Print command-line errors to stderr instead of stdout

3 files changed, 16 insertions, 13 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index e20d9bd..d3495ec 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -10,7 +10,8 @@ Change history for Coverage.py
 Unreleased
 ----------
 
-- Nothing yet.
+- Errors printed by the ``coverage`` command now go to stderr instead of
+  stdout.
 
 
 Version 4.4b1 --- 2017-04-04
diff --git a/coverage/cmdline.py b/coverage/cmdline.py
index a9af9f3..9c908b1 100644
--- a/coverage/cmdline.py
+++ b/coverage/cmdline.py
@@ -3,6 +3,8 @@
 
 """Command-line support for coverage.py."""
 
+from __future__ import print_function
+
 import glob
 import optparse
 import os.path
@@ -533,8 +535,8 @@ class CoverageScript(object):
         """Display an error message, or the named topic."""
         assert error or topic or parser
         if error:
-            print(error)
-            print("Use '%s help' for help." % (self.program_name,))
+            print(error, file=sys.stderr)
+            print("Use '%s help' for help." % (self.program_name,), file=sys.stderr)
         elif parser:
             print(parser.format_help().strip())
         else:
diff --git a/tests/test_cmdline.py b/tests/test_cmdline.py
index 000bf46..30fb06e 100644
--- a/tests/test_cmdline.py
+++ b/tests/test_cmdline.py
@@ -462,16 +462,16 @@ class CmdLineTest(BaseCmdLineTest):
 
     def test_bad_concurrency(self):
         self.command_line("run --concurrency=nothing", ret=ERR)
-        out = self.stdout()
-        self.assertIn("option --concurrency: invalid choice: 'nothing'", out)
+        err = self.stderr()
+        self.assertIn("option --concurrency: invalid choice: 'nothing'", err)
 
     def test_no_multiple_concurrency(self):
         # You can't use multiple concurrency values on the command line.
         # I would like to have a better message about not allowing multiple
         # values for this option, but optparse is not that flexible.
         self.command_line("run --concurrency=multiprocessing,gevent foo.py", ret=ERR)
-        out = self.stdout()
-        self.assertIn("option --concurrency: invalid choice: 'multiprocessing,gevent'", out)
+        err = self.stderr()
+        self.assertIn("option --concurrency: invalid choice: 'multiprocessing,gevent'", err)
 
     def test_multiprocessing_needs_config_file(self):
         # You can't use command-line args to add options to multiprocessing
@@ -480,7 +480,7 @@ class CmdLineTest(BaseCmdLineTest):
         self.command_line("run --concurrency=multiprocessing --branch foo.py", ret=ERR)
         self.assertIn(
             "Options affecting multiprocessing must be specified in a configuration file.",
-            self.stdout()
+            self.stderr()
         )
 
     def test_run_debug(self):
@@ -530,11 +530,11 @@ class CmdLineTest(BaseCmdLineTest):
 
     def test_run_nothing(self):
         self.command_line("run", ret=ERR)
-        self.assertIn("Nothing to do", self.stdout())
+        self.assertIn("Nothing to do", self.stderr())
 
     def test_cant_append_parallel(self):
         self.command_line("run --append --parallel-mode foo.py", ret=ERR)
-        self.assertIn("Can't append to data files in parallel mode.", self.stdout())
+        self.assertIn("Can't append to data files in parallel mode.", self.stderr())
 
     def test_xml(self):
         # coverage xml [-i] [--omit DIR,...] [FILE1 FILE2 ...]
@@ -662,9 +662,9 @@ class CmdLineStdoutTest(BaseCmdLineTest):
 
     def test_error(self):
         self.command_line("fooey kablooey", ret=ERR)
-        out = self.stdout()
-        self.assertIn("fooey", out)
-        self.assertIn("help", out)
+        err = self.stderr()
+        self.assertIn("fooey", err)
+        self.assertIn("help", err)
 
 
 class CmdMainTest(CoverageTest):