Closes #17540: Merged fix from 3.3.

Lib/logging/config.py

@@ -672,7 +672,8 @@ class DictConfigurator(BaseConfigurator):
         else:
             fmt = config.get('format', None)
             dfmt = config.get('datefmt', None)
-            result = logging.Formatter(fmt, dfmt)
+            style = config.get('style', '%')
+            result = logging.Formatter(fmt, dfmt, style)
         return result
 
     def configure_filter(self, config):
@@ -694,6 +695,7 @@ class DictConfigurator(BaseConfigurator):
 
     def configure_handler(self, config):
         """Configure a handler from a dictionary."""
+        config_copy = dict(config)  # for restoring in case of error
         formatter = config.pop('formatter', None)
         if formatter:
             try:
@@ -717,7 +719,7 @@ class DictConfigurator(BaseConfigurator):
                 try:
                     th = self.config['handlers'][config['target']]
                     if not isinstance(th, logging.Handler):
-                        config['class'] = cname # restore for deferred configuration
+                        config.update(config_copy)  # restore for deferred cfg
                         raise TypeError('target not configured yet')
                     config['target'] = th
                 except Exception as e:

Lib/test/test_logging.py

@@ -2449,7 +2449,8 @@ class ConfigDictTest(BaseTest):
         "version": 1,
         "formatters": {
             "mySimpleFormatter": {
-                "format": "%(asctime)s (%(name)s) %(levelname)s: %(message)s"
+                "format": "%(asctime)s (%(name)s) %(levelname)s: %(message)s",
+                "style": "$"
             }
         },
         "handlers": {
@@ -2851,6 +2852,8 @@ class ConfigDictTest(BaseTest):
         self.apply_config(self.out_of_order)
         handler = logging.getLogger('mymodule').handlers[0]
         self.assertIsInstance(handler.target, logging.Handler)
+        self.assertIsInstance(handler.formatter._style,
+                              logging.StringTemplateStyle)
 
     def test_baseconfig(self):
         d = {

Misc/NEWS

@@ -297,6 +297,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #17540: Added style to formatter configuration by dict.
+
 - Issue #16692: The ssl module now supports TLS 1.1 and TLS 1.2.  Initial
   patch by Michele Orrù.