what is solution to save and reload a object?

[DeaD_EyE]

[deadeye@nexus ~]$ ipython
Python 3.13.0 (main, Oct  8 2024, 16:30:59) [GCC 14.2.1 20240910]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.29.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import pickle
   ...: 
   ...: 
   ...: class Foo:
   ...:     def __init__(self, name):
   ...:         self.name = name
   ...: 
   ...:     def hello(self):
   ...:         print(f"Hello {self.name}")
   ...: 
   ...: 
   ...: with open("test_pickle.pickle", "wb") as fd:
   ...:     # pickle the resulting instance of Foo
   ...:     # and write it to the open file in binary mode
   ...:     pickle.dump(Foo("Maya"), fd)
   ...: 
   ...: 
   ...: # now the pitfall
   ...: # you decided to change you class, but you still have old instances
   ...: class Foo:
   ...:     def __init__(self, name):
   ...:         self.identity = name
   ...: 
   ...:     def hello(self):
   ...:         print(f"Hello {self.identity}")
   ...: 
   ...: 
   ...: # now Foo is a different class, whith a different attribute and method
   ...: # trying to load the instance
   ...: with open("test_pickle.pickle", "rb") as fd:
   ...:     foo_instance = pickle.load(fd)
   ...: 
   ...: foo_instance.hello()
---------------------------------------------------------------------------

Error:
AttributeError                            Traceback (most recent call last)
Cell In[1], line 33
     30 with open("test_pickle.pickle", "rb") as fd:
     31     foo_instance = pickle.load(fd)
---> 33 foo_instance.hello()

Cell In[1], line 25, in Foo.hello(self)
     24 def hello(self):
---> 25     print(f"Hello {self.identity}")

AttributeError: 'Foo' object has no attribute 'identity'

Btw. ipython is a better REPL. But often I use ptpython, which has a better way to show the signature of functions/methods incl. the docstrings.

Another solution could be to write the data of your class into a serializable format (json) and if you want to restore the instance, loading from the file and creatng a new instance of the slightly modified class.

import json


class Foo:
    def __init__(self, name):
        self.name = name

    def hello(self):
        print(f"Hello {self.name}")

    def save(self, file):
        with open(file, "w") as fd:
            json.dump({"name": self.name}, fd)

    @classmethod
    def load(cls, file):
        with open(file) as fd:
            data = json.load(fd)
            return cls(name=data.get("name", "DEFAULT"))


class FooNew:
    def __init__(self, identity):
        self.identity = identity

    def hello(self):
        print(f"Hello {self.identity}")

    def save(self, file):
        with open(file, "w") as fd:
            json.dump({"identity": self.identity}, fd)

    @classmethod
    def load(cls, file):
        """
        Load FooNew instances and beeing compatible to old Foo instances.
        """
        with open(file) as fd:
            data = json.load(fd)
        identity = data.get("identity") or data.get("name") or "DEFAULT"

        return cls(identity)


old_foo = Foo("Nick")
old_foo.save("old_foo.json")
new_foo = FooNew.load("old_foo.json")

print("Hello from old_foo")
old_foo.hello()

print("Hello from new_foo")
new_foo.hello()
print("Vars of old_foo", vars(old_foo))
print("Vars of new_foo", vars(new_foo))

Output:
Hello from old_foo
Hello Nick
Hello from new_foo
Hello Nick
Vars of old_foo {'name': 'Nick'}
Vars of new_foo {'identity': 'Nick'}