Closes #16110: fileConfig now accepts a pre-initialised ConfigParser instance.

Doc/library/logging.config.rst

@@ -76,11 +76,23 @@ in :mod:`logging` itself) and defining handlers which are declared either in
 
 .. function:: fileConfig(fname, defaults=None, disable_existing_loggers=True)
 
-   Reads the logging configuration from a :mod:`configparser`\-format file
-   named *fname*. This function can be called several times from an
-   application, allowing an end user to select from various pre-canned
-   configurations (if the developer provides a mechanism to present the choices
-   and load the chosen configuration).
+   Reads the logging configuration from a :mod:`configparser`\-format file.
+   This function can be called several times from an application, allowing an
+   end user to select from various pre-canned configurations (if the developer
+   provides a mechanism to present the choices and load the chosen
+   configuration).
+
+   :param fname: A filename, or a file-like object, or an instance derived
+                 from :class:`~configparser.RawConfigParser`. If a
+                 ``RawConfigParser``-derived instance is passed, it is used as
+                 is. Otherwise, a :class:`~configparser.Configparser` is
+                 instantiated, and the configuration read by it from the
+                 object passed in ``fname``. If that has a :meth:`readline`
+                 method, it is assumed to be a file-like object and read using
+                 :meth:`~configparser.ConfigParser.read_file`; otherwise,
+                 it is assumed to be a filename and passed to
+                 :meth:`~configparser.ConfigParser.read`.
+
 
    :param defaults: Defaults to be passed to the ConfigParser can be specified
                     in this argument.
@@ -94,6 +106,15 @@ in :mod:`logging` itself) and defining handlers which are declared either in
                                     their ancestors are explicitly named in the
                                     logging configuration.
 
+   .. versionchanged:: 3.4
+      An instance of a subclass of :class:`~configparser.RawConfigParser` is
+      now accepted as a value for ``fname``. This facilitates:
+
+      * Use of a configuration file where logging configuration is just part
+        of the overall application configuration.
+      * Use of a configuration read from a file, and then modified by the using
+        application (e.g. based on command-line parameters or other aspects
+        of the runtime environment) before being passed to ``fileConfig``.
 
 .. function:: listen(port=DEFAULT_LOGGING_CONFIG_PORT, verify=None)
 

Lib/logging/config.py

@@ -61,11 +61,14 @@ def fileConfig(fname, defaults=None, disable_existing_loggers=True):
     """
     import configparser
 
-    cp = configparser.ConfigParser(defaults)
-    if hasattr(fname, 'readline'):
-        cp.read_file(fname)
+    if isinstance(fname, configparser.RawConfigParser):
+        cp = fname
     else:
-        cp.read(fname)
+        cp = configparser.ConfigParser(defaults)
+        if hasattr(fname, 'readline'):
+            cp.read_file(fname)
+        else:
+            cp.read(fname)
 
     formatters = _create_formatters(cp)
 

Lib/test/test_logging.py

@@ -26,6 +26,7 @@ import logging.handlers
 import logging.config
 
 import codecs
+import configparser
 import datetime
 import pickle
 import io
@@ -1279,6 +1280,24 @@ class ConfigFileTest(BaseTest):
             # Original logger output is empty.
             self.assert_log_lines([])
 
+    def test_config0_using_cp_ok(self):
+        # A simple config file which overrides the default settings.
+        with captured_stdout() as output:
+            file = io.StringIO(textwrap.dedent(self.config0))
+            cp = configparser.ConfigParser()
+            cp.read_file(file)
+            logging.config.fileConfig(cp)
+            logger = logging.getLogger()
+            # Won't output anything
+            logger.info(self.next_message())
+            # Outputs a message
+            logger.error(self.next_message())
+            self.assert_log_lines([
+                ('ERROR', '2'),
+            ], stream=output)
+            # Original logger output is empty.
+            self.assert_log_lines([])
+
     def test_config1_ok(self, config=config1):
         # A config file defining a sub-parser as well.
         with captured_stdout() as output: