pythongh-76913: Add "merge extras" feature to LoggerAdapter

By default, LoggerAdapter objects ignores all `extra=` parameter used in
the individual log methods, which may be confusing for some users.

This commit is aimed at adding an option in the LoggerAdapter class to
allow instances / subclasses to merge both the adapter and individual
log call extra into a single entry

The default behavior is not changed

For example:

```
log = LoggerAdapter(..., extra={"component": "XYZ"})
log.info("return %r" % ret, extra={"duration": elapsed})
```

Author: romuald

Lib/logging/__init__.py

@@ -1880,7 +1880,9 @@ class LoggerAdapter(object):
     information in logging output.
     """
 
-    def __init__(self, logger, extra=None):
+    merge_extras = False
+
+    def __init__(self, logger, extra=None, merge_extras=None):
         """
         Initialize the adapter with a logger and a dict-like object which
         provides contextual information. This constructor signature allows
@@ -1890,9 +1892,21 @@ def __init__(self, logger, extra=None):
         following example:
 
         adapter = LoggerAdapter(someLogger, dict(p1=v1, p2="v2"))
+
+        By default, LoggerAdapter objects will drop the "extra" argument
+        passed on the individual log calls to use its own instead.
+
+        Initializing it with merge_extras=True will instead merge both
+        maps when logging, the indivicual call extra taking precedence
+        over the LoggerAdapter instance extra
+
+        .. versionchanged:: 3.13
+           Added the ``merge_extras`` parameter.
         """
         self.logger = logger
         self.extra = extra
+        if merge_extras is not None:
+            self.merge_extras = merge_extras
 
     def process(self, msg, kwargs):
         """
@@ -1904,7 +1918,10 @@ def process(self, msg, kwargs):
         Normally, you'll only need to override this one method in a
         LoggerAdapter subclass for your specific needs.
         """
-        kwargs["extra"] = self.extra
+        if self.merge_extras and "extra" in kwargs:
+            kwargs["extra"] = {**self.extra, **kwargs["extra"]}
+        else:
+            kwargs["extra"] = self.extra
         return msg, kwargs
 
     #

Lib/test/test_logging.py

@@ -5433,6 +5433,46 @@ def process(self, msg, kwargs):
         self.assertIs(adapter.manager, orig_manager)
         self.assertIs(self.logger.manager, orig_manager)
 
+    def test_extra_in_records(self):
+        self.adapter = logging.LoggerAdapter(logger=self.logger,
+                                             extra={'foo': '1'})
+
+        self.adapter.critical('foo should be here')
+        self.assertEqual(len(self.recording.records), 1)
+        record = self.recording.records[0]
+        self.assertTrue(hasattr(record, 'foo'))
+        self.assertEqual(record.foo, '1')
+
+    def test_extra_not_merged_by_default(self):
+        self.adapter.critical('foo should NOT be here', extra={'foo': 'nope'})
+        self.assertEqual(len(self.recording.records), 1)
+        record = self.recording.records[0]
+        self.assertFalse(hasattr(record, 'foo'))
+
+    def test_extra_merged(self):
+        self.adapter = logging.LoggerAdapter(logger=self.logger,
+                                             extra={'foo': '1'},
+                                             merge_extras=True)
+
+        self.adapter.critical('foo and bar should be here', extra={'bar': '2'})
+        self.assertEqual(len(self.recording.records), 1)
+        record = self.recording.records[0]
+        self.assertTrue(hasattr(record, 'foo'))
+        self.assertTrue(hasattr(record, 'bar'))
+        self.assertEqual(record.foo, '1')
+        self.assertEqual(record.bar, '2')
+
+    def test_extra_merged_log_call_has_precedence(self):
+        self.adapter = logging.LoggerAdapter(logger=self.logger,
+                                             extra={'foo': '1'},
+                                             merge_extras=True)
+
+        self.adapter.critical('foo shall be min', extra={'foo': '2'})
+        self.assertEqual(len(self.recording.records), 1)
+        record = self.recording.records[0]
+        self.assertTrue(hasattr(record, 'foo'))
+        self.assertEqual(record.foo, '2')
+
 
 class LoggerTest(BaseTest, AssertErrorMessage):