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Draft: DBC import reports wrong line numbers after a VAL_ line ​

Status: draft, not posted. Whether it goes to ebroecker/canmatrix is the maintainer's decision. Everything from the issue title down to Notes for CanViewer is the text to post; the notes are not.

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.

Issue title: DBC import: error line numbers are wrong after a VAL_ or VAL_TABLE_ line

Summary ​

When the DBC reader cannot read a line, it prints error with line no: N followed by the line. After the first VAL_ (or VAL_TABLE_) line in a file, N is wrong: the loops that read the value pairs reuse i, the reader's line counter, as their index, so the counter restarts from the number of pairs. Every line number printed or logged after that point is off, and by a different amount after each value table.

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;
  • the development branch at 8b54a41 (2026-10-28) prints the same: src/canmatrix/formats/dbc.py has not changed since 1.3.3.

Reproduction ​

python
import canmatrix.formats

DBC = b'''VERSION ""

BU_: ECU

BO_ 100 First: 8 ECU
 SG_ Mode : 0|8@1+ (1,0) [0|255] "" Vector__XXX
 SG_ Bad1 : x|8@1+ (1,0) [0|255] "" Vector__XXX

VAL_ 100 Mode 0 "Off" 1 "On" 2 "Auto" ;

BO_ 200 Second: 8 ECU
 SG_ Bad2 : y|8@1+ (1,0) [0|255] "" Vector__XXX
'''

print("canmatrix", canmatrix.__version__)
canmatrix.formats.loads_flat(DBC, "dbc")

Bad1 (line 7) and Bad2 (line 12) are deliberately unreadable: their start bits are not numbers.

Expected ​

text
canmatrix 1.3.3
error with line no: 7
b' SG_ Bad1 : x|8@1+ (1,0) [0|255] "" Vector__XXX\n'
error with line no: 12
b' SG_ Bad2 : y|8@1+ (1,0) [0|255] "" Vector__XXX\n'

Actual ​

text
canmatrix 1.3.3
error with line no: 7
b' SG_ Bad1 : x|8@1+ (1,0) [0|255] "" Vector__XXX\n'
error with line no: 5
b' SG_ Bad2 : y|8@1+ (1,0) [0|255] "" Vector__XXX\n'

Line 12 is reported as line 5: the VAL_ line on line 9 has three pairs, so its loop leaves i at 2, and the counter goes on from there (3, 4, 5).

Cause ​

load() in src/canmatrix/formats/dbc.py counts lines in i (line 490, i = 0, and line 513, i = i + 1). Three loops inside the same for line in f loop use i as their own index:

  • line 789, VAL_ for a signal: for i in range(math.floor(len(temp_list) / 2)):
  • line 799, VAL_ for an environment variable: the same loop;
  • line 814, VAL_TABLE_: for i in range(math.floor(len(temp_list) / 2)):

After any of them, i holds the last pair index, and both the print("error with line no: %d" % i) at line 965 and the logger.error("Error decoding line: %d (%s)" % (i, line)) calls use it.

Suggested fix ​

Give the three loops their own index:

diff
--- a/src/canmatrix/formats/dbc.py
+++ b/src/canmatrix/formats/dbc.py
@@ -788,7 +788,7 @@
                             sg = frame.signal_by_name(signal_name)
-                            for i in range(math.floor(len(temp_list) / 2)):
-                                val = temp_list[i * 2 + 1]
+                            for pair in range(math.floor(len(temp_list) / 2)):
+                                val = temp_list[pair * 2 + 1]
                                 val = val.replace('\\"', '"')
                                 if sg:
-                                    sg.add_values(temp_list[i * 2], val)
+                                    sg.add_values(temp_list[pair * 2], val)
                         except:
@@ -798,6 +798,6 @@
                             values = db.env_vars[signal_name]['values']
-                            for i in range(math.floor(len(temp_list) / 2)):
-                                val = temp_list[i * 2 + 1]
+                            for pair in range(math.floor(len(temp_list) / 2)):
+                                val = temp_list[pair * 2 + 1]
                                 val = val.replace('\\"', '"')
-                                values[temp_list[i * 2].strip()]=val
+                                values[temp_list[pair * 2].strip()]=val
                         except:
@@ -813,5 +813,5 @@
                     try:
-                        for i in range(math.floor(len(temp_list) / 2)):
-                            val = temp_list[i * 2 + 1]
-                            value_hash[temp_list[i * 2].strip()] = val.strip()
+                        for pair in range(math.floor(len(temp_list) / 2)):
+                            val = temp_list[pair * 2 + 1]
+                            value_hash[temp_list[pair * 2].strip()] = val.strip()
                     except:

With this patch applied to 1.3.3, the reproduction prints the Expected block, and tests/test_dbc.py still passes with lxml installed (68 passed, as for unpatched 1.3.3 installed with pip install canmatrix[kcd]==1.3.3 pytest). Counting with for i, line in enumerate(f, start=1): instead of i = i + 1 would also keep the counter right, but would leave the shadowing in place.

Notes for CanViewer (not for posting) ​

  • CanViewer reads .dbc with its own parser, so the app is not affected. The canmatrix DBC reader runs only in the mapper suite (app/src/lib/canmatrix/mapper.test.ts) over the 58 OEM DBC files.
  • canviewer_adapter.py already works around the bug: _locate() looks up the line canmatrix quotes and replaces its number with the real one. upstream-reports.test.ts pins that (lines 7 and 12 for this input). If upstream fixes the bug, the workaround stays harmless; it can then be reduced to the SYM case its docstring also names.
  • Before posting: search the open and closed issues for "line no" and "VAL_" for duplicates (not done yet). Check that the development branch still has the three loops.