class_docstr: An abstract class that takes a sentence and transforms it by replacing
  some of its words.
  
  letters_to_insert (string): letters allowed for insertion into words
  (used by some char-based transformations)
docstr: Helper function that returns a random single letter from the English
  alphabet that could be lowercase or uppercase.
code: def _get_random_letter(self):
      """Helper function that returns a random single letter from the English
      alphabet that could be lowercase or uppercase."""
      return random.choice(self.letters_to_insert)
  
file_path: TextAttack/textattack/transformations/word_swaps/word_swap.py
imports: import random
  import string
  from textattack.transformations import Transformation
label_desc: Use image node_id 3 for calling the WordSwap obj's underlying member method code with example usage: obj._get_random_letter() and returns: random