Stacked LoggerAdapters cause TypeError when inner adapter has extra=None

[danoan]

Bug report

Bug description:

When stacking LoggerAdapter instances in Python's logging module, an exception occurs if the inner adapter is initialized with extra=None. Specifically, the following code:

import logging

logging.basicConfig(level=logging.DEBUG, format="%(user)s %(message)s")

base_logger = logging.getLogger("base")

adapter_a = logging.LoggerAdapter(base_logger, extra={"user": "alice"}, merge_extra=True)
adapter_b = logging.LoggerAdapter(adapter_a)

# This raises: TypeError: 'NoneType' object is not a mapping
adapter_b.debug("Something")

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[Yzi-Li]

The default value of extra is None. Using extra={} will make it work.

adapter_b = logging.LoggerAdapter(adapter_a, extra={})

Maybe it's better to have extra automatically replaced with {} when merge_extra=True?

[serhiy-storchaka]

extra was made optional with default value None in 3.10 (#84933). And this change was not documented!

merge_extra was added in 3.13 (#76913). Since the former change was not documented and tested, no new tests for extra=None and merge_extra=True was added.

I wonder, why do you call LoggerAdapter() with a single argument if this is not documented?