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