Passing a NamedTuple **class** rather than instance to .dumps results in an error

[ng-henry]

simplejson has functionality to encode namedtuple instances using their _asdict() method. However, when passing the underlying namedtuple type to json.dumps, there's an error: ._asdict() missing 1 required positional argument: 'self'

simplejson/simplejson/_speedups.c

Lines 385 to 399 in 8dce5b0

	static int
	_is_namedtuple(PyObject *obj)
	{
	int rval = 0;
	/* We intentionally accept anything with a duck typed _asdict method rather
	* than requiring it to pass PyTuple_Check(obj). */
	PyObject *_asdict = PyObject_GetAttrString(obj, "_asdict");
	if (_asdict == NULL) {
	PyErr_Clear();
	return 0;
	}
	rval = PyCallable_Check(_asdict);
	Py_DECREF(_asdict);
	return rval;
	}

The reason is that _is_namedtuple only checks for the existence of a method named _asdict(). Calling _asdict() from the object instance requires no further arguments as self is already provided. However, calling _asdict() from the namedtuple class requires an additional argument, self.

To fix this, _is_namedtuple function should further check that the object is an instance rather than a class, or that the _asdict() function is indeed callable with zero arguments.

[etrepum]

What is the intended use case of passing such a nametuple class through the encoder? What do you expect to happen here other than perhaps a different error message?

[ng-henry]

I'm working on capturing all the arguments of a given function call and serializing them to JSON. Since the arguments can be anything, I'd expect simplejson to encode builtin objects and call the default function for all the other things it cannot encode.

For example, if I pass in a random class (not NamedTuple), then simplejson would call default and let user handle it themselves. But for NamedTuple, simplejson crashes and does not let user specify how to encode.

[etrepum]

This is implemented in v3.18.0