Specialisation fail `LOAD_ATTR_INSTANCE_VALUE` specialised when no instance variable

[johng]

Bug report

def test_load_attr_class_default_not_set_on_instance(self):
    class C:
        timeout = 30

        def set_timeout(self, value):
            self.timeout = value  # puts "timeout" in C's shared keys

    c = C()  # never sets timeout, so c.timeout is the class default

    @reset_code
    def get_timeout(n):
        for _ in range(n):
            value = c.timeout
        return value

    self.assertEqual(get_timeout(_testinternalcapi.SPECIALIZATION_THRESHOLD), 30)
    # Must load the class attribute, not specialize as an instance
    # attribute load that misses on every execution.
    self.assert_specialized(get_timeout, "LOAD_ATTR_NONDESCRIPTOR_WITH_VALUES")
    self.assert_no_opcode(get_timeout, "LOAD_ATTR_INSTANCE_VALUE")

    # Setting the attribute on the instance must be seen.
    c.set_timeout(5)
    self.assertEqual(get_timeout(1), 5)

This is due to instance_has_key returning True even if setting an instance field is not actually set. Json dumps shows this issue with many instance level attributes not set by default relying on class level defaults

cpython/Lib/json/encoder.py

Line 156 in 978a8be

if separators is not None:

For json.dumps this can add about a 5% tax due to missed optimisations although it depends on the payload size. threading.Condition is another example from the stdlib.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-158934: determine if instance or class attribute specialisation is done based off the instance slot value #158938