UnpicklingError: A load persistent id instruction was encountered, but no persistent_load function was specified.

This error occurs when attempting to unpickle an object that contains a reference to a persistent object, but no persistent_load function was defined for that object.

To fix this error, you need to define a persistent_load function for the object. The persistent_load function should take a single argument, which is the persistent ID of the object, and return the corresponding object.

Here's an example of how to define a persistent_load function for a custom class:

import pickle

class MyClass:
    def __init__(self, x):
        self.x = x

def persistent_load(persistent_id):
    if persistent_id == 'myclass':
        return MyClass

pickle.loads(b'\x80\x03c__main__\nMyClass\nq\x00.')
# => UnpicklingError: A load persistent id instruction was encountered, but no persistent_load function was specified.

pickle.loads(b'\x80\x03c__main__\nMyClass\nq\x00.', persistent_load=persistent_load)
# => <class '__main__.MyClass'>

In this example, we define a persistent_load function that returns the MyClass class when the persistent ID is 'myclass'. We then pass this function to the loads() method as the persistent_load argument. When we call loads() with this argument, the MyClass class is successfully unpickled.