code: def find_invalid_chars(self, text, context_size=20):
          
          result = defaultdict(list)
          for idx, char in enumerate(text):
              if char not in self.alphabet:
                  start = max(0, idx-context_size)
                  end   = min(len(text), idx+context_size)
                  result[char].append(text[start:end])
          return result
docs: Find invalid characters in text and store information about
          the findings.
  
          Parameters
          ----------
          context_size: int
              How many characters to return as the context.