Draft: JSON import drops is_fd

Status: draft, not posted, and probably not to be posted as a bug: the development branch already has the change. Pull request ebroecker/canmatrix#939 ("fix: preserve define defaults, signal groups, fd flag and multiplexing in json/yaml round trip", commit 2a63b57, merged 2026-10-28) makes from_dict apply is_fd. It is not in a release yet: 1.3.3 (2026-10-21) is the latest on PyPI, and it is the version CanViewer ships. The options are the maintainer's:

• wait for the next release, then update the wheel in app/src/lib/canmatrix/wheels.lock.json (the check of this draft then fails on purpose, and the draft retires);
• or post a short request for a release that includes ebroecker/canmatrix#939, quoting the reproduction as the reason.

The reproduction and the Actual block are checked by app/src/lib/canmatrix/upstream-reports.test.ts, which runs the script under the canmatrix wheel CanViewer ships and compares its output with the block below.

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Issue title: JSON import ignores is_fd, so CAN FD frames come back as classic CAN

Summary

The JSON exporter writes "is_fd": true for a CAN FD frame, in both the default and the jsonExportAll layout, but the JSON importer never reads the key. Every frame loaded from JSON has is_fd == False, so a JSON round trip, or a conversion from JSON to DBC, ARXML or any other format, turns CAN FD frames into classic CAN frames. Exported to DBC directly, the frame below gets BA_ "VFrameFormat" BO_ 291 14; (StandardCAN_FD); exported to DBC after a JSON round trip, it gets no VFrameFormat at all, so the DBC file describes a classic CAN frame.

Environment

• canmatrix 1.3.3, the latest release on PyPI (2026-10-21);
• CPython 3.13 on Linux, and Pyodide 314.0.7 (CPython 3.14) with the same wheel: identical output.

Reproduction

import io

import canmatrix
import canmatrix.formats

print("canmatrix", canmatrix.__version__)

db = canmatrix.CanMatrix()
frame = canmatrix.Frame("FdFrame", arbitration_id=canmatrix.ArbitrationId(0x123), size=8, is_fd=True)
frame.add_signal(canmatrix.Signal("S", start_bit=0, size=8))
db.add_frame(frame)

for export_all in (False, True):
    out = io.BytesIO()
    canmatrix.formats.dump(db, out, "json", jsonExportAll=export_all)
    text = out.getvalue()
    print("jsonExportAll=%s: written is_fd:" % export_all, b'"is_fd": true' in text)
    again = canmatrix.formats.loads_flat(text, "json")
    print("jsonExportAll=%s: read back is_fd:" % export_all, again.frames[0].is_fd)

Expected

canmatrix 1.3.3
jsonExportAll=False: written is_fd: True
jsonExportAll=False: read back is_fd: True
jsonExportAll=True: written is_fd: True
jsonExportAll=True: read back is_fd: True

Actual

canmatrix 1.3.3
jsonExportAll=False: written is_fd: True
jsonExportAll=False: read back is_fd: False
jsonExportAll=True: written is_fd: True
jsonExportAll=True: read back is_fd: False

Cause

to_dict() in src/canmatrix/formats/_dict_codec.py writes "is_fd": frame.is_fd (lines 97 and 177), but from_dict() copies only is_complex_multiplexed, mux_names, cycle_time, is_j1939, header_id and pdu_name (line 223), and nothing else sets is_fd.

Suggested fix

What the development branch does since #939, in from_dict():

if "is_fd" in frame:
    new_frame.is_fd = frame["is_fd"]

On the development branch at 8b54a41 the reproduction prints the Expected block (its __version__ there still reads 1.3.2).

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Notes for CanViewer (not for posting)

• CanViewer imports .json through canmatrix, so the app is affected: a CAN FD frame of up to 8 bytes in a JSON file shows as classic CAN. Frames longer than 8 bytes still show as CAN FD, because CanViewer's own rule (app/src/lib/dbc/canfd.ts) treats a DLC above 8 as FD. upstream-reports.test.ts pins the 8-byte case: no VFrameFormat, and isCanFd() is false.
• canviewer_adapter.py does not work around it, and should not guess: the flag is gone before the adapter sees the frame. Reading is_fd from the JSON text ourselves would be a workaround of our own; the release is the better fix.
• app/src/lib/canmatrix/mapper.test.ts notes the same ("it drops is_fd").