Should "py_make_scanner" return "scan_once" function rather than "_scan_once"?

Good catch c-fos!

The scan_once() wrapper was introduced in issue7451, but py_make_scanner() still returns unwrapped _scan_once().

Is it possible to write a test that catches this bug?

The possible test:

import unittest
from json.decoder import JSONDecoder

class Memo_Test(unittest.TestCase):
    def test_for_empty_memo(self):
        json_str = '{"a": 1}'
        decoder = JSONDecoder()
        decoder.decode(json_str)
        self.assertEqual(decoder.memo, {})

suite = unittest.TestSuite()
suite.addTest(Memo_Test("test_for_empty_memo"))
runner = unittest.TextTestRunner()
runner.run(suite)

But it works only when _json import fails

Test independent from _json library:

import unittest
from json.decoder import JSONDecoder
from json.scanner import py_make_scanner

class Memo_Test(unittest.TestCase):
    def test_for_empty_memo(self):
        json_str = '{"a": 1}'
        decoder = JSONDecoder()
        decoder.scan_once = py_make_scanner(decoder)
        result = decoder.decode(json_str)
        self.assertEqual(result, {"a":1})
        self.assertEqual(decoder.memo, {})

suite = unittest.TestSuite()
suite.addTest(Memo_Test("test_for_empty_memo"))
runner = unittest.TextTestRunner()
runner.run(suite)

Sorry for not fixing this issue earlier. The fix is trivial, but it needed tests. PR 7048 adds a simpler test.

New changeset 25fd6cc5b0ad311bb771ae47ae8173417730bb6a by Miss Islington (bot) in branch '3.7':
bpo-30877: Fix clearing a cache in the the JSON decoder. (GH-7048)
https://github.com/python/cpython/commit/25fd6cc5b0ad311bb771ae47ae8173417730bb6a

New changeset 2baee0aa77055755ac50e92e64bbccfea4108621 by Miss Islington (bot) in branch '3.6':
bpo-30877: Fix clearing a cache in the the JSON decoder. (GH-7048)
https://github.com/python/cpython/commit/2baee0aa77055755ac50e92e64bbccfea4108621