Motor-CAD JSON-RPC server sending invalid JSON (e.g. INF)

[vrzh]

🔍 Before submitting the issue

• I have searched among the existing issues
• I am using a Python virtual environment

🐞 Description of the bug

It seems reasonable that some Motor-CAD variables might take the value ∞. In which Motor-CAD seems to send the INF constant through the JSON-RPC interface. However, this is not valid JSON and a decode error is thrown on the Python side.

📝 Steps to reproduce

I believe I got into this situation when I stopped a solve early and then requested the value of the variable.

mcad.get_variable("IMSINGLELOADPOINT_POWERFACTOR")

Full stack trace:

Traceback (most recent call last):
  File "/lib/python3.10/site-packages/requests/models.py", line 971, in json
    return complexjson.loads(self.text, **kwargs)
  File "/usr/lib/python3/dist-packages/simplejson/__init__.py", line 525, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3/dist-packages/simplejson/decoder.py", line 370, in decode
    obj, end = self.raw_decode(s)
  File "/usr/lib/python3/dist-packages/simplejson/decoder.py", line 400, in raw_decode
    return self.scan_once(s, idx=_w(s, idx).end())
simplejson.errors.JSONDecodeError: Expecting value: line 1 column 58 (char 57)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/lib/python3.10/site-packages/ansys/motorcad/core/rpc_client_core.py", line 452, in send_and_receive
    response = requests.post(self._get_url(), json=payload).json()
  File "/lib/python3.10/site-packages/requests/models.py", line 975, in json
    raise RequestsJSONDecodeError(e.msg, e.doc, e.pos)
requests.exceptions.JSONDecodeError: Expecting value: line 1 column 58 (char 57)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "my-test.py", line 80, in <module>
    val = mcad.get_variable(v)
  File "/lib/python3.10/site-packages/ansys/motorcad/core/methods/rpc_methods_variables.py", line 67, in get_variable
    return self.connection.send_and_receive(method, params)
  File "/lib/python3.10/site-packages/ansys/motorcad/core/rpc_client_core.py", line 457, in send_and_receive
    self._raise_if_allowed("RPC Communication failed: " + str(e))
  File "/lib/python3.10/site-packages/ansys/motorcad/core/rpc_client_core.py", line 336, in _raise_if_allowed
    raise MotorCADError(error_message)
ansys.motorcad.core.rpc_client_core.MotorCADError: RPC Communication failed: Expecting value: line 1 column 58 (char 57)

Looking at the underlying JSON which is received:

{"jsonrpc":"2.0","id":-1,"result":{"success":0,"output":[INF],"errorMessage":"pymotorcad: GetVariable IMSINGLELOADPOINT_POWERFACTOR = INF"}}

💻 Which operating system are you using?

Windows

📀 Which ANSYS version are you using?

Ansys Motor-CAD v2023.2.1

🐍 Which Python version are you using?

3.10

📦 Installed packages

ansys-motorcad-core==0.2.0

[vrzh]

Here is what it looks like in the Motor-CAD UI:

Not sure what the best solution is, a change in MotorCAD to not send the INF constant, or somehow relax the JSON parsing on the Python side. For example, the simplejson has allow_nan and parse_constant which could be useful.

[jgsdavies]

Thanks for spotting this @vrzh. We will look into fixing this for a future release. Also feel free to submit a PR if you have a good solution already.

[vrzh]

Not sure what is the best solution, as it seems there is no JSON standard to represent these types of values.

One option is to change Motor-CAD to output Infinity instead of INF as this seems to be what Python JSON decoders typically accept:

# python3 -m pip install 'simplejson==3.19.1'

import simplejson
simplejson.loads('Infinity', allow_nan=True)

# python3 -m pip install 'simplejson<3.19.1'

import simplejson
simplejson.loads('Infinity')

# using built-in json package
import json
json.loads('Infinity')

It looks like pymotorcad uses the requests library. But, the problem is that requests library only uses simplejson if it is installed, otherwise it falls back to json.

Therefore pymotorcad would also have to check if simplejson is installed and what version it is (adding allow_nan=True for older versions of simplejson will raise an exception). Or, add simplejson as a dependency of pymotorcad with a fixed version (either the old or new one).

[jgsdavies]

Should be fixed for Motor-CAD 2024R2. Returns "None" instead of throwing exception