Show the tracer class in help output. #479

4 files changed, 21 insertions, 8 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 0d912b98..2167a534 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -30,9 +30,13 @@ Unreleased
   indents was fixed.  The index page also has slightly different styling, to
   try to make the clickable detail pages more apparent.
 
+- ``coverage --help`` and ``coverage --version`` now mention which tracer is
+  installed, to help diagnose problems (`issue 479`_).
+
 .. _issue 440: https://bitbucket.org/ned/coveragepy/issues/440/yielded-twisted-failure-marked-as-missed
 .. _issue 472: https://bitbucket.org/ned/coveragepy/issues/472/html-report-indents-incorrectly-for-one
 .. _issue 475: https://bitbucket.org/ned/coveragepy/issues/475/generator-expression-is-marked-as-not
+.. _issue 479: https://bitbucket.org/ned/coveragepy/issues/479/clarify-the-need-for-the-c-extension
 
 
 Version 4.1b2 --- 2016-01-23
diff --git a/coverage/__init__.py b/coverage/__init__.py
index d132e4a7..8085b2f0 100644
--- a/coverage/__init__.py
+++ b/coverage/__init__.py
@@ -10,10 +10,14 @@ http://nedbatchelder.com/code/coverage
 
 from coverage.version import __version__, __url__, version_info
 
+from coverage.collector import CTracer
 from coverage.control import Coverage, process_startup
 from coverage.data import CoverageData
 from coverage.misc import CoverageException
 from coverage.plugin import CoveragePlugin, FileTracer, FileReporter
+from coverage.pytracer import PyTracer
+
+tracer_class = CTracer or PyTracer
 
 # Backward compatibility.
 coverage = Coverage
diff --git a/coverage/cmdline.py b/coverage/cmdline.py
index 38f4b43a..9e6b088f 100644
--- a/coverage/cmdline.py
+++ b/coverage/cmdline.py
@@ -549,7 +549,7 @@ class CoverageScript(object):
             help_params['program_name'] = self.program_name
             help_msg = textwrap.dedent(HELP_TOPICS.get(topic, '')).strip()
             if help_msg:
-                print(help_msg % help_params)
+                print(help_msg.format(**help_params))
             else:
                 print("Don't know topic %r" % topic)
 
@@ -699,10 +699,10 @@ def unglob_args(args):
 
 HELP_TOPICS = {
     'help': """\
-        Coverage.py, version %(__version__)s
+        Coverage.py, version {__version__} with {tracer_class.__name__}
         Measure, collect, and report on code coverage in Python programs.
 
-        usage: %(program_name)s <command> [options] [args]
+        usage: {program_name} <command> [options] [args]
 
         Commands:
             annotate    Annotate source files with execution information.
@@ -714,17 +714,17 @@ HELP_TOPICS = {
             run         Run a Python program and measure code execution.
             xml         Create an XML report of coverage results.
 
-        Use "%(program_name)s help <command>" for detailed help on any command.
-        For full documentation, see %(__url__)s
+        Use "{program_name} help <command>" for detailed help on any command.
+        For full documentation, see {__url__}
     """,
 
     'minimum_help': """\
-        Code coverage for Python.  Use '%(program_name)s help' for help.
+        Code coverage for Python.  Use '{program_name} help' for help.
     """,
 
     'version': """\
-        Coverage.py, version %(__version__)s.
-        Documentation at %(__url__)s
+        Coverage.py, version {__version__} with {tracer_class.__name__}
+        Documentation at {__url__}
     """,
 }
 
diff --git a/tests/test_cmdline.py b/tests/test_cmdline.py
index c78e3468..ebda68c3 100644
--- a/tests/test_cmdline.py
+++ b/tests/test_cmdline.py
@@ -13,6 +13,7 @@ import mock
 
 import coverage
 import coverage.cmdline
+from coverage import env
 from coverage.config import CoverageConfig
 from coverage.data import CoverageData, CoverageDataFiles
 from coverage.misc import ExceptionDuringRun
@@ -599,6 +600,10 @@ class CmdLineStdoutTest(BaseCmdLineTest):
         self.command_line("--version")
         out = self.stdout()
         self.assertIn("ersion ", out)
+        if env.C_TRACER:
+            self.assertIn("with CTracer", out)
+        else:
+            self.assertIn("with PyTracer", out)
         self.assertLess(out.count("\n"), 4)
 
     def test_help(self):