feat: warnings are now real warnings

This makes coverage warnings visible when running test suites under
pytest.  But it also means some uninteresting warnings would show up in
our own test suite, so we had to catch or suppress those.

4 files changed, 15 insertions, 14 deletions

diff --git a/coverage/control.py b/coverage/control.py
index bf91d447..b13acf45 100644
--- a/coverage/control.py
+++ b/coverage/control.py
@@ -11,6 +11,7 @@ import os.path
 import platform
 import sys
 import time
+import warnings
 
 from coverage import env
 from coverage.annotate import AnnotateReporter
@@ -20,7 +21,7 @@ from coverage.context import should_start_context_test_function, combine_context
 from coverage.data import CoverageData, combine_parallel_data
 from coverage.debug import DebugControl, short_stack, write_formatted_info
 from coverage.disposition import disposition_debug_msg
-from coverage.exceptions import CoverageException
+from coverage.exceptions import CoverageException, CoverageWarning
 from coverage.files import PathAliases, abs_file, relative_filename, set_relative_directory
 from coverage.html import HtmlReporter
 from coverage.inorout import InOrOut
@@ -362,7 +363,7 @@ class Coverage:
             msg = f"{msg} ({slug})"
         if self._debug.should('pid'):
             msg = f"[{os.getpid()}] {msg}"
-        sys.stderr.write(f"Coverage.py warning: {msg}\n")
+        warnings.warn(msg, category=CoverageWarning, stacklevel=2)
 
         if once:
             self._no_warn_slugs.append(slug)
diff --git a/coverage/exceptions.py b/coverage/exceptions.py
index ed96fb21..6631e1ad 100644
--- a/coverage/exceptions.py
+++ b/coverage/exceptions.py
@@ -46,3 +46,8 @@ class StopEverything(BaseCoverageException):
 
     """
     pass
+
+
+class CoverageWarning(Warning):
+    """A warning from Coverage.py."""
+    pass
diff --git a/coverage/inorout.py b/coverage/inorout.py
index fae9ef18..32eb9079 100644
--- a/coverage/inorout.py
+++ b/coverage/inorout.py
@@ -356,10 +356,9 @@ class InOrOut:
                         )
                     break
             except Exception:
-                self.warn(
-                    "Disabling plug-in %r due to an exception:" % (plugin._coverage_plugin_name)
-                )
-                traceback.print_exc()
+                plugin_name = plugin._coverage_plugin_name
+                tb = traceback.format_exc()
+                self.warn(f"Disabling plug-in {plugin_name!r} due to an exception:\n{tb}")
                 plugin._coverage_enabled = False
                 continue
         else:
@@ -503,10 +502,8 @@ class InOrOut:
         # The module was in sys.modules, and seems like a module with code, but
         # we never measured it. I guess that means it was imported before
         # coverage even started.
-        self.warn(
-            "Module %s was previously imported, but not measured" % pkg,
-            slug="module-not-measured",
-        )
+        msg = f"Module {pkg} was previously imported, but not measured"
+        self.warn(msg, slug="module-not-measured")
 
     def find_possibly_unexecuted_files(self):
         """Find files in the areas of interest that might be untraced.
diff --git a/coverage/pytracer.py b/coverage/pytracer.py
index 51f08a1b..540df68c 100644
--- a/coverage/pytracer.py
+++ b/coverage/pytracer.py
@@ -254,10 +254,8 @@ class PyTracer:
             # has changed to None.
             dont_warn = (env.PYPY and env.PYPYVERSION >= (5, 4) and self.in_atexit and tf is None)
             if (not dont_warn) and tf != self._trace:   # pylint: disable=comparison-with-callable
-                self.warn(
-                    f"Trace function changed, measurement is likely wrong: {tf!r}",
-                    slug="trace-changed",
-                )
+                msg = f"Trace function changed, measurement is likely wrong: {tf!r}"
+                self.warn(msg, slug="trace-changed")
 
     def activity(self):
         """Has there been any activity?"""