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

Linked PRs

• gh-138162: Fix logging.LoggerAdapter with merge_extra=True and without the extra argument #140511
• [3.14] gh-138162: Fix logging.LoggerAdapter with merge_extra=True and without the extra argument (GH-140511) #140784
• [3.13] gh-138162: Fix logging.LoggerAdapter with merge_extra=True and without the extra argument (GH-140511) #140785

[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?

[christian-hnz]

Just ran into this issue, too.

In fact, the default for the extra parameter is not in the docs. But it's in the stub files. There you have

        def __init__(self, logger: _L, extra: Mapping[str, object] | None = None, merge_extra: bool = False) -> None: ...

Hence, many editors will show this.

This should probably be something like

@overload
def __init__(self, logger: _L, extra: None = None, merge_extra: Literal[False] = False) -> None: ...

@overload
def __init__(self, logger: _L, extra: Mapping[str, object], merge_extra: bool = False) -> None: ...

Or better IMO replace the extra=None with an empty dict.

[christian-hnz]

@serhiy-storchaka Thanks for fixing! There is one more edge case that isn't yet covered.

import logging

ad = logging.LoggerAdapter(logger=logging.getLogger(), extra={}, merge_extra=True)

ad.error("no error")  # OK
ad.error("error", extra=None)  # Raises an error as `kwargs["extra"]` is not a mapping.

The first add.error works as extra is handled via **kwargs in logging.LoggerAdapter.log.
Type checkers will not complain in either case.

Replacing "extra" in kwargs by kwargs.get("extra") is not None would solve this.