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

Make arg2 optional:

class MyException(Exception):     def __init__(self, arg1, arg2=None):         self.arg1 = arg1         self.arg2 = arg2         super(MyException, self).__init__(arg1) 

The base Exception class defines a .__reduce__() method to make the extension (C-based) type picklable and that method only expects one argument (which is .args); see the BaseException_reduce() function in the C source.

The easiest work-around is making extra arguments optional. The __reduce__ method also includes any additional object attributes beyond .args and .message and your instances are recreated properly:

>>> e = MyException('foo', 'bar') >>> e.__reduce__() (<class '__main__.MyException'>, ('foo',), {'arg1': 'foo', 'arg2': 'bar'}) >>> pickle.loads(pickle.dumps(e)) MyException('foo',) >>> e2 = pickle.loads(pickle.dumps(e)) >>> e2.arg1 'foo' >>> e2.arg2 'bar'