如何使用pickle从文件中获取类实例列表

Question

我有2个班：小组和学生。我创建了包含不同组的列表，每个组都包含包含学生列表的属性。然后我用这种方式用pickle将它保存到文件中：

encodedList = []
try:
    with open('test') as file:
        tfile = open( 'test', "r" )
        encodedList = pickle.load( tfile )
except IOError:
    tfile = open( 'test', "w" )
    pickle.dump( encodedList, tfile )
tfile.close()

这三条线很好用。再次启动程序后，我想从列表中的文件中获取所有这些信息，我按照这样的许多教程来做：

.factory('someFac', function (CONFIG, $injector, $http) {
   return $http.get('url').then(function (response) {
      var temp = response.data.answer;
      var answer = '';
      if(temp == bar){
          answer = barAnswer;
       }
      else if(temp == foo){
          answer = fooAnswer;
      }
      return $injector.get(answer);
   }
}

但是Programm崩溃了，给出了下一个错误：

我尝试以不同的方式从文件中读取此列表，但此错误始终相同，您能帮助我吗？

Answer 1

类定义必须在你unpickle之前出现：

class foo(): # works
    pass
encodedList = []
try:
    with open('test') as file:
        tfile = open( 'test', "r" )
        encodedList = pickle.load( tfile )
except IOError:
    tfile = open( 'test', "w" )
    pickle.dump( encodedList, tfile )
tfile.close()

它是如何失败的：

encodedList = []
try:
    with open('test') as file:
        tfile = open( 'test', "r" )
        encodedList = pickle.load( tfile )
except IOError:
    tfile = open( 'test', "w" )
    pickle.dump( encodedList, tfile )
tfile.close()


class foo(): # fails
    pass

输出将是AttributeError: 'module' object has no attribute 'foo'，这是您在自己的代码中看到的。如果类定义在另一个文件中，请在尝试unpickle之前添加导入

Answer 2

您可以使用pickle尝试不，而是使用更好的序列化工具，例如dill。使用dill，类定义与实例的pickle一起存储，因此如果您不知道要取消哪种类型的实例，则很容易。

>>> class Student(object):
...   def __init__(self, name):
...     self.name = name
...   def __repr__(self):
.. .    return "Student(%s)" % self.name
... 
>>> class Group(list):  
...   pass
... 
>>> myclass = Group([Student('Ted'), Student('Fred'), Student('Jane')])
>>>  
>>> import dill
>>> with open('myclass.pkl', 'w') as f:
...   dill.dump(myclass, f)
... 
>>> 

在挑选实例列表后，退出并开始新会话......

Python 2.7.10 (default, Sep  2 2015, 17:36:25) 
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import dill
>>> with open('myclass.pkl', 'r') as f:
...   myclass = dill.load(f)
... 
>>> myclass
[Student('Ted'), Student('Fred'), Student('Jane')]
>>> [student.name for student in myclass]
['Ted', 'Fred', 'Jane']
>>> type(myclass)
<class '__main__.Group'>

注意上面的答案实际上是存储一个列表子类的实例，即存储三个类实例。