Merge branch 'master' into verify-interfaces

3 files changed, 37 insertions, 16 deletions

diff --git a/tests/conftest.py b/tests/conftest.py
index 20926024..9dc37d38 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -17,7 +17,7 @@ import pytest
 
 from cryptography.hazmat.backends import _available_backends
 
-from .utils import check_backend_support, select_backends
+from .utils import check_backend_support, select_backends, skip_if_empty
 
 
 def pytest_generate_tests(metafunc):
@@ -25,22 +25,29 @@ def pytest_generate_tests(metafunc):
     selected_backends = select_backends(names, _available_backends())
 
     if "backend" in metafunc.fixturenames:
-        metafunc.parametrize("backend", selected_backends)
+        filtered_backends = []
+        for backend in selected_backends:
+            try:
+                required = metafunc.function.requires_backend_interface
+            except AttributeError:
+                # function does not have requires_backend_interface decorator
+                filtered_backends.append(backend)
+            else:
+                required_interfaces = tuple(
+                    mark.kwargs["interface"] for mark in required
+                )
+                if isinstance(backend, required_interfaces):
+                    filtered_backends.append(backend)
+
+        # If you pass an empty list to parametrize Bad Things(tm) happen
+        # as of pytest 2.6.4 when the test also has a parametrize decorator
+        skip_if_empty(filtered_backends, required_interfaces)
+
+        metafunc.parametrize("backend", filtered_backends)
 
 
 @pytest.mark.trylast
 def pytest_runtest_setup(item):
-    required = item.keywords.get("requires_backend_interface")
-    if required is not None and "backend" in item.funcargs:
-        required_interfaces = tuple(
-            mark.kwargs["interface"] for mark in required
-        )
-        if not isinstance(item.funcargs["backend"], required_interfaces):
-            pytest.skip("{0} backend does not support {1}".format(
-                item.funcargs["backend"],
-                ", ".join(iface.__name__ for iface in required_interfaces)
-            ))
-
     check_backend_support(item)
 
 
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 6c8d088b..8bb8422c 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -32,7 +32,7 @@ from .utils import (
     load_fips_ecdsa_key_pair_vectors, load_fips_ecdsa_signing_vectors,
     load_hash_vectors, load_kasvs_dh_vectors, load_nist_vectors,
     load_pkcs1_vectors, load_rsa_nist_vectors, load_vectors_from_file,
-    raises_unsupported_algorithm, select_backends
+    raises_unsupported_algorithm, select_backends, skip_if_empty
 )
 
 
@@ -82,6 +82,13 @@ def test_select_two_backends():
     assert selected_backends == [b1, b2]
 
 
+def test_skip_if_empty():
+    with pytest.raises(pytest.skip.Exception):
+        skip_if_empty([], [FakeInterface])
+
+    skip_if_empty(["notempty"], [FakeInterface])
+
+
 def test_check_backend_support_skip():
     supported = pretend.stub(
         kwargs={"only_if": lambda backend: False, "skip_message": "Nope"}
diff --git a/tests/utils.py b/tests/utils.py
index bc5bc1ea..acc6c141 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -40,8 +40,6 @@ def select_backends(names, backend_list):
     if names is None:
         return backend_list
     split_names = [x.strip() for x in names.split(',')]
-    # this must be duplicated and then removed to preserve the metadata
-    # pytest associates. Appending backends to a new list doesn't seem to work
     selected_backends = []
     for backend in backend_list:
         if backend.name in split_names:
@@ -55,6 +53,15 @@ def select_backends(names, backend_list):
         )
 
 
+def skip_if_empty(backend_list, required_interfaces):
+    if not backend_list:
+        pytest.skip(
+            "No backends provided supply the interface: {0}".format(
+                ", ".join(iface.__name__ for iface in required_interfaces)
+            )
+        )
+
+
 def check_backend_support(item):
     supported = item.keywords.get("supported")
     if supported and "backend" in item.funcargs: