只能将str（而不是“ NoneType”）连接到str

Question

我对python来说还比较陌生，我正在遵循一个教程，但是当我尝试使用非常小的示例文本运行此代码时，在第二次排序后会出现此错误。

  

TypeError：只能将str（而不是“ NoneType”）连接到str

使用较大的文本样本通常不会给我错误。我的问题是如何改善代码并更正此错误？

e2fsck -f /dev/sda2

输出是这样的

from random import choice
import sys

def generateModel(text, order):
    model = {}
    for i in range(0, len(text) - order):
        fragment = text[i:i+order]
        next_letter = text[i+order]
        if fragment not in model:
            model[fragment] = {}
        if next_letter not in model[fragment]:
            model[fragment][next_letter] = 1
        else:
            model[fragment][next_letter] += 1
    return model

def getNextCharacter(model, fragment):
    letters = []
    try:
      for letter in model[fragment].keys():
          for times in range(0, model[fragment][letter]):
              letters.append(letter)
      return choice(letters)
    except Exception as e:
      print(e)

def generateText(text, order, length):
    model = generateModel(text, order)
    currentFragment = text[0:order]
    output = ""
    for i in range(0, length-order):
      newCharacter = getNextCharacter(model, currentFragment)
      output += newCharacter
      currentFragment = currentFragment[1:] + newCharacter
    print(output)

text = "sample text"
if __name__ == "__main__":
  generateText(text, 3, 100)