I have a rather large project, including a class Foo which recently needed to be updated using the @property decorator to create custom getter and setter methods.

I also stored several instances of Foo on my hard drive that at some point I might need to reload. My problem is, that I cannot access the attributes decoreted with property on these old objects.

Consider the following example:

import pickle# define Class and create instance
class Foo:def __init__(self):self.val = 1
foo = Foo()# dump foo into file
with open("foo.pickle", 'wb') as handle:pickle.dump(foo, handle, pickle.HIGHEST_PROTOCOL)# overwrite and add @property in the class definition
class Foo:def __init__(self):self._val = "new_foo"@propertydef val(self):return self._val@val.setterdef val(self, val):self._val = valfoo_new = Foo()
print(foo_new.val)# reload foo
with open("foo.pickle", "rb") as handle:foo_old = pickle.load(handle)# try to access attributes
print(foo_old.val)

The last line raises:

NameError: name '_val' is not defined

What options do I have to still access the attributes of my archived instances?

Edit: Changed self.val to self._val in the constructor of the second Foo-definition.

The pickle documentation says:

When a class instance is unpickled, its __init__() method is usually not invoked.

Which is why the _val attribute wasn't defined You can workaround that by defining a __new__ method in the replacement Foo class and setting the instance attribute there:

import pickle# define Class and create instance
class Foo:def __init__(self):self.val = 1
foo = Foo()# dump foo into file
with open("foo.pickle", 'wb') as handle:pickle.dump(foo, handle, pickle.HIGHEST_PROTOCOL)# overwrite and add @property in the class definition
class Foo:def __new__(cls, val=None):inst = super().__new__(cls)inst._val = "new_foo"  if val is None else valreturn inst@propertydef val(self):return self._val@val.setterdef val(self, val):self._val = valfoo_new = Foo()
print(foo_new.val)  # -> new_foo# reload foo
with open("foo.pickle", "rb") as handle:foo_old = pickle.load(handle)print(foo_old.val)  # -> new_foo