class_docstr: Abstract base class for Active Learners.
code: def update(self, y):
      pass
  
imports: import numpy
  from abc import ABC, abstractmethod
  from pathlib import Path
  from scipy.sparse import csr_matrix
  from small_text.base import LABEL_IGNORED
  from small_text.exceptions import LearnerNotInitializedException
  from small_text.utils.data import list_length
  from small_text.utils.labels import concatenate, get_ignored_labels_mask, remove_by_index
  from small_text.version import __version__
label_desc: Use image node_id 2 for calling the ActiveLearner obj's underlying member method code with example usage: obj.update(y) without return types