[Python-checkins] [doc] Add a section on logging handler configuration order. (GH-101380)

[vsajip]

https://github.com/python/cpython/commit/b5c4d6064cc2ae7042d2dc7b533d74f6c4176a38
commit: b5c4d6064cc2ae7042d2dc7b533d74f6c4176a38
branch: main
author: Vinay Sajip <vinay_sajip at yahoo.co.uk>
committer: vsajip <vinay_sajip at yahoo.co.uk>
date: 2023-01-27T19:01:30Z
summary:

[doc] Add a section on logging handler configuration order. (GH-101380)

files:
M Doc/library/logging.config.rst

diff --git a/Doc/library/logging.config.rst b/Doc/library/logging.config.rst
index b4d0da1421dc..2daf2422ebd5 100644
--- a/Doc/library/logging.config.rst
+++ b/Doc/library/logging.config.rst
@@ -564,6 +564,29 @@ attribute ``baz`` set to ``'bozz'``.
    configuration machinery, but set as attribute values as-is.
 
 
+.. _handler-config-dict-order:
+
+Handler configuration order
+"""""""""""""""""""""""""""
+
+Handlers are configured in alphabetical order of their keys, and a configured
+handler replaces the configuration dictionary in (a working copy of) the
+``handlers`` dictionary in the schema. If you use a construct such as
+``cfg://handlers.foo``, then initially ``handlers['foo']`` points to the
+configuration dictionary for the handler named ``foo``, and later (once that
+handler has been configured) it points to the configured handler instance.
+Thus, ``cfg://handlers.foo`` could resolve to either a dictionary or a handler
+instance. In general, it is wise to name handlers in a way such that dependent
+handlers are configured _after_ any handlers they depend on; that allows
+something like ``cfg://handlers.foo`` to be used in configuring a handler that
+depends on handler ``foo``. If that dependent handler were named ``bar``,
+problems would result, because the configuration of ``bar`` would be attempted
+before that of ``foo``, and ``foo`` would not yet have been configured.
+However, if the dependent handler were named ``foobar``, it would be configured
+after ``foo``, with the result that ``cfg://handlers.foo`` would resolve to
+configured handler ``foo``, and not its configuration dictionary.
+
+
 .. _logging-config-dict-externalobj:
 
 Access to external objects