Inconsistent errors for JSON-encoding NaNs with allow_nan=False

[earwig]

BPO	41852
Nosy	@serhiy-storchaka, @earwig

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-09-24.16:44:20.041>
labels = ['type-bug', '3.8', '3.9', '3.10', 'extension-modules', 'library']
title = 'Inconsistent errors for JSON-encoding NaNs with allow_nan=False'
updated_at = <Date 2020-09-24.17:05:07.621>
user = 'https://github.com/earwig'

bugs.python.org fields:

activity = <Date 2020-09-24.17:05:07.621>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules', 'Library (Lib)']
creation = <Date 2020-09-24.16:44:20.041>
creator = 'earwig'
dependencies = []
files = []
hgrepos = []
issue_num = 41852
keywords = []
message_count = 1.0
messages = ['377450']
nosy_count = 2.0
nosy_names = ['serhiy.storchaka', 'earwig']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41852'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

• PR: gh-86018 Inconsistent errors for JSON-encoding NaNs with allow_nan=False #99172

[earwig]

json.dump vs. json.dumps have inconsistent error messages when encoding NaN with allow_nan=False:

>>> json.dumps(float('nan'), allow_nan=False)
ValueError: Out of range float values are not JSON compliant
>>> json.dump(float('nan'), sys.stdout, allow_nan=False)
ValueError: Out of range float values are not JSON compliant: nan

json.dump's error includes the value (which is useful); json.dumps's does not.

json.dumps uses this C implementation:

cpython/Modules/_json.c

Line 1340 in d67de0a

static PyObject *

json.dump uses this Python implementation:

cpython/Lib/json/encoder.py

Line 223 in d67de0a

def floatstr(o, allow_nan=self.allow_nan,

(Separately, is it expected that only one uses the C implementation? This seems it could have unexpected performance implications - without testing, I would've expected json.dump to take the "faster" path since it doesn't need to hold the entire encoded string in memory.)