internally make varnames() deal with classes's __init__,

although it's not needed by pytest itself atm.  Also
fix caching.  Fixes issue376.

CHANGELOG

@@ -48,6 +48,10 @@ Unreleased
 - xfail a test on pypy that checks wrong encoding/ascii (pypy does
   not error out). fixes issue385.
 
+- internally make varnames() deal with classes's __init__,
+  although it's not needed by pytest itself atm.  Also
+  fix caching.  Fixes issue376.
+
 Changes between 2.4.1 and 2.4.2
 -----------------------------------
 

_pytest/core.py

@@ -304,10 +304,24 @@ class MultiCall:
         return kwargs
 
 def varnames(func):
+    """ return argument name tuple for a function, method, class or callable.
+
+    In case of a class, its "__init__" method is considered.
+    For methods the "self" parameter is not included unless you are passing
+    an unbound method with Python3 (which has no supports for unbound methods)
+    """
+    cache = getattr(func, "__dict__", {})
     try:
-        return func._varnames
-    except AttributeError:
+        return cache["_varnames"]
+    except KeyError:
         pass
+    if inspect.isclass(func):
+        try:
+            func = func.__init__
+        except AttributeError:
+            return ()
+        ismethod = True
+    else:
         if not inspect.isfunction(func) and not inspect.ismethod(func):
             func = getattr(func, '__call__', func)
         ismethod = inspect.ismethod(func)
@@ -316,7 +330,10 @@ def varnames(func):
         x = rawcode.co_varnames[ismethod:rawcode.co_argcount]
     except AttributeError:
         x = ()
-    py.builtin._getfuncdict(func)['_varnames'] = x
+    try:
+        cache["_varnames"] = x
+    except TypeError:
+        pass
     return x
 
 class HookRelay:

testing/test_core.py

@@ -438,6 +438,15 @@ def test_varnames():
     assert varnames(A().f) == ('y',)
     assert varnames(B()) == ('z',)
 
+def test_varnames_class():
+    class C:
+        def __init__(self, x):
+            pass
+    class D:
+        pass
+    assert varnames(C) == ("x",)
+    assert varnames(D) == ()
+
 class TestMultiCall:
     def test_uses_copy_of_methods(self):
         l = [lambda: 42]
@@ -654,8 +663,7 @@ def test_default_markers(testdir):
 
 def test_importplugin_issue375(testdir):
     testdir.makepyfile(qwe="import aaaa")
-    with pytest.raises(ImportError) as excinfo:
-        importplugin("qwe")
+    excinfo = pytest.raises(ImportError, lambda: importplugin("qwe"))
     assert "qwe" not in str(excinfo.value)
     assert "aaaa" in str(excinfo.value)