class_docstr: Class implementing database connection.
  
  Example
  -------
  >>> AMPLIGRAPH_DATA_HOME='/your/path/to/datasets/'
  >>> # Initialize GraphDataLoader from .csv file
  >>> data = GraphDataLoader("data.csv", backend=SQLiteAdapter)
  >>> # Initialize GraphDataLoader from .txt file using indexer
  >>> # to map entities to integers
  >>> data = GraphDataLoader(AMPLIGRAPH_DATA_HOME + "fb15k/test.txt",
  >>>                        backend=SQLiteAdapter("database.db",
  >>>                        use_indexer=True))
  >>> for elem in data:
  >>>     print(elem)
  >>>     break
  [(1, 1, 2)]
  >>> # Populate the database with raw triples for training
  >>> with SQLiteAdapter("database.db") as backend:
  >>>     backend.populate(AMPLIGRAPH_DATA_HOME + "fb15k/train.txt",
  >>>                      dataset_type="train")
  >>> # Populate the database with indexed triples for training
  >>> with SQLiteAdapter("database.db", use_indexer=True) as backend:
  >>>     backend.populate(AMPLIGRAPH_DATA_HOME + "fb15k/train.txt",
  >>>                      dataset_type="train")
docstr: Check if the current data adapter has already been indexed.
  
  Returns
  -------
  Flag : bool
       Flag indicating whether indexing took place.
docstr_tok: ["Check","if","the","current","data","adapter","has","already","been","indexed",".","Returns","--","--","--","-","Flag",":","bool","Flag","indicating","whether","indexing","took","place","."]
code: def is_indexed(self):
      """Check if the current data adapter has already been indexed.
  
      Returns
      -------
      Flag : bool
           Flag indicating whether indexing took place.
  
      """
      if not hasattr(self, "mapper"):
          return False
      return True
  
imports: import logging
  import os
  import sqlite3
  import tempfile
  from sqlite3 import Error
  from urllib.request import pathname2url
  import numpy
  import pandas
  import tensorflow
  from ampligraph.utils.profiling import get_human_readable_size
  from .data_indexer import DataIndexer
label_desc: Use image node_id 17 for calling the SQLiteAdapter obj's underlying member method code with example usage: obj.is_indexed() and returns: True, False