Added defaults parameter for logging.Formatter

[bharel]

BPO	40884
Nosy	@vsajip, @bharel, @miss-islington, @remilapeyre
PRs	bpo-40884: Added defaults parameter for logging.Formatter #20668

^{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 = <Date 2020-06-29.17:09:05.435>
created_at = <Date 2020-06-05.23:29:51.873>
labels = ['type-feature', 'library', '3.10']
title = 'Added defaults parameter for logging.Formatter'
updated_at = <Date 2020-06-29.17:09:05.434>
user = 'https://github.com/bharel'

bugs.python.org fields:

activity = <Date 2020-06-29.17:09:05.434>
actor = 'vinay.sajip'
assignee = 'none'
closed = True
closed_date = <Date 2020-06-29.17:09:05.435>
closer = 'vinay.sajip'
components = ['Library (Lib)']
creation = <Date 2020-06-05.23:29:51.873>
creator = 'bar.harel'
dependencies = []
files = []
hgrepos = []
issue_num = 40884
keywords = ['patch']
message_count = 3.0
messages = ['370796', '370802', '371816']
nosy_count = 4.0
nosy_names = ['vinay.sajip', 'bar.harel', 'miss-islington', 'remi.lapeyre']
pr_nums = ['20668']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40884'
versions = ['Python 3.10']

[bharel]

TLDR; logging.Formatter('%(ip)s %(message)s', defaults={"ip": None})

Python's logging.Formatter allows the placement of custom fields, e.g.
logging.Formatter("%(ip)s %(message)").

If a handler has a formatter with a custom field, all log records that go through the handler must have the custom field set using extra={}.
Failure to do so will result in exceptions thrown inside the logging library.

Custom fields are common, and are even suggested by the Python logging cookbook, where they are attached to the root logger.

There is, however, no way to specify default values for the custom fields. Quite a few issues arise from it.

For example, if I've set a formatter on the root logger with the custom field "%(ip)s", all logging messages sent by the asyncio library, will cause exceptions to raise.

Adding default values is possible using LoggerAdapter but will causes other issues as well as not solve the aforementioned problem.

Adding default values is possible using Filters, but cause confusion, isn't simple, and permanently modify the record object itself, which can cause issues if more handlers or formatters are attached.

From a quick search, this feature was asked for many times in stackoverflow, and even spawned up a few libraries such as "logaugment" in order to solve it.

I believe the solution offered, by using defaults={} is simple enough to not need discussion over python-ideas, yet common enough to justify the addition to the standard library.

I've provided a reference PR. It does not cause backwards compatibility issues, complies with all formatter styles (%, {}, $), passes all tests and is simple enough to both use and understand.

Not sure if 3.9 is feature-closed for small additions like this.

[remilapeyre]

New features will go in Python 3.10 indeed.

[miss-islington]

New changeset 8f192d1 by Bar Harel in branch 'master':
bpo-40884: Added defaults parameter for logging.Formatter (GH-20668)
8f192d1