Provide "__str__" for "CmdError"

Let's provide the "__str__" method for "CmdError" to be able to see the
actual content on print(error).
Signed-off-by: NJunxiang Li <junli@redhat.com>
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/utils/process.py

@@ -82,6 +82,10 @@ class CmdError(Exception):
         self.result = result
         self.additional_text = additional_text
 
+    def __str__(self):
+        return ("Command '%s' failed.\nstdout: %r\nstderr: %r\nadditional_info: %s" %
+                (self.command, self.result.stdout, self.result.stderr, self.additional_text))
+
 
 def can_sudo(cmd=None):
     """

selftests/unit/test_utils_process.py

@@ -15,7 +15,7 @@ from avocado.utils import gdb
 from avocado.utils import process
 from avocado.utils import path
 
-from six import string_types
+from six import string_types, PY2
 
 
 def probe_binary(binary):
@@ -306,6 +306,23 @@ class CmdResultTests(unittest.TestCase):
         self.assertRaises(TypeError, lambda x: result.stderr_text)
 
 
+class CmdErrorTests(unittest.TestCase):
+
+    def test_nasty_str(self):
+        result = process.CmdResult("ls", b"unicode_follows: \xc5\xa1",
+                                   b"cp1250 follows: \xfd", 1, 2, 3,
+                                   "wrong_encoding")
+        err = process.CmdError("ls", result, "please don't crash")
+        if PY2:
+            prefix = ''
+        else:
+            prefix = 'b'
+        self.assertEqual(str(err), "Command 'ls' failed.\nstdout: "
+                         "%s'unicode_follows: \\xc5\\xa1'\nstderr: "
+                         "%s'cp1250 follows: \\xfd'\nadditional_info: "
+                         "please don't crash" % (prefix, prefix))
+
+
 class FDDrainerTests(unittest.TestCase):
 
     def test_drain_from_pipe_fd(self):