Provide more information in the fail-under message

Author: nedbat

coverage/cmdline.py

@@ -629,7 +629,10 @@ def command_line(self, argv):
             fail_under = self.coverage.get_option("report:fail_under")
             precision = self.coverage.get_option("report:precision")
             if should_fail_under(total, fail_under, precision):
-                print("fail-under has failed")
+                msg = "total of {total:.{p}f} is less than fail-under={fail_under:.{p}f}".format(
+                    total=total, fail_under=fail_under, p=precision,
+                )
+                print("Coverage failure:", msg)
                 return FAIL_UNDER
 
         return OK

tests/test_process.py

@@ -1236,13 +1236,19 @@ def test_report_43_is_ok(self):
     def test_report_43_is_not_ok(self):
         st, out = self.run_command_status("coverage report --fail-under=44")
         self.assertEqual(st, 2)
-        self.assertEqual(self.last_line_squeezed(out), "fail-under has failed")
+        self.assertEqual(
+            self.last_line_squeezed(out),
+            "Coverage failure: total of 43 is less than fail-under=44"
+        )
 
     def test_report_42p86_is_not_ok(self):
         self.make_file(".coveragerc", "[report]\nprecision = 2")
         st, out = self.run_command_status("coverage report --fail-under=42.88")
         self.assertEqual(st, 2)
-        self.assertEqual(self.last_line_squeezed(out), "fail-under has failed")
+        self.assertEqual(
+            self.last_line_squeezed(out),
+            "Coverage failure: total of 42.86 is less than fail-under=42.88"
+        )
 
 
 class FailUnderNoFilesTest(CoverageTest):