Fix issue #3372

AUTHORS

@@ -188,6 +188,7 @@ Tareq Alayan
 Ted Xiao
 Thomas Grainger
 Thomas Hisch
+Tim Strazny
 Tom Dalton
 Tom Viner
 Trevor Bekolay

_pytest/python_api.py

@@ -584,10 +584,11 @@ def raises(expected_exception, *args, **kwargs):
 
     """
     __tracebackhide__ = True
-    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))
+    if not issubclass(expected_exception, BaseException):
+        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

changelog/3372.bugfix.rst

@@ -0,0 +1 @@
+``pytest.raises`` now works with exception classes that look like iterables.

testing/python/raises.py

@@ -1,3 +1,4 @@
+from _pytest.outcomes import Failed
 import pytest
 import sys
 
@@ -147,3 +148,20 @@ class TestRaises(object):
         with pytest.raises(ValueError):
             with pytest.raises(IndexError, match='nomatch'):
                 int('asdf')
+
+    def test_raises_exception_looks_iterable(self):
+        from six import add_metaclass
+
+        class Meta(type(object)):
+            def __getitem__(self, item):
+                return 1/0
+
+            def __len__(self):
+                return 1
+
+        @add_metaclass(Meta)
+        class ClassLooksIterableException(Exception):
+            pass
+
+        with pytest.raises(Failed, match="DID NOT RAISE <class 'raises.ClassLooksIterableException'>"):
+            pytest.raises(ClassLooksIterableException, lambda: None)