Improving error message with trailing comma in json

[kno10]

Bug report

Bug description:

import json
json.loads('{"a":1,}')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.11/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/json/decoder.py", line 353, in raw_decode
    obj, end = self.scan_once(s, idx)
               ^^^^^^^^^^^^^^^^^^^^^^
json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 1 column 8 (char 7)

The error is the trailing comma.

Initial pull proposal: #113047 but apparently this needs more attention than I can contribute.
The actual fix needs to be further down, possibly in line 206

cpython/Lib/json/decoder.py

Lines 199 to 208 in 25061f5

	if nextchar == '}':
	break
	elif nextchar != ',':
	raise JSONDecodeError("Expecting ',' delimiter", s, end - 1)
	end = _w(s, end).end()
	nextchar = s[end:end + 1]
	end += 1
	if nextchar != '"':
	raise JSONDecodeError(
	"Expecting property name enclosed in double quotes", s, end - 1)

we already know that we have seen a comma, and can insert a more helpful error message in line 206.

if nextchar == '}':
    raise JSONDecodeError("No trailing commas allowed in JSON objects.", s, end - 1)

at this location, the previous character must have been a comma (line 201). Whitespace has been removed, so this will also catch ,\n}

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Linked PRs

• gh-113149: improve error when json has trailing comma #113227

[serhiy-storchaka]

You are welcome. Looks like it will work. Do you want to create a PR? A similar special case for a trailing comma can be added for parsing JSON array.