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stlink_probe._get_board_id() raises TypeError, not KeyError, when mbed.htm exists but has no TargetID

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@alwaysmy is already working on this.

Since Oct 13, 2026.

  • #2056 by @alwaysmy — open

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
embedded-iot

Research direction

Start in pyocd/probe/stlink_probe.py at the _get_board_id() lookup, around line 94, where info['target_id_mbed_htm'][0:4] is guarded only by except KeyError. Then check how stlink/detect/base.py stores target_id_mbed_htm as None. Done when a volume with an unparsable mbed.htm no longer aborts pyocd list, and the ST-Link shows n/a in the Target column.

Written by the indexing model from the issue text.

Description

Summary

StlinkProbe._get_board_id() raises an uncaught TypeError when an MSD volume contains an
mbed.htm whose TargetID cannot be parsed. Probe enumeration then aborts entirely, so
pyocd list prints one error line and exits 1 without listing any probe — including probes
that have nothing to do with the ST-Link.

The except KeyError guarding that line was added to fix #695, but it only covers the
"no mbed.htm file" case. The "file present but unparsable" case stores None in the same
key and raises TypeError, which slips past it.

Environment
  • pyocd 0.36.0 on Windows, Python 3.14
  • Behaviour is identical on 0.45.1 and current main (checked by reading the released wheels
    and the branch source)
  • ST-Link whose firmware does not answer get_board_id()
  • A DAPLink MSD volume is also present, whose mbed.htm is a redirect page with no TargetID
Symptom
PS> pyocd list
0002715 C Error: 'NoneType' object is not subscriptable [__main__]

No traceback by default (debug.traceback is off), exit code 1, no probe listed.

With -O debug.traceback=True:

  File ".../pyocd/subcommands/list_cmd.py", line 94, in invoke
    ConnectHelper.list_connected_probes()
  File ".../pyocd/core/helpers.py", line 123, in list_connected_probes
    ConnectHelper._print_probe_list(allProbes)
  File ".../pyocd/core/helpers.py", line 283, in _print_probe_list
    board_info = probe.associated_board_info
  File ".../pyocd/probe/stlink_probe.py", line 142, in associated_board_info
    if (self.board_id is not None) and (self.board_id in BOARD_ID_TO_INFO):
  File ".../pyocd/probe/stlink_probe.py", line 73, in board_id
    self._board_id = self._get_board_id()
  File ".../pyocd/probe/stlink_probe.py", line 94, in _get_board_id
    this_board_id = info['target_id_mbed_htm'][0:4]
TypeError: 'NoneType' object is not subscriptable
Root cause

stlink/detect/base.py stores None in the key whenever parsing fails:

htm_target_id, daplink_info = self._read_htm_ids(device["mount_point"])
if htm_target_id:
    device["target_id"] = htm_target_id
else:
    ...                                        # falls back to the USB id
device["target_id_mbed_htm"] = htm_target_id   # None is stored here as well

_read_htm_ids() is documented to return None on failure, and _target_id_from_htm()
recognises only ?code=<hex> and ?auth=<hex>. A volume whose mbed.htm is just a redirect
page therefore yields target_id_mbed_htm = None while the key is always present.

stlink_probe.py guards only the key-absent case:

try:
    this_board_id = info['target_id_mbed_htm'][0:4]
except KeyError:

so "key present, value None" escapes as TypeError.

Confirmed on this machine with the detector output for the single enumerated volume:

mount_point            = 'D:'
target_id_mbed_htm     = None          <-- key present, value None
target_id_usb_id       = '<hex>'
vendor_id / product_id = 0d28 / 0204
Amplification since 0.35.0

The loop slices for every enumerated MSD volume and only breaks on a matching usb_id,
so the offending volume does not need to belong to the ST-Link at all. Any volume with an
unparsable mbed.htm breaks enumeration of every ST-Link on the machine.

version usb_id gate before the slice exception caught triggered here
0.20.0 / 0.21.0 yes none no
0.22.0 – 0.34.3 yes (the #695 fix) KeyError no
0.35.0 – 0.45.1, main removed KeyError yes
Suggested fix

Either is sufficient; the first is the smaller change.

except (KeyError, TypeError):

or stop storing None in base.py:

if htm_target_id is not None:
    device["target_id_mbed_htm"] = htm_target_id

With the first fix applied locally, board_id returns None and pyocd list shows n/a
in the Target column for the ST-Link instead of losing the whole listing.

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