testutils: Add a context manager to capture stderr

and use it to test the help output

3 files changed, 35 insertions, 3 deletions

diff --git a/tests/11_test_dch_main.py b/tests/11_test_dch_main.py
index 4f27131..538d2e8 100644
--- a/tests/11_test_dch_main.py
+++ b/tests/11_test_dch_main.py
@@ -4,7 +4,7 @@
 
 from . import context
 from .testutils import (DebianGitTestRepo, OsReleaseFile,
-                        get_dch_default_urgency)
+                        get_dch_default_urgency, capture_stderr)
 
 from gbp.scripts import dch
 
@@ -191,7 +191,9 @@ class TestScriptDch(DebianGitTestRepo):
     def test_dch_main_new_upstream_version_with_snapshot_release(self):
         """Test dch.py like gbp dch script does: new upstream version - snapshot - release"""
         options = ["--snapshot", "--release"]
-        self.assertRaises(SystemExit, self.run_dch, options)
+        with capture_stderr() as c:
+            self.assertRaises(SystemExit, self.run_dch, options)
+        self.assertTrue("'--snapshot' and '--release' are incompatible options" in c.output())
 
 
     def test_dch_main_new_upstream_version_with_distribution(self):
diff --git a/tests/testutils/__init__.py b/tests/testutils/__init__.py
index c94af64..092d377 100644
--- a/tests/testutils/__init__.py
+++ b/tests/testutils/__init__.py
@@ -13,9 +13,10 @@ from gbp.deb.changelog import ChangeLog
 
 from . gbplogtester import GbpLogTester
 from . debiangittestrepo import DebianGitTestRepo
+from . capture import capture_stderr
 
 __all__ = ['GbpLogTester', 'DebianGitTestRepo', 'OsReleaseFile',
-           'MockedChangeLog', 'get_dch_default_urgency' ]
+           'MockedChangeLog', 'get_dch_default_urgency', 'capture_stderr' ]
 
 class OsReleaseFile(object):
     """Repesents a simple file with key-value pairs"""
diff --git a/tests/testutils/capture.py b/tests/testutils/capture.py
new file mode 100644
index 0000000..60d5a12
--- /dev/null
+++ b/tests/testutils/capture.py
@@ -0,0 +1,29 @@
+# vim: set fileencoding=utf-8 :
+
+import sys
+from contextlib import contextmanager
+from StringIO import StringIO
+
+
+class StderrCapture(StringIO):
+    def save(self):
+        self.safed = sys.stderr
+        sys.stderr = self
+
+    def restore(self):
+        if self.safed is not None:
+            sys.stderr = self.safed
+            self.safed = None
+
+    def output(self):
+        self.seek(0)
+        return self.read()
+
+
+@contextmanager
+def capture_stderr():
+    """Capture an output and return it's content"""
+    c = StderrCapture()
+    c.save()
+    yield c
+    c.restore()