logging: use dummy handler when CLI logging is disabled instead of None

This makes the code cleaner by removing conditionals and making the CLI
and file logging completely analogous.

Doesn't affect performance.

src/_pytest/logging.py

@@ -11,6 +11,7 @@ from typing import Generator
 from typing import List
 from typing import Mapping
 from typing import Optional
+from typing import Union
 
 import pytest
 from _pytest import nodes
@@ -529,11 +530,24 @@ class LoggingPlugin:
         self.log_file_handler.setFormatter(log_file_formatter)
 
         # CLI/live logging.
+        self.log_cli_level = get_log_level_for_setting(
+            config, "log_cli_level", "log_level"
+        )
         if self._log_cli_enabled():
-            self._setup_cli_logging()
+            terminal_reporter = config.pluginmanager.get_plugin("terminalreporter")
+            capture_manager = config.pluginmanager.get_plugin("capturemanager")
+            # if capturemanager plugin is disabled, live logging still works.
+            self.log_cli_handler = _LiveLoggingStreamHandler(
+                terminal_reporter, capture_manager
+            )  # type: Union[_LiveLoggingStreamHandler, _LiveLoggingNullHandler]
         else:
-            self.log_cli_handler = None
-            self.live_logs_context = nullcontext
+            self.log_cli_handler = _LiveLoggingNullHandler()
+        log_cli_formatter = self._create_formatter(
+            get_option_ini(config, "log_cli_format", "log_format"),
+            get_option_ini(config, "log_cli_date_format", "log_date_format"),
+            get_option_ini(config, "log_auto_indent"),
+        )
+        self.log_cli_handler.setFormatter(log_cli_formatter)
 
     def _create_formatter(self, log_format, log_date_format, auto_indent):
         # color option doesn't exist if terminal plugin is disabled
@@ -553,30 +567,6 @@ class LoggingPlugin:
 
         return formatter
 
-    def _setup_cli_logging(self):
-        config = self._config
-        terminal_reporter = config.pluginmanager.get_plugin("terminalreporter")
-        if terminal_reporter is None:
-            # terminal reporter is disabled e.g. by pytest-xdist.
-            return
-
-        capture_manager = config.pluginmanager.get_plugin("capturemanager")
-        # if capturemanager plugin is disabled, live logging still works.
-        log_cli_handler = _LiveLoggingStreamHandler(terminal_reporter, capture_manager)
-
-        log_cli_formatter = self._create_formatter(
-            get_option_ini(config, "log_cli_format", "log_format"),
-            get_option_ini(config, "log_cli_date_format", "log_date_format"),
-            get_option_ini(config, "log_auto_indent"),
-        )
-        log_cli_handler.setFormatter(log_cli_formatter)
-
-        log_cli_level = get_log_level_for_setting(config, "log_cli_level", "log_level")
-        self.log_cli_handler = log_cli_handler
-        self.live_logs_context = lambda: catching_logs(
-            log_cli_handler, level=log_cli_level
-        )
-
     def set_log_path(self, fname):
         """Public method, which can set filename parameter for
         Logging.FileHandler(). Also creates parent directory if
@@ -608,19 +598,25 @@ class LoggingPlugin:
             old_stream.close()
 
     def _log_cli_enabled(self):
-        """Return True if log_cli should be considered enabled, either explicitly
-        or because --log-cli-level was given in the command-line.
-        """
-        return self._config.getoption(
+        """Return whether live logging is enabled."""
+        enabled = self._config.getoption(
             "--log-cli-level"
         ) is not None or self._config.getini("log_cli")
+        if not enabled:
+            return False
+
+        terminal_reporter = self._config.pluginmanager.get_plugin("terminalreporter")
+        if terminal_reporter is None:
+            # terminal reporter is disabled e.g. by pytest-xdist.
+            return False
+
+        return True
 
     @pytest.hookimpl(hookwrapper=True, tryfirst=True)
     def pytest_collection(self) -> Generator[None, None, None]:
-        if self.log_cli_handler is not None:
         self.log_cli_handler.set_when("collection")
 
-        with self.live_logs_context():
+        with catching_logs(self.log_cli_handler, level=self.log_cli_level):
             with catching_logs(self.log_file_handler, level=self.log_file_level):
                 yield
 
@@ -641,7 +637,6 @@ class LoggingPlugin:
 
     @pytest.hookimpl(hookwrapper=True)
     def pytest_runtest_setup(self, item):
-        if self.log_cli_handler is not None:
         self.log_cli_handler.set_when("setup")
 
         empty = {}  # type: Dict[str, LogCaptureHandler]
@@ -650,14 +645,12 @@ class LoggingPlugin:
 
     @pytest.hookimpl(hookwrapper=True)
     def pytest_runtest_call(self, item):
-        if self.log_cli_handler is not None:
         self.log_cli_handler.set_when("call")
 
         yield from self._runtest_for(item, "call")
 
     @pytest.hookimpl(hookwrapper=True)
     def pytest_runtest_teardown(self, item):
-        if self.log_cli_handler is not None:
         self.log_cli_handler.set_when("teardown")
 
         yield from self._runtest_for(item, "teardown")
@@ -666,40 +659,34 @@ class LoggingPlugin:
 
     @pytest.hookimpl
     def pytest_runtest_logstart(self):
-        if self.log_cli_handler is not None:
         self.log_cli_handler.reset()
         self.log_cli_handler.set_when("start")
 
     @pytest.hookimpl
     def pytest_runtest_logfinish(self):
-        if self.log_cli_handler is not None:
         self.log_cli_handler.set_when("finish")
 
     @pytest.hookimpl
     def pytest_runtest_logreport(self):
-        if self.log_cli_handler is not None:
         self.log_cli_handler.set_when("logreport")
 
     @pytest.hookimpl(hookwrapper=True, tryfirst=True)
     def pytest_sessionfinish(self):
-        if self.log_cli_handler is not None:
         self.log_cli_handler.set_when("sessionfinish")
 
-        with self.live_logs_context():
-            try:
+        with catching_logs(self.log_cli_handler, level=self.log_cli_level):
             with catching_logs(self.log_file_handler, level=self.log_file_level):
                 yield
-            finally:
+
         # Close the FileHandler explicitly.
         # (logging.shutdown might have lost the weakref?!)
         self.log_file_handler.close()
 
     @pytest.hookimpl(hookwrapper=True, tryfirst=True)
     def pytest_sessionstart(self):
-        if self.log_cli_handler is not None:
         self.log_cli_handler.set_when("sessionstart")
 
-        with self.live_logs_context():
+        with catching_logs(self.log_cli_handler, level=self.log_cli_level):
             with catching_logs(self.log_file_handler, level=self.log_file_level):
                 yield
 
@@ -715,7 +702,7 @@ class LoggingPlugin:
             # setting verbose flag is needed to avoid messy test progress output
             self._config.option.verbose = 1
 
-        with self.live_logs_context():
+        with catching_logs(self.log_cli_handler, level=self.log_cli_level):
             with catching_logs(self.log_file_handler, level=self.log_file_level):
                 yield  # run all the tests
 
@@ -768,4 +755,14 @@ class _LiveLoggingStreamHandler(logging.StreamHandler):
             if not self._section_name_shown and self._when:
                 self.stream.section("live log " + self._when, sep="-", bold=True)
                 self._section_name_shown = True
-            logging.StreamHandler.emit(self, record)
+            super().emit(record)
+
+
+class _LiveLoggingNullHandler(logging.NullHandler):
+    """A handler used when live logging is disabled."""
+
+    def reset(self):
+        pass
+
+    def set_when(self, when):
+        pass