sysfce2 · pull · Nov 6, 2025 · Nov 6, 2025
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
@@ -164,6 +164,7 @@ Mathieu Kniewallner
 Matt Bachmann
 Matthew Boehm
 Matthew Desmarais
+Matthias Schoettle
 Matus Valo
 Max Linke
 Mayank Singhal

diff --git a/coverage/core.py b/coverage/core.py
@@ -72,13 +72,13 @@ def _debug(msg: str) -> None:
         # Check the conditions that preclude us from using sys.monitoring.
         reason_no_sysmon = ""
         if not env.PYBEHAVIOR.pep669:
-            reason_no_sysmon = "isn't available in this version"
+            reason_no_sysmon = "sys.monitoring isn't available in this version"
         elif config.branch and not env.PYBEHAVIOR.branch_right_left:
-            reason_no_sysmon = "can't measure branches in this version"
+            reason_no_sysmon = "sys.monitoring can't measure branches in this version"
         elif dynamic_contexts:
-            reason_no_sysmon = "doesn't yet support dynamic contexts"
+            reason_no_sysmon = "it doesn't yet support dynamic contexts"
         elif any((bad := c) in config.concurrency for c in ["greenlet", "eventlet", "gevent"]):
-            reason_no_sysmon = f"doesn't support concurrency={bad}"
+            reason_no_sysmon = f"it doesn't support concurrency={bad}"
 
         core_name: str | None = None
         if config.timid:
@@ -92,7 +92,9 @@ def _debug(msg: str) -> None:
         if core_name == "sysmon" and reason_no_sysmon:
             _debug(f"core.py: raising ConfigError because sysmon not usable: {reason_no_sysmon}")
             raise ConfigError(
-                f"Can't use core=sysmon: sys.monitoring {reason_no_sysmon}", skip_tests=True
+                f"Can't use core=sysmon: {reason_no_sysmon}",
+                skip_tests=True,
+                slug="no-sysmon",
             )
 
         if core_name is None:

diff --git a/doc/config.rst b/doc/config.rst
@@ -333,15 +333,21 @@ Before version 4.2, this option only accepted a single string.
 [run] core
 ..........
 
-(string) Specify which trace function implementation to use. Valid values are:
-"pytrace" for the pure Python implementation, "ctrace" for the C implementation
-(default until Python 3.13),
-or "sysmon" (Python 3.12+ only) for the
-:mod:`sys.monitoring <python:sys.monitoring>`
-implementation (default with Python 3.14+).
-
-This was previously only available as the COVERAGE_CORE environment variable.
-Note that the "sysmon" core does not yet support plugins or dynamic contexts.
+(string, default depends on the Python version) Specify which measurement core
+to use. Valid values are:
+
+- ``ctrace``: the C implementation of a sys.settrace function.  This was the
+  default until Python 3.13.
+
+- ``sysmon``: the :mod:`sys.monitoring <python:sys.monitoring>` implementation.
+  Only available in Python 3.12+, and the default in Python 3.14+.  The sysmon
+  core does not yet support plugins, dynamic contexts, or some concurrency
+  libraries. In Python 3.12 and 3.13, it does not support branch coverage.
+
+- ``pytrace``: the pure Python implementation of a sys.settrace function.
+
+This setting was previously only available as the COVERAGE_CORE environment
+variable.
 
 .. versionadded:: 7.9
 

diff --git a/doc/messages.rst b/doc/messages.rst
@@ -52,6 +52,22 @@ Can't combine (branch or statement) coverage data with (statement or branch) dat
   incompatible.  You'll need to ensure that all of your data files are
   collected with the same settings.
 
+.. _error_no_sysmon:
+
+Can't use core=sysmon: sys.monitoring isn't available in this version
+  You requested the sys.monitoring measurement core, but are running on
+  Python 3.11 or lower where it isn't available.
+
+Can't use core=sysmon: sys.monitoring can't measure branches in this version
+  You requested the sys.monitoring measurement core and also branch coverage.
+  This isn't supported until the Python 3.14.
+
+Can't use core=sysmon: it doesn't yet support dynamic contexts
+  You requested the sys.monitoring measurement core and also dynamic contexts.
+  This isn't supported by coverage.py yet.
+
+Can't use core=sysmon: it doesn't support concurrency=ZZZ
+
 
 .. _cmd_warnings:
 .. _warnings:
@@ -138,23 +154,6 @@ Couldn't import C tracer (no-ctracer)
   imported.  The reason is included in the warning message.  The Python tracer
   will be used instead.
 
-.. _warning_no_sysmon:
-
-sys.monitoring isn't available in this version, using default core (no-sysmon)
-  You requested to use the sys.monitoring measurement core, but are running on
-  Python 3.11 or lower where it isn't available.  A default core will be used
-  instead.
-
-sys.monitoring can't measure branches in this version, using default core (no-sysmon)
-  You requested the sys.monitoring measurement core and also branch coverage.
-  This isn't supported until the later alphas of Python 3.14.  A default core
-  will be used instead.
-
-sys.monitoring doesn't yet support dynamic contexts, using default core (no-sysmon)
-  You requested the sys.monitoring measurement core and also dynamic contexts.
-  This isn't supported by coverage.py yet.  A default core will be used
-  instead.
-
 
 Disabling warnings
 ------------------

diff --git a/metacov.ini b/metacov.ini
@@ -9,7 +9,6 @@
 [run]
 branch = true
 data_file = ${COVERAGE_METAFILE-.metacov}
-disable_warnings = no-sysmon
 parallel = true
 relative_files = true
 source =

diff --git a/pyproject.toml b/pyproject.toml
@@ -131,8 +131,6 @@ filterwarnings = [
 
     ## Pytest warns if it can't collect things that seem to be tests. This should be an error.
     "error::pytest.PytestCollectionWarning",
-
-    "ignore:.*no-sysmon"
 ]
 
 # xfail tests that pass should fail the test suite

diff --git a/tests/conftest.py b/tests/conftest.py
@@ -57,8 +57,6 @@ def set_warnings() -> None:
     # https://github.com/python/cpython/issues/105539
     warnings.filterwarnings("ignore", r"unclosed database", category=ResourceWarning)
 
-    warnings.filterwarnings("ignore", r".*no-sysmon")
-
     # We have a test that has a return in a finally: test_bug_1891.
     warnings.filterwarnings("ignore", "'return' in a 'finally' block", category=SyntaxWarning)
 

diff --git a/tests/test_concurrency.py b/tests/test_concurrency.py
@@ -183,21 +183,17 @@ def cant_trace_msg(concurrency: str, the_module: ModuleType | None) -> str | Non
         concurrency = ",".join(parts)
 
     if testenv.SYS_MON and concurrency:
-        expected_out = (
-            f"Can't use core=sysmon: sys.monitoring doesn't support concurrency={concurrency}\n"
-        )
+        expected_out = f"Can't use core=sysmon: it doesn't support concurrency={concurrency};"
     elif the_module is None:
         # We don't even have the underlying module installed, we expect
         # coverage to alert us to this fact.
-        expected_out = (
-            f"Couldn't trace with concurrency={concurrency}, the module isn't installed.\n"
-        )
+        expected_out = f"Couldn't trace with concurrency={concurrency}, the module isn't installed."
     elif testenv.C_TRACER or concurrency == "thread" or concurrency == "":
         expected_out = None
     else:
         expected_out = (
             f"Can't support concurrency={concurrency} with {testenv.REQUESTED_TRACER_CLASS}, "
-            + "only threads are supported.\n"
+            + "only threads are supported."
         )
     return expected_out
 
@@ -230,7 +226,7 @@ def try_some_code(
         expected_cant_trace = cant_trace_msg(concurrency, the_module)
 
         if expected_cant_trace is not None:
-            assert out == expected_cant_trace
+            assert expected_cant_trace in out
             pytest.skip(f"Can't test: {expected_cant_trace}")
         else:
             # We can fully measure the code if we are using the C tracer, which
@@ -406,7 +402,7 @@ def test_missing_module(self, module: str) -> None:
         self.make_file("prog.py", "a = 1")
         sys.modules[module] = None  # type: ignore[assignment]
         if testenv.SYS_MON:
-            msg = rf"Can't use core=sysmon: sys.monitoring doesn't support concurrency={module}"
+            msg = rf"Can't use core=sysmon: it doesn't support concurrency={module}"
         else:
             msg = rf"Couldn't trace with concurrency={module}, the module isn't installed."
         with pytest.raises(ConfigError, match=msg):
@@ -501,8 +497,7 @@ def try_multiprocessing_code(
         expected_cant_trace = cant_trace_msg(concurrency, the_module)
 
         if expected_cant_trace is not None:
-            print(out)
-            assert out == expected_cant_trace
+            assert expected_cant_trace in out
             pytest.skip(f"Can't test: {expected_cant_trace}")
         else:
             assert out.rstrip() == expected_out
@@ -579,9 +574,6 @@ def test_multiprocessing_with_branching(self, start_method: str) -> None:
         code = (SQUARE_OR_CUBE_WORK + MULTI_CODE).format(NPROCS=nprocs, UPTO=upto)
         total = sum(x * x if x % 2 else x * x * x for x in range(upto))
         expected_out = f"{nprocs} pids, total = {total}"
-        expect_warn = (
-            env.PYBEHAVIOR.pep669 and (not env.PYBEHAVIOR.branch_right_left) and testenv.SYS_MON
-        )
         self.make_file("multi.py", code)
         self.make_file(
             "multi.rc",
@@ -590,8 +582,7 @@ def test_multiprocessing_with_branching(self, start_method: str) -> None:
             concurrency = multiprocessing
             branch = True
             omit = */site-packages/*
-            """
-            + ("disable_warnings = no-sysmon" if expect_warn else ""),
+            """,
         )
 
         out = self.run_command(f"coverage run --rcfile=multi.rc multi.py {start_method}")

diff --git a/tests/test_core.py b/tests/test_core.py
@@ -83,13 +83,9 @@ def test_core_request_sysmon(self) -> None:
         if status == 0:
             assert out.endswith("123 456\n")
             core = re_line(r" core:", out).strip()
-            warns = re_lines(r"\(no-sysmon\)", out)
             assert core == "core: SysMonitor"
-            assert not warns
         else:
-            assert out.endswith(
-                "Can't use core=sysmon: sys.monitoring isn't available in this version\n"
-            )
+            assert "Can't use core=sysmon: sys.monitoring isn't available in this version;" in out
 
     def test_core_request_sysmon_no_dyncontext(self) -> None:
         # Use config core= for this test just to be different.
@@ -103,12 +99,9 @@ def test_core_request_sysmon_no_dyncontext(self) -> None:
         )
         out = self.run_command("coverage run --debug=sys numbers.py", status=1)
         if env.PYBEHAVIOR.pep669:
-            assert (
-                "Can't use core=sysmon: sys.monitoring doesn't yet support dynamic contexts\n"
-                in out
-            )
+            assert "Can't use core=sysmon: it doesn't yet support dynamic contexts;" in out
         else:
-            assert "Can't use core=sysmon: sys.monitoring isn't available in this version\n" in out
+            assert "Can't use core=sysmon: sys.monitoring isn't available in this version;" in out
 
     def test_core_request_sysmon_no_branches(self) -> None:
         # Use config core= for this test just to be different.
@@ -124,19 +117,17 @@ def test_core_request_sysmon_no_branches(self) -> None:
             status = 0
         elif env.PYBEHAVIOR.pep669:
             status = 1
-            msg = "Can't use core=sysmon: sys.monitoring can't measure branches in this version\n"
+            msg = "Can't use core=sysmon: sys.monitoring can't measure branches in this version;"
         else:
             status = 1
-            msg = "Can't use core=sysmon: sys.monitoring isn't available in this version\n"
+            msg = "Can't use core=sysmon: sys.monitoring isn't available in this version;"
         out = self.run_command("coverage run --debug=sys numbers.py", status=status)
         if status == 0:
             assert out.endswith("123 456\n")
             core = re_line(r" core:", out).strip()
-            warns = re_lines(r"\(no-sysmon\)", out)
             assert core == "core: SysMonitor"
-            assert not warns
         else:
-            assert out.endswith(msg)  # pylint: disable=possibly-used-before-assignment
+            assert msg in out  # pylint: disable=possibly-used-before-assignment
 
     def test_core_request_nosuchcore(self) -> None:
         # Test the coverage misconfigurations in-process with pytest. Running a

diff --git a/tests/test_process.py b/tests/test_process.py
@@ -1545,7 +1545,6 @@ def f2():
             ".coveragerc",
             """\
             [run]
-            disable_warnings = no-sysmon
             patch = subprocess
             """,
         )
-Original file line number
+Diff line change
@@ Expand Up / @@ -1545,7 +1545,6 @@ def f2(): @@
                 ".coveragerc",
                 """\
                 [run]
-                disable_warnings = no-sysmon
                 patch = subprocess
                 """,
             )
@@ Expand Down @@