text: """
     Extensions to SQLAlchemy for altering existing tables.
  
     At the moment, this isn't so much based off of ANSI as much as
     things that just happen to work with multiple databases.
  """
  
  import sqlalchemy as sa
  from sqlalchemy.schema import SchemaVisitor
  from sqlalchemy.engine.default import DefaultDialect
  from sqlalchemy.sql import ClauseElement
  from sqlalchemy.schema import (ForeignKeyConstraint,
                                 PrimaryKeyConstraint,
                                 CheckConstraint,
                                 UniqueConstraint,
                                 Index)
  
  from sqlalchemy_migrate_hotoffthehamster import exceptions
  import sqlalchemy.sql.compiler
  from sqlalchemy_migrate_hotoffthehamster.changeset import constraint
  from sqlalchemy_migrate_hotoffthehamster.changeset import util
  from six.moves import StringIO
  
  from sqlalchemy.schema import AddConstraint, DropConstraint
  from sqlalchemy.sql.compiler import DDLCompiler
  SchemaGenerator = SchemaDropper = DDLCompiler
  
  
  class AlterTableVisitor(SchemaVisitor):
      """Common operations for ``ALTER TABLE`` statements."""
  
      # engine.Compiler looks for .statement
      # when it spawns off a new compiler
      statement = ClauseElement()
  
      def append(self, s):
          """Append content to the SchemaIterator's query buffer."""
  
          self.buffer.write(s)
  
      def execute(self):
          """Execute the contents of the SchemaIterator's buffer."""
          try:
              return self.connection.execute(self.buffer.getvalue())
          finally:
              self.buffer.seek(0)
              self.buffer.truncate()
  
      def __init__(self, dialect, connection, **kw):
          self.connection = connection
          self.buffer = StringIO()
          self.preparer = dialect.identifier_preparer
          self.dialect = dialect
  
      def traverse_single(self, elem):
          ret = super(AlterTableVisitor, self).traverse_single(elem)
          if ret:
              # adapt to 0.6 which uses a string-returning
              # object
              self.append(" %s" % ret)
  
      def _to_table(self, param):
          """Returns the table object for the given param object."""
          if isinstance(param, (sa.Column, sa.Index, sa.schema.Constraint)):
              ret = param.table
          else:
              ret = param
          return ret
  
      def start_alter_table(self, param):
          """Returns the start of an ``ALTER TABLE`` SQL-Statement.
  
          Use the param object to determine the table name and use it
          for building the SQL statement.
  
          :param param: object to determine the table from
          :type param: :class:`sqlalchemy.Column`, :class:`sqlalchemy.Index`,
            :class:`sqlalchemy.schema.Constraint`, :class:`sqlalchemy.Table`,
            or string (table name)
          """
          table = self._to_table(param)
          self.append('\nALTER TABLE %s ' % self.preparer.format_table(table))
          return table
  
  
  class ANSIColumnGenerator(AlterTableVisitor, SchemaGenerator):
      """Extends ansisql generator for column creation (alter table add col)"""
  
      def visit_column(self, column):
          """Create a column (table already exists).
  
          :param column: column object
          :type column: :class:`sqlalchemy.Column` instance
          """
          if column.default is not None:
              self.traverse_single(column.default)
  
          table = self.start_alter_table(column)
          self.append("ADD ")
          self.append(self.get_column_specification(column))
  
          for cons in column.constraints:
              self.traverse_single(cons)
          self.execute()
  
          # ALTER TABLE STATEMENTS
  
          # add indexes and unique constraints
          if column.index_name:
              Index(column.index_name,column).create()
          elif column.unique_name:
              constraint.UniqueConstraint(column,
                                          name=column.unique_name).create()
  
          # SA bounds FK constraints to table, add manually
          for fk in column.foreign_keys:
              self.add_foreignkey(fk.constraint)
  
          # add primary key constraint if needed
          if column.primary_key_name:
              cons = constraint.PrimaryKeyConstraint(column,
                                                     name=column.primary_key_name)
              cons.create()
  
      def add_foreignkey(self, fk):
          self.connection.execute(AddConstraint(fk))
  
  class ANSIColumnDropper(AlterTableVisitor, SchemaDropper):
      """Extends ANSI SQL dropper for column dropping (``ALTER TABLE
      DROP COLUMN``).
      """
  
      def visit_column(self, column):
          """Drop a column from its table.
  
          :param column: the column object
          :type column: :class:`sqlalchemy.Column`
          """
          table = self.start_alter_table(column)
          self.append('DROP COLUMN %s' % self.preparer.format_column(column))
          self.execute()
  
  
  class ANSISchemaChanger(AlterTableVisitor, SchemaGenerator):
      """Manages changes to existing schema elements.
  
      Note that columns are schema elements; ``ALTER TABLE ADD COLUMN``
      is in SchemaGenerator.
  
      All items may be renamed. Columns can also have many of their properties -
      type, for example - changed.
  
      Each function is passed a tuple, containing (object, name); where
      object is a type of object you'd expect for that function
      (ie. table for visit_table) and name is the object's new
      name. NONE means the name is unchanged.
      """
  
      def visit_table(self, table):
          """Rename a table. Other ops aren't supported."""
          self.start_alter_table(table)
          self.append("RENAME TO %s" % self.preparer.quote(table.new_name))
          self.execute()
  
      def visit_index(self, index):
          """Rename an index"""
          if hasattr(self, '_validate_identifier'):
              # SA <= 0.6.3
              self.append("ALTER INDEX %s RENAME TO %s" % (
                      self.preparer.quote(
                          self._validate_identifier(
                              index.name, True)),
                      self.preparer.quote(
                          self._validate_identifier(
                              index.new_name, True))))
          elif hasattr(self, '_index_identifier'):
              # SA >= 0.6.5, < 0.8
              self.append("ALTER INDEX %s RENAME TO %s" % (
                      self.preparer.quote(
                          self._index_identifier(
                              index.name)),
                      self.preparer.quote(
                          self._index_identifier(
                              index.new_name))))
          else:
              # SA >= 0.8
              class NewName(object):
                  """Map obj.name -> obj.new_name"""
                  def __init__(self, index):
                      self.name = index.new_name
                      self._obj = index
  
                  def __getattr__(self, attr):
                      if attr == 'name':
                          return getattr(self, attr)
                      return getattr(self._obj, attr)
  
              self.append("ALTER INDEX %s RENAME TO %s" % (
                      self._prepared_index_name(index),
                      self._prepared_index_name(NewName(index))))
  
          self.execute()
  
      def visit_column(self, delta):
          """Rename/change a column."""
          # ALTER COLUMN is implemented as several ALTER statements
          keys = delta.keys()
          if 'type' in keys:
              self._run_subvisit(delta, self._visit_column_type)
          if 'nullable' in keys:
              self._run_subvisit(delta, self._visit_column_nullable)
          if 'server_default' in keys:
              # Skip 'default': only handle server-side defaults, others
              # are managed by the app, not the db.
              self._run_subvisit(delta, self._visit_column_default)
          if 'name' in keys:
              self._run_subvisit(delta, self._visit_column_name, start_alter=False)
  
      def _run_subvisit(self, delta, func, start_alter=True):
          """Runs visit method based on what needs to be changed on column"""
          table = self._to_table(delta.table)
          col_name = delta.current_name
          if start_alter:
              self.start_alter_column(table, col_name)
          ret = func(table, delta.result_column, delta)
          self.execute()
  
      def start_alter_column(self, table, col_name):
          """Starts ALTER COLUMN"""
          self.start_alter_table(table)
          self.append("ALTER COLUMN %s " % self.preparer.quote(col_name))
  
      def _visit_column_nullable(self, table, column, delta):
          nullable = delta['nullable']
          if nullable:
              self.append("DROP NOT NULL")
          else:
              self.append("SET NOT NULL")
  
      def _visit_column_default(self, table, column, delta):
          default_text = self.get_column_default_string(column)
          if default_text is not None:
              self.append("SET DEFAULT %s" % default_text)
          else:
              self.append("DROP DEFAULT")
  
      def _visit_column_type(self, table, column, delta):
          type_ = delta['type']
          type_text = str(type_.compile(dialect=self.dialect))
          self.append("TYPE %s" % type_text)
  
      def _visit_column_name(self, table, column, delta):
          self.start_alter_table(table)
          col_name = self.preparer.quote(delta.current_name)
          new_name = self.preparer.format_column(delta.result_column)
          self.append('RENAME COLUMN %s TO %s' % (col_name, new_name))
  
  
  class ANSIConstraintCommon(AlterTableVisitor):
      """
      Migrate's constraints require a separate creation function from
      SA's: Migrate's constraints are created independently of a table;
      SA's are created at the same time as the table.
      """
  
      def get_constraint_name(self, cons):
          """Gets a name for the given constraint.
  
          If the name is already set it will be used otherwise the
          constraint's :meth:`autoname <sqlalchemy_migrate_hotoffthehamster.changeset.constraint.ConstraintChangeset.autoname>`
          method is used.
  
          :param cons: constraint object
          """
          if cons.name is not None:
              ret = cons.name
          else:
              ret = cons.name = cons.autoname()
          return ret
  
      def visit_migrate_primary_key_constraint(self, *p, **k):
          self._visit_constraint(*p, **k)
  
      def visit_migrate_foreign_key_constraint(self, *p, **k):
          self._visit_constraint(*p, **k)
  
      def visit_migrate_check_constraint(self, *p, **k):
          self._visit_constraint(*p, **k)
  
      def visit_migrate_unique_constraint(self, *p, **k):
          self._visit_constraint(*p, **k)
  
  class ANSIConstraintGenerator(ANSIConstraintCommon, SchemaGenerator):
      def _visit_constraint(self, constraint):
          constraint.name = self.get_constraint_name(constraint)
          self.append(self.process(AddConstraint(constraint)))
          self.execute()
  
  class ANSIConstraintDropper(ANSIConstraintCommon, SchemaDropper):
      def _visit_constraint(self, constraint):
          constraint.name = self.get_constraint_name(constraint)
          self.append(self.process(DropConstraint(constraint, cascade=constraint.cascade)))
          self.execute()
  
  
  class ANSIDialect(DefaultDialect):
      columngenerator = ANSIColumnGenerator
      columndropper = ANSIColumnDropper
      schemachanger = ANSISchemaChanger
      constraintgenerator = ANSIConstraintGenerator
      constraintdropper = ANSIConstraintDropper
  