args attribute of an exception subclass's type changes from string to tuple

Question

This happens on both python2.6 and python3:

class Error(Exception):
    def __init__(self, args):
            print(type(args))
            print(type(self.args)) # From BaseException
            self.args = args
            print(type(self.args))

Error("foo")

This results in:

<type 'str'>
<type 'tuple'>
<type 'tuple'>
Error('f', 'o', 'o')

For some reason, the args attribute is coerced into a tuple. Might the fact that it's defined in C have something to do with it? https://github.com/python/cpython/blob/master/Objects/exceptions.c

The name of the args argument is unrelated. Changing it to 'a' results in the same behavior, as long as it is assigned to self.args.

Answer 1

Looking at the code you link to, there is a setter defined for the "args" attribute. Look for BaseException_set_args - it is being set (elsewhere in the linked code) as the setter for args. So when you write self.args = args, you're really calling the function BaseException_set_args, with args as the argument.

If you then look in BaseException_set_args, it is coercing the argument into a tuple. If you give try to set self.args to something that can't be converted to a tuple (e.g. try Error(23)), you'll get a TypeError.