Python AttributeError求助：'str'对象无'get_message_text'属性

问题概述 #

尝试用Message类实现凯撒密码时，运行代码触发AttributeError，报错行是original = self.get_message_text()，错误提示'str' object has no attribute 'get_message_text'。

完整报错信息 #

Traceback (most recent call last):
File ~.spyder-py3\ps5.py:239 in
print(Message.apply_shift('tweLve',12))
File ~.spyder-py3\ps5.py:129 in apply_shift
original = self.get_message_text()
AttributeError: 'str' object has no attribute 'get_message_text'

完整代码 #

import string

class Message(object):

    def __init__(self, text):

        self.message_text = text
        #self.valid_words = load_words(WORDLIST_FILENAME) - commented out bc irrelevant here


    def get_message_text(self):
        '''
        Used to safely access self.message_text outside of the class
    
        Returns: self.message_text
        '''
        return self.message_text

    def build_shift_dict(self, shift):
        letters_lower, letters_upper = string.ascii_lowercase, string.ascii_uppercase
    
    
        new_dict={}
        for letter in letters_upper:
            new_dict[letter] = letters_upper[(letters_upper.index(letter) + shift) % 26]
        for letter in letters_lower:
            new_dict[letter] = letters_lower[(letters_lower.index(letter) + shift) % 26]
    
        return new_dict

    def apply_shift(self, shift):
    
        original = self.get_message_text()
        new = ''
        for letter in original:
            new += (self.build_shift_dict(shift))
        
        #print(new) - a test print
        return new

print(Message.apply_shift('tweLve',12))

问题原因及修复方案 #

核心错误：实例方法调用方式不对 #

你直接用类Message调用了apply_shift方法，而不是先创建类的实例。Python中，实例方法的第一个参数self默认指向类实例，如果你直接用类调用，这个self会被你传入的字符串'tweLve'替代，导致self变成了字符串对象，自然没有get_message_text这个方法。

两步修复： #

1. 修正调用方式：先创建Message的实例，再调用apply_shift：

   msg = Message('tweLve')
   print(msg.apply_shift(12))
   

2. 修复apply_shift的逻辑错误：原代码里new += (self.build_shift_dict(shift))会把整个字典拼到字符串里，完全错误。应该根据每个字母从移位字典中取对应字符，非字母保持原样：

   def apply_shift(self, shift):
       original = self.get_message_text()
       shift_dict = self.build_shift_dict(shift)  # 提前生成字典，避免循环里重复创建
       new = ''
       for letter in original:
           new += shift_dict.get(letter, letter)
       return new
   

修复后的完整代码 #

import string

class Message(object):

    def __init__(self, text):
        self.message_text = text
        #self.valid_words = load_words(WORDLIST_FILENAME) - commented out bc irrelevant here

    def get_message_text(self):
        '''
        Used to safely access self.message_text outside of the class
    
        Returns: self.message_text
        '''
        return self.message_text

    def build_shift_dict(self, shift):
        letters_lower, letters_upper = string.ascii_lowercase, string.ascii_uppercase
        new_dict = {}
        for letter in letters_upper:
            new_dict[letter] = letters_upper[(letters_upper.index(letter) + shift) % 26]
        for letter in letters_lower:
            new_dict[letter] = letters_lower[(letters_lower.index(letter) + shift) % 26]
        return new_dict

    def apply_shift(self, shift):
        original = self.get_message_text()
        shift_dict = self.build_shift_dict(shift)
        new = ''
        for letter in original:
            new += shift_dict.get(letter, letter)
        return new

# 正确调用
msg = Message('tweLve')
print(msg.apply_shift(12))

内容的提问来源于stack exchange，提问作者BergholtStuttley