Show an error if --confcutdir is not a valid directory

Fixes #2078

CHANGELOG.rst

@@ -3,7 +3,10 @@
 
 *
 
-*
+* An error message is now displayed if ``--confcutdir`` is not a valid directory, avoiding
+  subtle bugs (`#2078`_).
+  Thanks `@nicoddemus`_ for the PR.
+
 
 * Cope gracefully with a .pyc file with no matching .py file (`#2038`_). Thanks
   `@nedbat`_.
@@ -15,6 +18,7 @@
 .. _@nedbat: https://github.com/nedbat
 
 .. _#2038: https://github.com/pytest-dev/pytest/issues/2038
+.. _#2078: https://github.com/pytest-dev/pytest/issues/2078
 
 
 3.0.4

_pytest/config.py

@@ -996,6 +996,7 @@ def _warn_about_missing_assertion(self, mode):
                                  "(are you using python -O?)\n")
 
     def _preparse(self, args, addopts=True):
+        import pytest
         self._initini(args)
         if addopts:
             args[:] = shlex.split(os.environ.get('PYTEST_ADDOPTS', '')) + args
@@ -1007,7 +1008,10 @@ def _preparse(self, args, addopts=True):
         self.pluginmanager.load_setuptools_entrypoints(entrypoint_name)
         self.pluginmanager.consider_env()
         self.known_args_namespace = ns = self._parser.parse_known_args(args, namespace=self.option.copy())
-        if self.known_args_namespace.confcutdir is None and self.inifile:
+        confcutdir = self.known_args_namespace.confcutdir
+        if confcutdir and not os.path.isdir(confcutdir):
+            raise pytest.UsageError('--confcutdir must be a directory, given: {0}'.format(confcutdir))
+        if confcutdir is None and self.inifile:
             confcutdir = py.path.local(self.inifile).dirname
             self.known_args_namespace.confcutdir = confcutdir
         try:

testing/test_config.py

@@ -294,6 +294,15 @@ def pytest_addoption(parser):
         assert len(l) == 2
         assert l == ["456", "123"]
 
+    def test_confcutdir_check_isdir(self, testdir):
+        """Give an error if --confcutdir is not a valid directory (#2078)"""
+        with pytest.raises(pytest.UsageError):
+            testdir.parseconfig('--confcutdir', testdir.tmpdir.join('file').ensure(file=1))
+        with pytest.raises(pytest.UsageError):
+            testdir.parseconfig('--confcutdir', testdir.tmpdir.join('inexistant'))
+        config = testdir.parseconfig('--confcutdir', testdir.tmpdir.join('dir').ensure(dir=1))
+        assert config.getoption('confcutdir') == str(testdir.tmpdir.join('dir'))
+
 
 class TestConfigFromdictargs:
     def test_basic_behavior(self):