Encoding list of objects as JSON object

Question

I would like to extend the standard library Python JSON encoder to encode lists of complex numbers into a representation correctly parsed by JSONLAB, the unofficial JSON toolbox for MATLAB. Briefly, a single Python complex number (x + yj) should be encoded as a JSON complex number object,

{"_ArrayType_": "double",
 "_ArraySize_": [1, 1],
 "_ArrayIsComplex_": 1,
 "_ArrayData_": [x, y]}

and a list of complex numbers [(x1 + y1j), (x2+y2j), (x3+y3j)] should also become a single JSON complex number object,

{"_ArrayType_": "double",
 "_ArraySize_": [1, 3],
 "_ArrayIsComplex_": 1,
 "_ArrayData_": [[x1, y1], [x2, y2], [x3, y3]]}

I have been able to correctly encode a single complex number, like so:

class ComplexEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, complex):
            return {"_ArrayType_": "double", "_ArraySize_": [1, 1],
                    "_ArrayIsComplex_": 1, "_ArrayData_": [obj.real, obj.imag]}
        return json.JSONEncoder.default(self, obj)

I tried a straightforward extension to the case of a complex number list, but it does not quite work:

class ComplexEncoder(json.JSONEncoder):
    def default(self, obj):
        # if obj is a list containing at least one complex number
        if isinstance(obj, list) and sum(map(lambda x: isinstance(x, complex),
                                             obj)):
            data = [[elem.real, elem.imag] for elem in obj]
            return {"_ArrayType_": "double", "_ArraySize_": [1, len(obj)],
                    "_ArrayIsComplex_": 1, "_ArrayData_": data}
        elif isinstance(obj, complex):
            return {"_ArrayType_": "double", "_ArraySize_": [1, 1],
                    "_ArrayIsComplex_": 1, "_ArrayData_": [obj.real, obj.imag]}
        return json.JSONEncoder.default(self, obj)

If this class is used to parse a Python list of complex numbers, the result is a JSON list of 1x1 JSON complex number objects, rather than a single JSON complex number object. It seems that the Python object is parsed "bottom-up," rather than "top-down," with the complex numbers converted into 1x1 JSON complex number objects before it is noticed that they're in a list.

How best to encode lists of complex numbers into complex number objects compatible with JSONLAB?

borntyping borntyping · Accepted Answer · 2013-10-16T20:08:31

From taking a brief look through the simplejson source code, there seems to be no easy way to encode the JSON in any way other than bottom up. I'd suggest two possible solutions:

Using a non-iterable or non-list object (i.e. something that the json module doesn't parse) to contain the list of complex numbers, so that it will be encoded by default() and not json.JSONEncoder.
Encode the lists of complex numbers before encoding the rest, so that the json encoder doesn't' have to deal with encoding parent objects before child objects.

Encoding list of objects as JSON object

1 Answers