How to make a custom exception class with multiple init args pickleable

Question

Why does my custom Exception class below not serialize/unserialize correctly using the pickle module?

import pickle

class MyException(Exception):
    def __init__(self, arg1, arg2):
        self.arg1 = arg1
        self.arg2 = arg2

        super(MyException, self).__init__(arg1)

e = MyException("foo", "bar")

str = pickle.dumps(e)
obj = pickle.loads(str)

This code throws the following error:

Traceback (most recent call last):
File "test.py", line 13, in <module>
   obj = pickle.loads(str)
File "/usr/lib/python2.7/pickle.py", line 1382, in loads
   return Unpickler(file).load()
File "/usr/lib/python2.7/pickle.py", line 858, in load
   dispatch[key](self)
File "/usr/lib/python2.7/pickle.py", line 1133, in load_reduce
   value = func(*args)
TypeError: __init__() takes exactly 3 arguments (2 given)

I'm sure this problem stems from a lack of knowledge on my part of how to make a class pickle-friendly. Interestingly, this problem doesn't occur when my class doesn't extend Exception.

Thanks for any help. Kyle

EDIT: Fixing my call to super per shx2 EDIT: Cleaning up title/content

Answer 1

The current answers break down if you're using both arguments to construct an error message to pass to the parent Exception class. I believe the best way is to simply override the __reduce__ method in your exception. The __reduce__ method should return a two item tuple. The first item in the tuple is your class. The second item is a tuple containing the arguments to pass to your class's __init__ method.

import pickle

class MyException(Exception):
    def __init__(self, arg1, arg2):
        self.arg1 = arg1
        self.arg2 = arg2

        super(MyException, self).__init__('arg1: {}, arg2: {}'.format(arg1, arg2))

    def __reduce__(self):
        return (MyException, (self.arg1, self.arg2))


original = MyException('foo', 'bar')
print repr(original)
print original.arg1
print original.arg2

reconstituted = pickle.loads(pickle.dumps(original))
print repr(reconstituted)
print reconstituted.arg1
print reconstituted.arg2

More info about __reduce__ here.

Answer 2

I like Martijn's answer, but I think a better way is to pass all arguments to the Exception base class:

class MyException(Exception):
    def __init__(self, arg1, arg2):
        super(MyException, self).__init__(arg1, arg2)        
        self.arg1 = arg1
        self.arg2 = arg2

The base Exception class' __reduce__ method will include all the args. By not making all of the extra arguments optional, you can ensure that the exception is constructed correctly.

Answer 3

I simply do this

class MyCustomException(Exception):
    def __init__(self):
        self.value = 'Message about my error'

    def __str__(self):
        return repr(self.value)

... somewhere in code ...
raise MyCustomException

Answer 4

I was able to get a custom exception with keyword-only arguments to work with pickling like this:

class CustomException(Exception):
    def __init__(self, arg1, *, kwonly):
        super().__init__(arg1, kwonly) # makes self.args == (arg1, kwonly)
        self.arg1 = arg1
        self.kwonly = kwonly

    def __str__(self):
        # Logic here to turn the args into the real exception message
        return f"{self.arg1} ({self.kwonly})"

    # __repr__ and __eq__ are not needed but are nice for testing
    def __repr__(self):
        return f"{self.__class__.__name__}({self.arg1!r}, kwonly={self.kwonly!r})"

    def __eq__(self, other):
        return isinstance(other, CustomException) and self.args == other.args

    # This is what makes pickling work
    @classmethod
    def _new(cls, arg1, kwonly):
        return cls(arg1, kwonly=kwonly)

    def __reduce__(self):
        return (self._new, (self.arg1, self.kwonly))

Example:

import pickle

w = CustomException('arg1', kwonly='kwonly')
w2 = pickle.loads(pickle.dumps(w))
assert w == w2

By the way, this also works with custom warning classes, since those are also Exception subclasses.