shearichard wrote:
> Hi - I've got a straightforward class I want to serialize as JSON
> (actually I want to a serialize a list of them but I believe that's
> irrelevant).
>
> I've subclassed JSONEncoder and defined my own version of the
> 'default' method ( based upon what I read at
> http://docs.python.org/library/json.html ) but when I then try to
> serialize the class I get the (fairly weird) error message : "TypeError:
> __init__() got an unexpected keyword argument 'indent'".
>
> I suspect I'm doing something pretty fundamentally wrong but I don't
> know what - can anyone tell me what's wrong (or alternatively tell me
> how to log this as a bug ;-)
>
> Here's my test case :
>
> import json
> class SuperPeople(object):
> pass
> class People(SuperPeople, json.JSONEncoder):
> def __init__(self, name, age):
> self.__name = name
> self.__age = age
> def default(self, obj):
> if isinstance(obj, People):
> return [obj.__name, obj.__age]
> else:
> return json.JSONEncoder.default(self, obj)
>
> def main():
> lstPeople = []
> lstPeople.append(People("Mary", 50))
> lstPeople.append(People("Joe", 40))
> lstPeople.append(People("Sue", 30))
>
> print json.dumps(lstPeople, cls=People)
>
>
> if __name__ == "__main__":
> main()
>
>
> ... and this is what the stacktrace looks like ....
>
> Traceback (most recent call last):
> File "testJSON.py", line 24, in <module>
> main()
> File "testJSON.py", line 20, in main
> json.dumps(lstPeople, cls=People)
> File "C:\bin\installed\Python2.6\lib\json\__init__.py", line 237, in
> dumps
> **kw).encode(obj)
> TypeError: __init__() got an unexpected keyword argument 'indent'
>
>
> ... I'm running Python 2.6 on Win32.
>
> All suggestions welcomed .
You pass the encoder *class* to json.dumps(), so the function has to
instantiate it. It does that with the arguments that an encoder class must
accept. There's no way for it to expect that an encoder requires a name and
an age.
The solution is to separate the encoder and the class that shall be encoded.
Here's one way:
import json
class People(object):
def __init__(self, name, age):
self.__name = name
self.__age = age
def get_json_state(self):
return [self.__name, self.__age]
class PeopleEncoder(json.JSONEncoder):
def default(self, obj):
if isinstance(obj, People):
return obj.get_json_state()
else:
return json.JSONEncoder.default(self, obj)
def main():
lstPeople = []
lstPeople.append(People("Mary", 50))
lstPeople.append(People("Joe", 40))
lstPeople.append(People("Sue", 30))
print json.dumps(lstPeople, cls=PeopleEncoder)
if __name__ == "__main__":
main()
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