wip: better core choices and warnings

Author: nedbat

coverage/control.py

@@ -549,6 +549,7 @@ def _init_for_start(self) -> None:
         self._core = Core(
             warn=self._warn,
             config=self.config,
+            dynamic_contexts=(should_start_context is not None),
             metacov=self._metacov,
         )
         self._collector = Collector(

coverage/core.py

@@ -18,8 +18,8 @@
 from coverage.sysmon import SysMonitor
 from coverage.types import (
     TFileDisposition,
-    Tracer,
     TWarnFn,
+    Tracer,
 )
 
 
@@ -53,36 +53,40 @@ class Core:
     packed_arcs: bool
     systrace: bool
 
-    def __init__(self, warn: TWarnFn, config: CoverageConfig, metacov: bool) -> None:
-        core_name: str | None
+    def __init__(
+        self,
+        warn: TWarnFn,
+        config: CoverageConfig,
+        dynamic_contexts: bool,
+        metacov: bool,
+    ) -> 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"
+        elif config.branch and not env.PYBEHAVIOR.branch_right_left:
+            reason_no_sysmon = "can't measure branches in this version"
+        elif dynamic_contexts:
+            reason_no_sysmon = "doesn't yet support dynamic contexts"
+
+        core_name: str | None = None
         if config.timid:
             core_name = "pytrace"
-        else:
+
+        if core_name is None:
             core_name = os.getenv("COVERAGE_CORE")
 
-            if core_name == "sysmon":
-                if not env.PYBEHAVIOR.pep669:
-                    warn(
-                        "sys.monitoring isn't available in this version, using default core",
-                        slug="no-sysmon",
-                    )
-                    core_name = None
-
-                if config.branch and not env.PYBEHAVIOR.branch_right_left:
-                    warn(
-                        "sys.monitoring can't measure branches in this vesion, using default core",
-                        slug="no-sysmon",
-                    )
-                    core_name = None
-
-            if not core_name:
-                # Once we're comfortable with sysmon as a default:
-                # if env.PYBEHAVIOR.pep669 and self.should_start_context is None:
-                #     core_name = "sysmon"
-                if HAS_CTRACER:
-                    core_name = "ctrace"
-                else:
-                    core_name = "pytrace"
+        if core_name == "sysmon" and reason_no_sysmon:
+            warn(f"sys.monitoring {reason_no_sysmon}, using default core", slug="no-sysmon")
+            core_name = None
+
+        if core_name is None:
+            if not reason_no_sysmon:
+                core_name = "sysmon"
+            elif HAS_CTRACER:
+                core_name = "ctrace"
+            else:
+                core_name = "pytrace"
 
         self.tracer_kwargs = {}