Consolidate behavior by using filterfalse and always_iterable

_pytest/python_api.py

@@ -2,7 +2,8 @@ import math
 import sys
 
 import py
-from six.moves import zip
+from six.moves import zip, filterfalse
+from more_itertools.more import always_iterable
 
 from _pytest.compat import isclass
 from _pytest.outcomes import fail
@@ -566,14 +567,10 @@ def raises(expected_exception, *args, **kwargs):
 
     """
     __tracebackhide__ = True
-    msg = ("exceptions must be old-style classes or"
-           " derived from BaseException, not %s")
-    if isinstance(expected_exception, tuple):
-        for exc in expected_exception:
-            if not isclass(exc):
-                raise TypeError(msg % type(exc))
-    elif not isclass(expected_exception):
-        raise TypeError(msg % type(expected_exception))
+    for exc in filterfalse(isclass, always_iterable(expected_exception)):
+        msg = ("exceptions must be old-style classes or"
+               " derived from BaseException, not %s")
+        raise TypeError(msg % type(exc))
 
     message = "DID NOT RAISE {0}".format(expected_exception)
     match_expr = None

setup.py

@@ -60,6 +60,7 @@ def main():
         'six>=1.10.0',
         'setuptools',
         'attrs>=17.2.0',
+        'more_itertools>=4.0.0',
     ]
     # if _PYTEST_SETUP_SKIP_PLUGGY_DEP is set, skip installing pluggy;
     # used by tox.ini to test with pluggy master