err)
+        else:
+            return self.default
+
+    def fget(self, instance):
+        attr_name = self.attr_name
+        column_value = getattr(instance, attr_name)
+        if column_value is None:
+            return self._fget_default()
+        try:
+            value = column_value[self.index]
+        except (KeyError, IndexError) as err:
+            return self._fget_default(err)
+        else:
+            return value
+
+    def fset(self, instance, value):
+        attr_name = self.attr_name
+        column_value = getattr(instance, attr_name, None)
+        if column_value is None:
+            column_value = self.datatype()
+            setattr(instance, attr_name, column_value)
+        column_value[self.index] = value
+        setattr(instance, attr_name, column_value)
+        if attr_name in inspect(instance).mapper.attrs:
+            flag_modified(instance, attr_name)
+
+    def fdel(self, instance):
+        attr_name = self.attr_name
+        column_value = getattr(instance, attr_name)
+        if column_value is None:
+            raise AttributeError(self.attr_name)
+        try:
+            del column_value[self.index]
+        except KeyError as err:
+            util.raise_(AttributeError(self.attr_name), replace_context=err)
+        else:
+            setattr(instance, attr_name, column_value)
+            flag_modified(instance, attr_name)
+
+    def expr(self, model):
+        column = getattr(model, self.attr_name)
+        index = self.index
+        if self.onebased:
+            index += 1
+        return column[index]