avocado.utils.path: introduce check_readable()

It's a common use case to want to make sure a file can be read, or if
not fail as early as possible with a relevant error message.

The avocado.utils.iso9600 is doing that itself, but for the sake of
being useful to a wider audience, let's make that a utility itself.

As always, the benefits are also increased test coverage and
documentation.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/utils/iso9660.py

@@ -34,6 +34,7 @@ import sys
 import tempfile
 
 from . import process
+from . import path as utils_path
 
 
 try:
@@ -129,25 +130,8 @@ class BaseIso9660(object):
     """
 
     def __init__(self, path):
+        utils_path.check_readable(path)
         self.path = path
-        self._verify_path(path)
-
-    @staticmethod
-    def _verify_path(path):
-        """
-        Verify that the current set path is accessible
-
-        :param path: the path for test
-        :type path: str
-        :raise OSError: path does not exist or path could not be read
-        :rtype: None
-        """
-        if not os.path.exists(path):
-            raise OSError('File or device path does not exist: %s' %
-                          path)
-        if not os.access(path, os.R_OK):
-            raise OSError('File or device path could not be read: %s' %
-                          path)
 
     def read(self, path):
         """

avocado/utils/path.py

@@ -190,3 +190,22 @@ def usable_ro_dir(directory):
             pass
 
     return False
+
+
+def check_readable(path):
+    """
+    Verify that the given path exists and is readable
+
+    This should be used where an assertion makes sense, and is useful
+    because it can provide a better message in the exception it
+    raises.
+
+    :param path: the path to test
+    :type path: str
+    :raise OSError: path does not exist or path could not be read
+    :rtype: None
+    """
+    if not os.path.exists(path):
+        raise OSError('File "%s" does not exist' % path)
+    if not os.access(path, os.R_OK):
+        raise OSError('File "%s" can not be read' % path)

selftests/unit/test_utils_path.py

+import os
+import unittest
+
+try:
+    from unittest import mock
+except ImportError:
+    import mock
+
+from avocado.utils import path
+
+
+class Path(unittest.TestCase):
+
+    def test_check_readable_exists(self):
+        with mock.patch('avocado.utils.path.os.path.exists',
+                        return_value=False) as mocked_exists:
+            with self.assertRaises(OSError) as cm:
+                path.check_readable(os.devnull)
+            self.assertEqual('File "%s" does not exist' % os.devnull,
+                             cm.exception.args[0])
+            mocked_exists.assert_called_with(os.devnull)
+
+    def test_check_readable_access(self):
+        with mock.patch('avocado.utils.path.os.access',
+                        return_value=False) as mocked_access:
+            with self.assertRaises(OSError) as cm:
+                path.check_readable(os.devnull)
+            self.assertEqual('File "%s" can not be read' % os.devnull,
+                             cm.exception.args[0])
+            mocked_access.assert_called_with(os.devnull, os.R_OK)
+
+
+if __name__ == '__main__':
+    unittest.main()