simplejson: alternate encoder not called enought

Chris Rebert clp2 at
Sun Mar 22 10:16:07 CET 2009

On Sat, Mar 21, 2009 at 2:11 PM, Pierre Hanser <hanser at> wrote:
> hello
> I'm trying to use simplejson to encode some
> python objects using simplejson dumps method.
> The dumps method accept a cls parameter to specify
> an alternate encoder. But it seems that this alternate
> encoder is called only as a last resort, if object type
> is not int, string, and all other basic types.
> My problem is that i wanted to specify an encoding
> for a dbus.Boolean object which inherits from int.
> As int is handled by the standard encoder, the alternate
> encoder is never called and a dbus.Boolean is converted
> to 0 or 1 instead of true or false.
> Could someone knowledgeabled enought confirm my diagnostic?

Indeed, this does appear to be the case:

import json

class Foo(int):
    def __init__(self, val):
        int.__init__(self, val)
        self.val = val

class MyEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, Foo):
            return ["foo", obj.val]
        return json.JSONEncoder.default(self, obj)

print "Expect:", json.dumps(["foo", 42])
print "Got:   ", json.dumps(Foo(42), cls=MyEncoder)

# Expect: ["foo", 42]
# Got:    42

The docs are not entirely clear as to whether this should be expected or not.
I'd file a bug. If this behavior is deemed correct, then at the least
the docs should be clarified to indicate that the default encodings
apply to certain builtin types *and their subclasses*.


I have a blog:

More information about the Python-list mailing list