Add --log-file-mode option to the logging plugin, enabling appending to log-files

AUTHORS

@@ -56,6 +56,7 @@ Babak Keyvani
 Barney Gale
 Ben Brown
 Ben Gartner
+Ben Leith
 Ben Webb
 Benjamin Peterson
 Benjamin Schubert

changelog/11978.improvement.rst

@@ -0,0 +1,3 @@
+Add ``--log-file-mode`` option to the logging plugin, enabling appending to log-files. This option accepts either ``"w"`` or ``"a"`` and defaults to ``"w"``.
+
+Previously, the mode was hard-coded to be ``"w"`` which truncates the file before logging.

doc/en/how-to/logging.rst

@@ -206,8 +206,9 @@ option names are:
 * ``log_cli_date_format``
 
 If you need to record the whole test suite logging calls to a file, you can pass
-``--log-file=/path/to/log/file``. This log file is opened in write mode which
+``--log-file=/path/to/log/file``. This log file is opened in write mode by default which
 means that it will be overwritten at each run tests session.
+If you'd like the file opened in append mode instead, then you can pass ``--log-file-mode=a``.
 Note that relative paths for the log-file location, whether passed on the CLI or declared in a
 config file, are always resolved relative to the current working directory.
 
@@ -223,12 +224,13 @@ All of the log file options can also be set in the configuration INI file. The
 option names are:
 
 * ``log_file``
+* ``log_file_mode``
 * ``log_file_level``
 * ``log_file_format``
 * ``log_file_date_format``
 
 You can call ``set_log_path()`` to customize the log_file path dynamically. This functionality
-is considered **experimental**.
+is considered **experimental**. Note that ``set_log_path()`` respects the ``log_file_mode`` option.
 
 .. _log_colors:
 

src/_pytest/logging.py

@@ -298,6 +298,13 @@
         default=None,
         help="Path to a file when logging will be written to",
     )
+    add_option_ini(
+        "--log-file-mode",
+        dest="log_file_mode",
+        default="w",
+        choices=["w", "a"],
+        help="Log file open mode",
+    )
     add_option_ini(
         "--log-file-level",
         dest="log_file_level",
@@ -669,7 +676,10 @@
             if not os.path.isdir(directory):
                 os.makedirs(directory)
 
-        self.log_file_handler = _FileHandler(log_file, mode="w", encoding="UTF-8")
+        self.log_file_mode = get_option_ini(config, "log_file_mode") or "w"
+        self.log_file_handler = _FileHandler(
+            log_file, mode=self.log_file_mode, encoding="UTF-8"
+        )
         log_file_format = get_option_ini(config, "log_file_format", "log_format")
         log_file_date_format = get_option_ini(
             config, "log_file_date_format", "log_date_format"
@@ -746,7 +756,7 @@
             fpath.parent.mkdir(exist_ok=True, parents=True)
 
         # https://github.com/python/mypy/issues/11193
-        stream: io.TextIOWrapper = fpath.open(mode="w", encoding="UTF-8")  # type: ignore[assignment]
+        stream: io.TextIOWrapper = fpath.open(mode=self.log_file_mode, encoding="UTF-8")  # type: ignore[assignment]
         old_stream = self.log_file_handler.setStream(stream)
         if old_stream:
             old_stream.close()

testing/logging/test_reporting.py

@@ -661,6 +661,73 @@ def test_log_file(request):
         assert "This log message won't be shown" not in contents
 
 
+def test_log_file_mode_cli(pytester: Pytester) -> None:
+    # Default log file level
+    pytester.makepyfile(
+        """
+        import pytest
+        import logging
+        def test_log_file(request):
+            plugin = request.config.pluginmanager.getplugin('logging-plugin')
+            assert plugin.log_file_handler.level == logging.WARNING
+            logging.getLogger('catchlog').info("This log message won't be shown")
+            logging.getLogger('catchlog').warning("This log message will be shown")
+            print('PASSED')
+    """
+    )
+
+    log_file = str(pytester.path.joinpath("pytest.log"))
+
+    with open(log_file, mode="w", encoding="utf-8") as wfh:
+        wfh.write("A custom header\n")
+
+    result = pytester.runpytest(
+        "-s",
+        f"--log-file={log_file}",
+        "--log-file-mode=a",
+        "--log-file-level=WARNING",
+    )
+
+    # fnmatch_lines does an assertion internally
+    result.stdout.fnmatch_lines(["test_log_file_mode_cli.py PASSED"])
+
+    # make sure that we get a '0' exit code for the testsuite
+    assert result.ret == 0
+    assert os.path.isfile(log_file)
+    with open(log_file, encoding="utf-8") as rfh:
+        contents = rfh.read()
+        assert "A custom header" in contents
+        assert "This log message will be shown" in contents
+        assert "This log message won't be shown" not in contents
+
+
+def test_log_file_mode_cli_invalid(pytester: Pytester) -> None:
+    # Default log file level
+    pytester.makepyfile(
+        """
+        import pytest
+        import logging
+        def test_log_file(request):
+            plugin = request.config.pluginmanager.getplugin('logging-plugin')
+            assert plugin.log_file_handler.level == logging.WARNING
+            logging.getLogger('catchlog').info("This log message won't be shown")
+            logging.getLogger('catchlog').warning("This log message will be shown")
+    """
+    )
+
+    log_file = str(pytester.path.joinpath("pytest.log"))
+
+    result = pytester.runpytest(
+        "-s",
+        f"--log-file={log_file}",
+        "--log-file-mode=b",
+        "--log-file-level=WARNING",
+    )
+
+    # make sure that we get a '4' exit code for the testsuite
+    assert result.ret == ExitCode.USAGE_ERROR
+
+
 def test_log_file_cli_level(pytester: Pytester) -> None:
     # Default log file level
     pytester.makepyfile(
@@ -741,6 +808,47 @@ def test_log_file(request):
         assert "This log message won't be shown" not in contents
 
 
+def test_log_file_mode_ini(pytester: Pytester) -> None:
+    log_file = str(pytester.path.joinpath("pytest.log"))
+
+    pytester.makeini(
+        f"""
+        [pytest]
+        log_file={log_file}
+        log_file_mode=a
+        log_file_level=WARNING
+        """
+    )
+    pytester.makepyfile(
+        """
+        import pytest
+        import logging
+        def test_log_file(request):
+            plugin = request.config.pluginmanager.getplugin('logging-plugin')
+            assert plugin.log_file_handler.level == logging.WARNING
+            logging.getLogger('catchlog').info("This log message won't be shown")
+            logging.getLogger('catchlog').warning("This log message will be shown")
+            print('PASSED')
+    """
+    )
+
+    with open(log_file, mode="w", encoding="utf-8") as wfh:
+        wfh.write("A custom header\n")
+
+    result = pytester.runpytest("-s")
+
+    # fnmatch_lines does an assertion internally
+    result.stdout.fnmatch_lines(["test_log_file_mode_ini.py PASSED"])
+
+    assert result.ret == ExitCode.OK
+    assert os.path.isfile(log_file)
+    with open(log_file, encoding="utf-8") as rfh:
+        contents = rfh.read()
+        assert "A custom header" in contents
+        assert "This log message will be shown" in contents
+        assert "This log message won't be shown" not in contents
+
+
 def test_log_file_ini_level(pytester: Pytester) -> None:
     log_file = str(pytester.path.joinpath("pytest.log"))
 
@@ -1060,6 +1168,66 @@ def test_second():
         assert "message from test 2" in content
 
 
+def test_log_set_path_with_log_file_mode(pytester: Pytester) -> None:
+    report_dir_base = str(pytester.path)
+
+    pytester.makeini(
+        """
+        [pytest]
+        log_file_level = DEBUG
+        log_cli=true
+        log_file_mode=a
+        """
+    )
+    pytester.makeconftest(
+        f"""
+            import os
+            import pytest
+            @pytest.hookimpl(wrapper=True, tryfirst=True)
+            def pytest_runtest_setup(item):
+                config = item.config
+                logging_plugin = config.pluginmanager.get_plugin("logging-plugin")
+                report_file = os.path.join({report_dir_base!r}, item._request.node.name)
+                logging_plugin.set_log_path(report_file)
+                return (yield)
+        """
+    )
+    pytester.makepyfile(
+        """
+            import logging
+            logger = logging.getLogger("testcase-logger")
+            def test_first():
+                logger.info("message from test 1")
+                assert True
+
+            def test_second():
+                logger.debug("message from test 2")
+                assert True
+        """
+    )
+
+    test_first_log_file = os.path.join(report_dir_base, "test_first")
+    test_second_log_file = os.path.join(report_dir_base, "test_second")
+    with open(test_first_log_file, mode="w", encoding="utf-8") as wfh:
+        wfh.write("A custom header for test 1\n")
+
+    with open(test_second_log_file, mode="w", encoding="utf-8") as wfh:
+        wfh.write("A custom header for test 2\n")
+
+    result = pytester.runpytest()
+    assert result.ret == ExitCode.OK
+
+    with open(test_first_log_file, encoding="utf-8") as rfh:
+        content = rfh.read()
+        assert "A custom header for test 1" in content
+        assert "message from test 1" in content
+
+    with open(test_second_log_file, encoding="utf-8") as rfh:
+        content = rfh.read()
+        assert "A custom header for test 2" in content
+        assert "message from test 2" in content
+
+
 def test_colored_captured_log(pytester: Pytester) -> None:
     """Test that the level names of captured log messages of a failing test
     are colored."""