old_contents: __all__ = [
      'ensure_only_one_row',
      'insert_or_ignore',
  ]
  
  
  def ensure_only_one_row(rows):
      row = rows.fetchone()
      if row is None or rows.fetchone() is not None:
          raise KeyError
      return row
  
  
  def insert_or_ignore(conn, table, values):
      conn.execute(table.insert().prefix_with('OR IGNORE'), values)
  
new_contents: __all__ = [
      'add_if_not_exists_clause',
      'ensure_only_one_row',
      'insert_or_ignore',
  ]
  
  from garage import asserts
  
  from sqlalchemy.schema import CreateIndex
  
  
  def add_if_not_exists_clause(index, engine):
      # `sqlalchemy.Index.create()` does not take `checkfirst` for reasons
      # that I am unaware of, and here is a hack for sidestep that.
      stmt = str(CreateIndex(index).compile(engine))
      stmt = stmt.replace('CREATE INDEX', 'CREATE INDEX IF NOT EXISTS', 1)
      asserts.postcond('IF NOT EXISTS' in stmt, stmt)
      return stmt
  
  
  def ensure_only_one_row(rows):
      row = rows.fetchone()
      if row is None or rows.fetchone() is not None:
          raise KeyError
      return row
  
  
  def insert_or_ignore(conn, table, values):
      conn.execute(table.insert().prefix_with('OR IGNORE'), values)
  
subject: Add a hack for appending "IF NOT EXISTS" clause to "CREATE INDEX"