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Improving error message with trailing comma in json #113149

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kno10 opened this issue Dec 14, 2023 · 1 comment · Fixed by #113227
Closed

Improving error message with trailing comma in json #113149

kno10 opened this issue Dec 14, 2023 · 1 comment · Fixed by #113227
Labels
3.13 new features, bugs and security fixes stdlib Python modules in the Lib dir type-feature A feature request or enhancement

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@kno10
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kno10 commented Dec 14, 2023 •
edited by bedevere-app bot

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
@kno10 kno10 added the type-bug An unexpected behavior, bug, or error label Dec 14, 2023
@sunmy2019 sunmy2019 added type-feature A feature request or enhancement stdlib Python modules in the Lib dir labels Dec 15, 2023
@sunmy2019 sunmy2019 changed the title Misleading error message with trailing comma in json Improving error message with trailing comma in json Dec 15, 2023
@sunmy2019 sunmy2019 removed the type-bug An unexpected behavior, bug, or error label Dec 15, 2023
@serhiy-storchaka
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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.

@serhiy-storchaka serhiy-storchaka added the 3.13 new features, bugs and security fixes label Dec 16, 2023
@bedevere-app bedevere-app bot mentioned this issue Dec 17, 2023
Merged
serhiy-storchaka pushed a commit that referenced this issue Dec 17, 2023
@carsonRadtke
gh-113149: Improve error message when JSON has trailing comma (GH-113227
cfa25fe 
)
ryan-duve pushed a commit to ryan-duve/cpython that referenced this issue Dec 26, 2023
@carsonRadtke @ryan-duve
pythongh-113149: Improve error message when JSON has trailing comma (p…
1aec257 
…ythonGH-113227)
aisk pushed a commit to aisk/cpython that referenced this issue Feb 11, 2024
@carsonRadtke @aisk
pythongh-113149: Improve error message when JSON has trailing comma (p…
dd63886 
…ythonGH-113227)
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3.13 new features, bugs and security fixes stdlib Python modules in the Lib dir type-feature A feature request or enhancement
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gh-113149: improve error when json has trailing comma carsonRadtke/cpython
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@serhiy-storchaka @kno10 @sunmy2019