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