text: # sql/ddl.py
  # Copyright (C) 2009-2020 the SQLAlchemy authors and contributors
  # <see AUTHORS file>
  #
  # This module is part of SQLAlchemy and is released under
  # the MIT License: http://www.opensource.org/licenses/mit-license.php
  """
  Provides the hierarchy of DDL-defining schema items as well as routines
  to invoke them for a create/drop call.
  
  """
  
  from . import roles
  from .base import _bind_or_error
  from .base import _generative
  from .base import Executable
  from .base import SchemaVisitor
  from .elements import ClauseElement
  from .. import exc
  from .. import util
  from ..util import topological
  
  
  class _DDLCompiles(ClauseElement):
      def _compiler(self, dialect, **kw):
          """Return a compiler appropriate for this ClauseElement, given a
          Dialect."""
  
          return dialect.ddl_compiler(dialect, self, **kw)
  
      def _compile_w_cache(self, *arg, **kw):
          raise NotImplementedError()
  
  
  class DDLElement(roles.DDLRole, Executable, _DDLCompiles):
      """Base class for DDL expression constructs.
  
      This class is the base for the general purpose :class:`.DDL` class,
      as well as the various create/drop clause constructs such as
      :class:`.CreateTable`, :class:`.DropTable`, :class:`.AddConstraint`,
      etc.
  
      :class:`.DDLElement` integrates closely with SQLAlchemy events,
      introduced in :ref:`event_toplevel`.  An instance of one is
      itself an event receiving callable::
  
          event.listen(
              users,
              'after_create',
              AddConstraint(constraint).execute_if(dialect='postgresql')
          )
  
      .. seealso::
  
          :class:`.DDL`
  
          :class:`.DDLEvents`
  
          :ref:`event_toplevel`
  
          :ref:`schema_ddl_sequences`
  
      """
  
      _execution_options = Executable._execution_options.union(
          {"autocommit": True}
      )
  
      target = None
      on = None
      dialect = None
      callable_ = None
  
      def _execute_on_connection(
          self, connection, multiparams, params, execution_options
      ):
          return connection._execute_ddl(
              self, multiparams, params, execution_options
          )
  
      @util.deprecated_20(
          ":meth:`.DDL.execute`",
          alternative="All statement execution in SQLAlchemy 2.0 is performed "
          "by the :meth:`_engine.Connection.execute` method of "
          ":class:`_engine.Connection`, "
          "or in the ORM by the :meth:`.Session.execute` method of "
          ":class:`.Session`.",
      )
      def execute(self, bind=None, target=None):
          """Execute this DDL immediately.
  
          Executes the DDL statement in isolation using the supplied
          :class:`.Connectable` or
          :class:`.Connectable` assigned to the ``.bind``
          property, if not supplied. If the DDL has a conditional ``on``
          criteria, it will be invoked with None as the event.
  
          :param bind:
            Optional, an ``Engine`` or ``Connection``. If not supplied, a valid
            :class:`.Connectable` must be present in the
            ``.bind`` property.
  
          :param target:
            Optional, defaults to None.  The target :class:`_schema.SchemaItem`
            for the execute call.   This is equivalent to passing the
            :class:`_schema.SchemaItem` to the :meth:`.DDLElement.against`
            method and then invoking :meth:`_schema.DDLElement.execute`
            upon the resulting :class:`_schema.DDLElement` object.  See
            :meth:`.DDLElement.against` for further detail.
  
          """
  
          if bind is None:
              bind = _bind_or_error(self)
  
          if self._should_execute(target, bind):
              return bind.execute(self.against(target))
          else:
              bind.engine.logger.info("DDL execution skipped, criteria not met.")
  
      @_generative
      def against(self, target):
          """Return a copy of this :class:`_schema.DDLElement` which will include
          the given target.
  
          This essentially applies the given item to the ``.target`` attribute
          of the returned :class:`_schema.DDLElement` object.  This target
          is then usable by event handlers and compilation routines in order to
          provide services such as tokenization of a DDL string in terms of a
          particular :class:`_schema.Table`.
  
          When a :class:`_schema.DDLElement` object is established as an event
          handler for the :meth:`_events.DDLEvents.before_create` or
          :meth:`_events.DDLEvents.after_create` events, and the event
          then occurs for a given target such as a :class:`_schema.Constraint`
          or :class:`_schema.Table`, that target is established with a copy
          of the :class:`_schema.DDLElement` object using this method, which
          then proceeds to the :meth:`_schema.DDLElement.execute` method
          in order to invoke the actual DDL instruction.
  
          :param target: a :class:`_schema.SchemaItem` that will be the subject
           of a DDL operation.
  
          :return: a copy of this :class:`_schema.DDLElement` with the
           ``.target`` attribute assigned to the given
           :class:`_schema.SchemaItem`.
  
          .. seealso::
  
              :class:`_schema.DDL` - uses tokenization against the "target" when
              processing the DDL string.
  
          """
  
          self.target = target
  
      @_generative
      def execute_if(self, dialect=None, callable_=None, state=None):
          r"""Return a callable that will execute this
          :class:`_ddl.DDLElement` conditionally within an event handler.
  
          Used to provide a wrapper for event listening::
  
              event.listen(
                          metadata,
                          'before_create',
                          DDL("my_ddl").execute_if(dialect='postgresql')
                      )
  
          :param dialect: May be a string, tuple or a callable
            predicate.  If a string, it will be compared to the name of the
            executing database dialect::
  
              DDL('something').execute_if(dialect='postgresql')
  
            If a tuple, specifies multiple dialect names::
  
              DDL('something').execute_if(dialect=('postgresql', 'mysql'))
  
          :param callable\_: A callable, which will be invoked with
            four positional arguments as well as optional keyword
            arguments:
  
              :ddl:
                This DDL element.
  
              :target:
                The :class:`_schema.Table` or :class:`_schema.MetaData`
                object which is the
                target of this event. May be None if the DDL is executed
                explicitly.
  
              :bind:
                The :class:`_engine.Connection` being used for DDL execution
  
              :tables:
                Optional keyword argument - a list of Table objects which are to
                be created/ dropped within a MetaData.create_all() or drop_all()
                method call.
  
              :state:
                Optional keyword argument - will be the ``state`` argument
                passed to this function.
  
              :checkfirst:
               Keyword argument, will be True if the 'checkfirst' flag was
               set during the call to ``create()``, ``create_all()``,
               ``drop()``, ``drop_all()``.
  
            If the callable returns a True value, the DDL statement will be
            executed.
  
          :param state: any value which will be passed to the callable\_
            as the ``state`` keyword argument.
  
          .. seealso::
  
              :class:`.DDLEvents`
  
              :ref:`event_toplevel`
  
          """
          self.dialect = dialect
          self.callable_ = callable_
          self.state = state
  
      def _should_execute(self, target, bind, **kw):
          if isinstance(self.dialect, util.string_types):
              if self.dialect != bind.engine.name:
                  return False
          elif isinstance(self.dialect, (tuple, list, set)):
              if bind.engine.name not in self.dialect:
                  return False
          if self.callable_ is not None and not self.callable_(
              self, target, bind, state=self.state, **kw
          ):
              return False
  
          return True
  
      def __call__(self, target, bind, **kw):
          """Execute the DDL as a ddl_listener."""
  
          if self._should_execute(target, bind, **kw):
              return bind.execute(self.against(target))
  
      def bind(self):
          if self._bind:
              return self._bind
  
      def _set_bind(self, bind):
          self._bind = bind
  
      bind = property(bind, _set_bind)
  
      def _generate(self):
          s = self.__class__.__new__(self.__class__)
          s.__dict__ = self.__dict__.copy()
          return s
  
  
  class DDL(DDLElement):
      """A literal DDL statement.
  
      Specifies literal SQL DDL to be executed by the database.  DDL objects
      function as DDL event listeners, and can be subscribed to those events
      listed in :class:`.DDLEvents`, using either :class:`_schema.Table` or
      :class:`_schema.MetaData` objects as targets.
      Basic templating support allows
      a single DDL instance to handle repetitive tasks for multiple tables.
  
      Examples::
  
        from sqlalchemy import event, DDL
  
        tbl = Table('users', metadata, Column('uid', Integer))
        event.listen(tbl, 'before_create', DDL('DROP TRIGGER users_trigger'))
  
        spow = DDL('ALTER TABLE %(table)s SET secretpowers TRUE')
        event.listen(tbl, 'after_create', spow.execute_if(dialect='somedb'))
  
        drop_spow = DDL('ALTER TABLE users SET secretpowers FALSE')
        connection.execute(drop_spow)
  
      When operating on Table events, the following ``statement``
      string substitutions are available::
  
        %(table)s  - the Table name, with any required quoting applied
        %(schema)s - the schema name, with any required quoting applied
        %(fullname)s - the Table name including schema, quoted if needed
  
      The DDL's "context", if any, will be combined with the standard
      substitutions noted above.  Keys present in the context will override
      the standard substitutions.
  
      """
  
      __visit_name__ = "ddl"
  
      @util.deprecated_params(
          bind=(
              "2.0",
              "The :paramref:`_ddl.DDL.bind` argument is deprecated and "
              "will be removed in SQLAlchemy 2.0.",
          ),
      )
      def __init__(self, statement, context=None, bind=None):
          """Create a DDL statement.
  
          :param statement:
            A string or unicode string to be executed.  Statements will be
            processed with Python's string formatting operator.  See the
            ``context`` argument and the ``execute_at`` method.
  
            A literal '%' in a statement must be escaped as '%%'.
  
            SQL bind parameters are not available in DDL statements.
  
          :param context:
            Optional dictionary, defaults to None.  These values will be
            available for use in string substitutions on the DDL statement.
  
          :param bind:
            Optional. A :class:`.Connectable`, used by
            default when ``execute()`` is invoked without a bind argument.
  
  
          .. seealso::
  
              :class:`.DDLEvents`
  
              :ref:`event_toplevel`
  
          """
  
          if not isinstance(statement, util.string_types):
              raise exc.ArgumentError(
                  "Expected a string or unicode SQL statement, got '%r'"
                  % statement
              )
  
          self.statement = statement
          self.context = context or {}
  
          self._bind = bind
  
      def __repr__(self):
          return "<%s@%s; %s>" % (
              type(self).__name__,
              id(self),
              ", ".join(
                  [repr(self.statement)]
                  + [
                      "%s=%r" % (key, getattr(self, key))
                      for key in ("on", "context")
                      if getattr(self, key)
                  ]
              ),
          )
  
  
  class _CreateDropBase(DDLElement):
      """Base class for DDL constructs that represent CREATE and DROP or
      equivalents.
  
      The common theme of _CreateDropBase is a single
      ``element`` attribute which refers to the element
      to be created or dropped.
  
      """
  
      def __init__(self, element, bind=None):
          self.element = element
          self.bind = bind
  
      def _create_rule_disable(self, compiler):
          """Allow disable of _create_rule using a callable.
  
          Pass to _create_rule using
          util.portable_instancemethod(self._create_rule_disable)
          to retain serializability.
  
          """
          return False
  
  
  class CreateSchema(_CreateDropBase):
      """Represent a CREATE SCHEMA statement.
  
      The argument here is the string name of the schema.
  
      """
  
      __visit_name__ = "create_schema"
  
      def __init__(self, name, quote=None, **kw):
          """Create a new :class:`.CreateSchema` construct."""
  
          self.quote = quote
          super(CreateSchema, self).__init__(name, **kw)
  
  
  class DropSchema(_CreateDropBase):
      """Represent a DROP SCHEMA statement.
  
      The argument here is the string name of the schema.
  
      """
  
      __visit_name__ = "drop_schema"
  
      def __init__(self, name, quote=None, cascade=False, **kw):
          """Create a new :class:`.DropSchema` construct."""
  
          self.quote = quote
          self.cascade = cascade
          super(DropSchema, self).__init__(name, **kw)
  
  
  class CreateTable(_CreateDropBase):
      """Represent a CREATE TABLE statement."""
  
      __visit_name__ = "create_table"
  
      def __init__(
          self, element, bind=None, include_foreign_key_constraints=None
      ):
          """Create a :class:`.CreateTable` construct.
  
          :param element: a :class:`_schema.Table` that's the subject
           of the CREATE
          :param on: See the description for 'on' in :class:`.DDL`.
          :param bind: See the description for 'bind' in :class:`.DDL`.
          :param include_foreign_key_constraints: optional sequence of
           :class:`_schema.ForeignKeyConstraint` objects that will be included
           inline within the CREATE construct; if omitted, all foreign key
           constraints that do not specify use_alter=True are included.
  
           .. versionadded:: 1.0.0
  
          """
          super(CreateTable, self).__init__(element, bind=bind)
          self.columns = [CreateColumn(column) for column in element.columns]
          self.include_foreign_key_constraints = include_foreign_key_constraints
  
  
  class _DropView(_CreateDropBase):
      """Semi-public 'DROP VIEW' construct.
  
      Used by the test suite for dialect-agnostic drops of views.
      This object will eventually be part of a public "view" API.
  
      """
  
      __visit_name__ = "drop_view"
  
  
  class CreateColumn(_DDLCompiles):
      """Represent a :class:`_schema.Column`
      as rendered in a CREATE TABLE statement,
      via the :class:`.CreateTable` construct.
  
      This is provided to support custom column DDL within the generation
      of CREATE TABLE statements, by using the
      compiler extension documented in :ref:`sqlalchemy.ext.compiler_toplevel`
      to extend :class:`.CreateColumn`.
  
      Typical integration is to examine the incoming :class:`_schema.Column`
      object, and to redirect compilation if a particular flag or condition
      is found::
  
          from sqlalchemy import schema
          from sqlalchemy.ext.compiler import compiles
  
          @compiles(schema.CreateColumn)
          def compile(element, compiler, **kw):
              column = element.element
  
              if "special" not in column.info:
                  return compiler.visit_create_column(element, **kw)
  
              text = "%s SPECIAL DIRECTIVE %s" % (
                      column.name,
                      compiler.type_compiler.process(column.type)
                  )
              default = compiler.get_column_default_string(column)
              if default is not None:
                  text += " DEFAULT " + default
  
              if not column.nullable:
                  text += " NOT NULL"
  
              if column.constraints:
                  text += " ".join(
                              compiler.process(const)
                              for const in column.constraints)
              return text
  
      The above construct can be applied to a :class:`_schema.Table`
      as follows::
  
          from sqlalchemy import Table, Metadata, Column, Integer, String
          from sqlalchemy import schema
  
          metadata = MetaData()
  
          table = Table('mytable', MetaData(),
                  Column('x', Integer, info={"special":True}, primary_key=True),
                  Column('y', String(50)),
                  Column('z', String(20), info={"special":True})
              )
  
          metadata.create_all(conn)
  
      Above, the directives we've added to the :attr:`_schema.Column.info`
      collection
      will be detected by our custom compilation scheme::
  
          CREATE TABLE mytable (
                  x SPECIAL DIRECTIVE INTEGER NOT NULL,
                  y VARCHAR(50),
                  z SPECIAL DIRECTIVE VARCHAR(20),
              PRIMARY KEY (x)
          )
  
      The :class:`.CreateColumn` construct can also be used to skip certain
      columns when producing a ``CREATE TABLE``.  This is accomplished by
      creating a compilation rule that conditionally returns ``None``.
      This is essentially how to produce the same effect as using the
      ``system=True`` argument on :class:`_schema.Column`, which marks a column
      as an implicitly-present "system" column.
  
      For example, suppose we wish to produce a :class:`_schema.Table`
      which skips
      rendering of the PostgreSQL ``xmin`` column against the PostgreSQL
      backend, but on other backends does render it, in anticipation of a
      triggered rule.  A conditional compilation rule could skip this name only
      on PostgreSQL::
  
          from sqlalchemy.schema import CreateColumn
  
          @compiles(CreateColumn, "postgresql")
          def skip_xmin(element, compiler, **kw):
              if element.element.name == 'xmin':
                  return None
              else:
                  return compiler.visit_create_column(element, **kw)
  
  
          my_table = Table('mytable', metadata,
                      Column('id', Integer, primary_key=True),
                      Column('xmin', Integer)
                  )
  
      Above, a :class:`.CreateTable` construct will generate a ``CREATE TABLE``
      which only includes the ``id`` column in the string; the ``xmin`` column
      will be omitted, but only against the PostgreSQL backend.
  
      """
  
      __visit_name__ = "create_column"
  
      def __init__(self, element):
          self.element = element
  
  
  class DropTable(_CreateDropBase):
      """Represent a DROP TABLE statement."""
  
      __visit_name__ = "drop_table"
  
  
  class CreateSequence(_CreateDropBase):
      """Represent a CREATE SEQUENCE statement."""
  
      __visit_name__ = "create_sequence"
  
  
  class DropSequence(_CreateDropBase):
      """Represent a DROP SEQUENCE statement."""
  
      __visit_name__ = "drop_sequence"
  
  
  class CreateIndex(_CreateDropBase):
      """Represent a CREATE INDEX statement."""
  
      __visit_name__ = "create_index"
  
  
  class DropIndex(_CreateDropBase):
      """Represent a DROP INDEX statement."""
  
      __visit_name__ = "drop_index"
  
  
  class AddConstraint(_CreateDropBase):
      """Represent an ALTER TABLE ADD CONSTRAINT statement."""
  
      __visit_name__ = "add_constraint"
  
      def __init__(self, element, *args, **kw):
          super(AddConstraint, self).__init__(element, *args, **kw)
          element._create_rule = util.portable_instancemethod(
              self._create_rule_disable
          )
  
  
  class DropConstraint(_CreateDropBase):
      """Represent an ALTER TABLE DROP CONSTRAINT statement."""
  
      __visit_name__ = "drop_constraint"
  
      def __init__(self, element, cascade=False, **kw):
          self.cascade = cascade
          super(DropConstraint, self).__init__(element, **kw)
          element._create_rule = util.portable_instancemethod(
              self._create_rule_disable
          )
  
  
  class SetTableComment(_CreateDropBase):
      """Represent a COMMENT ON TABLE IS statement."""
  
      __visit_name__ = "set_table_comment"
  
  
  class DropTableComment(_CreateDropBase):
      """Represent a COMMENT ON TABLE '' statement.
  
      Note this varies a lot across database backends.
  
      """
  
      __visit_name__ = "drop_table_comment"
  
  
  class SetColumnComment(_CreateDropBase):
      """Represent a COMMENT ON COLUMN IS statement."""
  
      __visit_name__ = "set_column_comment"
  
  
  class DropColumnComment(_CreateDropBase):
      """Represent a COMMENT ON COLUMN IS NULL statement."""
  
      __visit_name__ = "drop_column_comment"
  
  
  class DDLBase(SchemaVisitor):
      def __init__(self, connection):
          self.connection = connection
  
  
  class SchemaGenerator(DDLBase):
      def __init__(
          self, dialect, connection, checkfirst=False, tables=None, **kwargs
      ):
          super(SchemaGenerator, self).__init__(connection, **kwargs)
          self.checkfirst = checkfirst
          self.tables = tables
          self.preparer = dialect.identifier_preparer
          self.dialect = dialect
          self.memo = {}
  
      def _can_create_table(self, table):
          self.dialect.validate_identifier(table.name)
          effective_schema = self.connection.schema_for_object(table)
          if effective_schema:
              self.dialect.validate_identifier(effective_schema)
          return not self.checkfirst or not self.dialect.has_table(
              self.connection, table.name, schema=effective_schema
          )
  
      def _can_create_index(self, index):
          effective_schema = self.connection.schema_for_object(index.table)
          if effective_schema:
              self.dialect.validate_identifier(effective_schema)
          return not self.checkfirst or not self.dialect.has_index(
              self.connection,
              index.table.name,
              index.name,
              schema=effective_schema,
          )
  
      def _can_create_sequence(self, sequence):
          effective_schema = self.connection.schema_for_object(sequence)
  
          return self.dialect.supports_sequences and (
              (not self.dialect.sequences_optional or not sequence.optional)
              and (
                  not self.checkfirst
                  or not self.dialect.has_sequence(
                      self.connection, sequence.name, schema=effective_schema
                  )
              )
          )
  
      def visit_metadata(self, metadata):
          if self.tables is not None:
              tables = self.tables
          else:
              tables = list(metadata.tables.values())
  
          collection = sort_tables_and_constraints(
              [t for t in tables if self._can_create_table(t)]
          )
  
          seq_coll = [
              s
              for s in metadata._sequences.values()
              if s.column is None and self._can_create_sequence(s)
          ]
  
          event_collection = [t for (t, fks) in collection if t is not None]
          metadata.dispatch.before_create(
              metadata,
              self.connection,
              tables=event_collection,
              checkfirst=self.checkfirst,
              _ddl_runner=self,
          )
  
          for seq in seq_coll:
              self.traverse_single(seq, create_ok=True)
  
          for table, fkcs in collection:
              if table is not None:
                  self.traverse_single(
                      table,
                      create_ok=True,
                      include_foreign_key_constraints=fkcs,
                      _is_metadata_operation=True,
                  )
              else:
                  for fkc in fkcs:
                      self.traverse_single(fkc)
  
          metadata.dispatch.after_create(
              metadata,
              self.connection,
              tables=event_collection,
              checkfirst=self.checkfirst,
              _ddl_runner=self,
          )
  
      def visit_table(
          self,
          table,
          create_ok=False,
          include_foreign_key_constraints=None,
          _is_metadata_operation=False,
      ):
          if not create_ok and not self._can_create_table(table):
              return
  
          table.dispatch.before_create(
              table,
              self.connection,
              checkfirst=self.checkfirst,
              _ddl_runner=self,
              _is_metadata_operation=_is_metadata_operation,
          )
  
          for column in table.columns:
              if column.default is not None:
                  self.traverse_single(column.default)
  
          if not self.dialect.supports_alter:
              # e.g., don't omit any foreign key constraints
              include_foreign_key_constraints = None
  
          self.connection.execute(
              # fmt: off
              CreateTable(
                  table,
                  include_foreign_key_constraints=  # noqa
                      include_foreign_key_constraints,  # noqa
              )
              # fmt: on
          )
  
          if hasattr(table, "indexes"):
              for index in table.indexes:
                  self.traverse_single(index, create_ok=True)
  
          if self.dialect.supports_comments and not self.dialect.inline_comments:
              if table.comment is not None:
                  self.connection.execute(SetTableComment(table))
  
              for column in table.columns:
                  if column.comment is not None:
                      self.connection.execute(SetColumnComment(column))
  
          table.dispatch.after_create(
              table,
              self.connection,
              checkfirst=self.checkfirst,
              _ddl_runner=self,
              _is_metadata_operation=_is_metadata_operation,
          )
  
      def visit_foreign_key_constraint(self, constraint):
          if not self.dialect.supports_alter:
              return
          self.connection.execute(AddConstraint(constraint))
  
      def visit_sequence(self, sequence, create_ok=False):
          if not create_ok and not self._can_create_sequence(sequence):
              return
          self.connection.execute(CreateSequence(sequence))
  
      def visit_index(self, index, create_ok=False):
          if not create_ok and not self._can_create_index(index):
              return
          self.connection.execute(CreateIndex(index))
  
  
  class SchemaDropper(DDLBase):
      def __init__(
          self, dialect, connection, checkfirst=False, tables=None, **kwargs
      ):
          super(SchemaDropper, self).__init__(connection, **kwargs)
          self.checkfirst = checkfirst
          self.tables = tables
          self.preparer = dialect.identifier_preparer
          self.dialect = dialect
          self.memo = {}
  
      def visit_metadata(self, metadata):
          if self.tables is not None:
              tables = self.tables
          else:
              tables = list(metadata.tables.values())
  
          try:
              unsorted_tables = [t for t in tables if self._can_drop_table(t)]
              collection = list(
                  reversed(
                      sort_tables_and_constraints(
                          unsorted_tables,
                          filter_fn=lambda constraint: False
                          if not self.dialect.supports_alter
                          or constraint.name is None
                          else None,
                      )
                  )
              )
          except exc.CircularDependencyError as err2:
              if not self.dialect.supports_alter:
                  util.warn(
                      "Can't sort tables for DROP; an "
                      "unresolvable foreign key "
                      "dependency exists between tables: %s; and backend does "
                      "not support ALTER.  To restore at least a partial sort, "
                      "apply use_alter=True to ForeignKey and "
                      "ForeignKeyConstraint "
                      "objects involved in the cycle to mark these as known "
                      "cycles that will be ignored."
                      % (", ".join(sorted([t.fullname for t in err2.cycles])))
                  )
                  collection = [(t, ()) for t in unsorted_tables]
              else:
                  util.raise_(
                      exc.CircularDependencyError(
                          err2.args[0],
                          err2.cycles,
                          err2.edges,
                          msg="Can't sort tables for DROP; an "
                          "unresolvable foreign key "
                          "dependency exists between tables: %s.  Please ensure "
                          "that the ForeignKey and ForeignKeyConstraint objects "
                          "involved in the cycle have "
                          "names so that they can be dropped using "
                          "DROP CONSTRAINT."
                          % (
                              ", ".join(
                                  sorted([t.fullname for t in err2.cycles])
                              )
                          ),
                      ),
                      from_=err2,
                  )
  
          seq_coll = [
              s
              for s in metadata._sequences.values()
              if s.column is None and self._can_drop_sequence(s)
          ]
  
          event_collection = [t for (t, fks) in collection if t is not None]
  
          metadata.dispatch.before_drop(
              metadata,
              self.connection,
              tables=event_collection,
              checkfirst=self.checkfirst,
              _ddl_runner=self,
          )
  
          for table, fkcs in collection:
              if table is not None:
                  self.traverse_single(
                      table, drop_ok=True, _is_metadata_operation=True
                  )
              else:
                  for fkc in fkcs:
                      self.traverse_single(fkc)
  
          for seq in seq_coll:
              self.traverse_single(seq, drop_ok=True)
  
          metadata.dispatch.after_drop(
              metadata,
              self.connection,
              tables=event_collection,
              checkfirst=self.checkfirst,
              _ddl_runner=self,
          )
  
      def _can_drop_table(self, table):
          self.dialect.validate_identifier(table.name)
          effective_schema = self.connection.schema_for_object(table)
          if effective_schema:
              self.dialect.validate_identifier(effective_schema)
          return not self.checkfirst or self.dialect.has_table(
              self.connection, table.name, schema=effective_schema
          )
  
      def _can_drop_index(self, index):
          effective_schema = self.connection.schema_for_object(index.table)
          if effective_schema:
              self.dialect.validate_identifier(effective_schema)
          return not self.checkfirst or self.dialect.has_index(
              self.connection,
              index.table.name,
              index.name,
              schema=effective_schema,
          )
  
      def _can_drop_sequence(self, sequence):
          effective_schema = self.connection.schema_for_object(sequence)
          return self.dialect.supports_sequences and (
              (not self.dialect.sequences_optional or not sequence.optional)
              and (
                  not self.checkfirst
                  or self.dialect.has_sequence(
                      self.connection, sequence.name, schema=effective_schema
                  )
              )
          )
  
      def visit_index(self, index, drop_ok=False):
          if not drop_ok and not self._can_drop_index(index):
              return
  
          self.connection.execute(DropIndex(index))
  
      def visit_table(self, table, drop_ok=False, _is_metadata_operation=False):
          if not drop_ok and not self._can_drop_table(table):
              return
  
          table.dispatch.before_drop(
              table,
              self.connection,
              checkfirst=self.checkfirst,
              _ddl_runner=self,
              _is_metadata_operation=_is_metadata_operation,
          )
  
          self.connection.execute(DropTable(table))
  
          # traverse client side defaults which may refer to server-side
          # sequences. noting that some of these client side defaults may also be
          # set up as server side defaults (see http://docs.sqlalchemy.org/en/
          # latest/core/defaults.html#associating-a-sequence-as-the-server-side-
          # default), so have to be dropped after the table is dropped.
          for column in table.columns:
              if column.default is not None:
                  self.traverse_single(column.default)
  
          table.dispatch.after_drop(
              table,
              self.connection,
              checkfirst=self.checkfirst,
              _ddl_runner=self,
              _is_metadata_operation=_is_metadata_operation,
          )
  
      def visit_foreign_key_constraint(self, constraint):
          if not self.dialect.supports_alter:
              return
          self.connection.execute(DropConstraint(constraint))
  
      def visit_sequence(self, sequence, drop_ok=False):
  
          if not drop_ok and not self._can_drop_sequence(sequence):
              return
          self.connection.execute(DropSequence(sequence))
  
  
  def sort_tables(
      tables,
      skip_fn=None,
      extra_dependencies=None,
  ):
      """Sort a collection of :class:`_schema.Table` objects based on
      dependency.
  
      This is a dependency-ordered sort which will emit :class:`_schema.Table`
      objects such that they will follow their dependent :class:`_schema.Table`
      objects.
      Tables are dependent on another based on the presence of
      :class:`_schema.ForeignKeyConstraint`
      objects as well as explicit dependencies
      added by :meth:`_schema.Table.add_is_dependent_on`.
  
      .. warning::
  
          The :func:`._schema.sort_tables` function cannot by itself
          accommodate automatic resolution of dependency cycles between
          tables, which are usually caused by mutually dependent foreign key
          constraints. When these cycles are detected, the foreign keys
          of these tables are omitted from consideration in the sort.
          A warning is emitted when this condition occurs, which will be an
          exception raise in a future release.   Tables which are not part
          of the cycle will still be returned in dependency order.
  
          To resolve these cycles, the
          :paramref:`_schema.ForeignKeyConstraint.use_alter` parameter may be
          applied to those constraints which create a cycle.  Alternatively,
          the :func:`_schema.sort_tables_and_constraints` function will
          automatically return foreign key constraints in a separate
          collection when cycles are detected so that they may be applied
          to a schema separately.
  
          .. versionchanged:: 1.3.17 - a warning is emitted when
             :func:`_schema.sort_tables` cannot perform a proper sort due to
             cyclical dependencies.  This will be an exception in a future
             release.  Additionally, the sort will continue to return
             other tables not involved in the cycle in dependency order
             which was not the case previously.
  
      :param tables: a sequence of :class:`_schema.Table` objects.
  
      :param skip_fn: optional callable which will be passed a
       :class:`_schema.ForeignKey` object; if it returns True, this
       constraint will not be considered as a dependency.  Note this is
       **different** from the same parameter in
       :func:`.sort_tables_and_constraints`, which is
       instead passed the owning :class:`_schema.ForeignKeyConstraint` object.
  
      :param extra_dependencies: a sequence of 2-tuples of tables which will
       also be considered as dependent on each other.
  
      .. seealso::
  
          :func:`.sort_tables_and_constraints`
  
          :attr:`_schema.MetaData.sorted_tables` - uses this function to sort
  
  
      """
  
      if skip_fn is not None:
  
          def _skip_fn(fkc):
              for fk in fkc.elements:
                  if skip_fn(fk):
                      return True
              else:
                  return None
  
      else:
          _skip_fn = None
  
      return [
          t
          for (t, fkcs) in sort_tables_and_constraints(
              tables,
              filter_fn=_skip_fn,
              extra_dependencies=extra_dependencies,
              _warn_for_cycles=True,
          )
          if t is not None
      ]
  
  
  def sort_tables_and_constraints(
      tables, filter_fn=None, extra_dependencies=None, _warn_for_cycles=False
  ):
      """Sort a collection of :class:`_schema.Table`  /
      :class:`_schema.ForeignKeyConstraint`
      objects.
  
      This is a dependency-ordered sort which will emit tuples of
      ``(Table, [ForeignKeyConstraint, ...])`` such that each
      :class:`_schema.Table` follows its dependent :class:`_schema.Table`
      objects.
      Remaining :class:`_schema.ForeignKeyConstraint`
      objects that are separate due to
      dependency rules not satisfied by the sort are emitted afterwards
      as ``(None, [ForeignKeyConstraint ...])``.
  
      Tables are dependent on another based on the presence of
      :class:`_schema.ForeignKeyConstraint` objects, explicit dependencies
      added by :meth:`_schema.Table.add_is_dependent_on`,
      as well as dependencies
      stated here using the :paramref:`~.sort_tables_and_constraints.skip_fn`
      and/or :paramref:`~.sort_tables_and_constraints.extra_dependencies`
      parameters.
  
      :param tables: a sequence of :class:`_schema.Table` objects.
  
      :param filter_fn: optional callable which will be passed a
       :class:`_schema.ForeignKeyConstraint` object,
       and returns a value based on
       whether this constraint should definitely be included or excluded as
       an inline constraint, or neither.   If it returns False, the constraint
       will definitely be included as a dependency that cannot be subject
       to ALTER; if True, it will **only** be included as an ALTER result at
       the end.   Returning None means the constraint is included in the
       table-based result unless it is detected as part of a dependency cycle.
  
      :param extra_dependencies: a sequence of 2-tuples of tables which will
       also be considered as dependent on each other.
  
      .. versionadded:: 1.0.0
  
      .. seealso::
  
          :func:`.sort_tables`
  
  
      """
  
      fixed_dependencies = set()
      mutable_dependencies = set()
  
      if extra_dependencies is not None:
          fixed_dependencies.update(extra_dependencies)
  
      remaining_fkcs = set()
      for table in tables:
          for fkc in table.foreign_key_constraints:
              if fkc.use_alter is True:
                  remaining_fkcs.add(fkc)
                  continue
  
              if filter_fn:
                  filtered = filter_fn(fkc)
  
                  if filtered is True:
                      remaining_fkcs.add(fkc)
                      continue
  
              dependent_on = fkc.referred_table
              if dependent_on is not table:
                  mutable_dependencies.add((dependent_on, table))
  
          fixed_dependencies.update(
              (parent, table) for parent in table._extra_dependencies
          )
  
      try:
          candidate_sort = list(
              topological.sort(
                  fixed_dependencies.union(mutable_dependencies),
                  tables,
                  deterministic_order=True,
              )
          )
      except exc.CircularDependencyError as err:
          if _warn_for_cycles:
              util.warn(
                  "Cannot correctly sort tables; there are unresolvable cycles "
                  'between tables "%s", which is usually caused by mutually '
                  "dependent foreign key constraints.  Foreign key constraints "
                  "involving these tables will not be considered; this warning "
                  "may raise an error in a future release."
                  % (", ".join(sorted(t.fullname for t in err.cycles)),)
              )
          for edge in err.edges:
              if edge in mutable_dependencies:
                  table = edge[1]
                  if table not in err.cycles:
                      continue
                  can_remove = [
                      fkc
                      for fkc in table.foreign_key_constraints
                      if filter_fn is None or filter_fn(fkc) is not False
                  ]
                  remaining_fkcs.update(can_remove)
                  for fkc in can_remove:
                      dependent_on = fkc.referred_table
                      if dependent_on is not table:
                          mutable_dependencies.discard((dependent_on, table))
          candidate_sort = list(
              topological.sort(
                  fixed_dependencies.union(mutable_dependencies),
                  tables,
                  deterministic_order=True,
              )
          )
  
      return [
          (table, table.foreign_key_constraints.difference(remaining_fkcs))
          for table in candidate_sort
      ] + [(None, list(remaining_fkcs))]
  