fix: better defaulting of core

Author: nedbat

CHANGES.rst

@@ -23,7 +23,13 @@ upgrading your version of coverage.py.
 Unreleased
 ----------
 
-Nothing yet.
+- The "no-ctracer" warning is not issued for Python pre-release versions.
+  Coverage doesn't ship compiled wheels for those versions, so this was far too
+  noisy.
+
+- On Python 3.14+, the "sysmon" core is now the default if it's supported for
+  your configuration.  Plugins and dynamic contexts are still not supported
+  with it.
 
 
 .. start-releases

coverage/core.py

@@ -88,9 +88,11 @@ def __init__(
         if core_name is None:
             if env.SYSMON_DEFAULT and not reason_no_sysmon:
                 core_name = "sysmon"
-            elif CTRACER_FILE:
-                core_name = "ctrace"
             else:
+                core_name = "ctrace"
+
+        if core_name == "ctrace":
+            if not CTRACER_FILE:
                 if IMPORT_ERROR and env.SHIPPING_WHEELS:
                     warn(f"Couldn't import C tracer: {IMPORT_ERROR}", slug="no-ctracer", once=True)
                 core_name = "pytrace"

tests/test_process.py

@@ -1133,8 +1133,8 @@ def test_core_default(self) -> None:
         self.make_file("numbers.py", "print(123, 456)")
         out = self.run_command("coverage run --debug=sys numbers.py")
         assert out.endswith("123 456\n")
-        warns = re_lines(r"\(no-ctracer\)", out)
         core = re_line(r" core:", out).strip()
+        warns = re_lines(r"\(no-ctracer\)", out)
         if env.SYSMON_DEFAULT:
             assert core == "core: SysMonitor"
             assert not warns
@@ -1154,6 +1154,18 @@ def test_core_request_ctrace(self) -> None:
         core = re_line(r" core:", out).strip()
         assert core == "core: CTracer"
 
+    @pytest.mark.skipif(has_ctracer, reason="CTracer needs to be missing")
+    def test_core_request_ctrace_but_missing(self) -> None:
+        self.del_environ("COVERAGE_CORE")
+        self.make_file(".coveragerc", "[run]\ncore = ctrace\n")
+        self.make_file("numbers.py", "print(123, 456)")
+        out = self.run_command("coverage run --debug=sys,pybehave numbers.py")
+        assert out.endswith("123 456\n")
+        core = re_line(r" core:", out).strip()
+        assert core == "core: PyTracer"
+        warns = re_lines(r"\(no-ctracer\)", out)
+        assert bool(warns) == env.SHIPPING_WHEELS
+
     def test_core_request_pytrace(self) -> None:
         self.set_environ("COVERAGE_CORE", "pytrace")
         self.make_file("numbers.py", "print(123, 456)")