- Add new Warning class, ImportWarning

 - Warn-raise ImportWarning when importing would have picked up a directory
   as package, if only it'd had an __init__.py. This swaps two tests (for
   case-ness and __init__-ness), but case-test is not really more expensive,
   and it's not in a speed-critical section.

 - Test for the new warning by importing a common non-package directory on
   sys.path: site-packages

 - In regrtest.py, silence warnings generated by the build-environment
   because Modules/ (which is added to sys.path for Setup-created modules)
   has 'zlib' and '_ctypes' directories without __init__.py's.

Include/pyerrors.h

@@ -108,6 +108,7 @@ PyAPI_DATA(PyObject *) PyExc_SyntaxWarning;
 PyAPI_DATA(PyObject *) PyExc_OverflowWarning;
 PyAPI_DATA(PyObject *) PyExc_RuntimeWarning;
 PyAPI_DATA(PyObject *) PyExc_FutureWarning;
+PyAPI_DATA(PyObject *) PyExc_ImportWarning;
 
 
 /* Convenience functions */

Lib/test/exception_hierarchy.txt

@@ -44,3 +44,4 @@ BaseException
            +-- UserWarning
            +-- FutureWarning
 	   +-- OverflowWarning [not generated by the interpreter]
+	   +-- ImportWarning

Lib/test/regrtest.py

@@ -138,6 +138,12 @@ if sys.maxint > 0x7fffffff:
     warnings.filterwarnings("ignore", "hex/oct constants", FutureWarning,
                             "<string>")
 
+# Ignore ImportWarnings that only occur in the source tree,
+# (because of modules with the same name as source-directories in Modules/)
+for mod in ("ctypes", "gzip", "test.test_zipimport", "test.test_zlib"):
+    warnings.filterwarnings(module=".*%s$" % (mod,),
+                            action="ignore", category=ImportWarning)
+
 # MacOSX (a.k.a. Darwin) has a default stack size that is too small
 # for deeply recursive regular expressions.  We see this as crashes in
 # the Python test suite when running test_re.py and test_sre.py.  The

Lib/test/test_import.py

@@ -205,3 +205,20 @@ def test_import_name_binding():
     assert y is test.test_support, y.__name__
 
 test_import_name_binding()
+
+def test_import_initless_directory_warning():
+    import warnings
+    oldfilters = warnings.filters[:]
+    warnings.simplefilter('error', ImportWarning);
+    try:
+        # Just a random non-package directory we always expect to be
+        # somewhere in sys.path...
+        __import__("site-packages")
+    except ImportWarning:
+        pass
+    else:
+        raise AssertionError
+    finally:
+        warnings.filters = oldfilters
+
+test_import_initless_directory_warning()

Python/exceptions.c

@@ -1647,6 +1647,8 @@ PyDoc_STRVAR(FutureWarning__doc__,
 "Base class for warnings about constructs that will change semantically "
 "in the future.");
 
+PyDoc_STRVAR(ImportWarning__doc__,
+"Base class for warnings about probable mistakes in module imports");
 
 
 /* module global functions */
@@ -1719,6 +1721,7 @@ PyObject *PyExc_SyntaxWarning;
 PyObject *PyExc_OverflowWarning;
 PyObject *PyExc_RuntimeWarning;
 PyObject *PyExc_FutureWarning;
+PyObject *PyExc_ImportWarning;
 
 
 
@@ -1818,6 +1821,8 @@ static struct {
   RuntimeWarning__doc__},
  {"FutureWarning", &PyExc_FutureWarning, &PyExc_Warning,
   FutureWarning__doc__},
+ {"ImportWarning", &PyExc_ImportWarning, &PyExc_Warning,
+  ImportWarning__doc__},
  /* Sentinel */
  {NULL}
 };

Python/import.c

@@ -1271,19 +1271,42 @@ find_module(char *fullname, char *subname, PyObject *path, char *buf,
 #ifdef HAVE_STAT
 		if (stat(buf, &statbuf) == 0 &&         /* it exists */
 		    S_ISDIR(statbuf.st_mode) &&         /* it's a directory */
-		    find_init_module(buf) &&            /* it has __init__.py */
-		    case_ok(buf, len, namelen, name)) { /* and case matches */
-			Py_XDECREF(copy);
-			return &fd_package;
+		    case_ok(buf, len, namelen, name)) { /* case matches */
+			if (find_init_module(buf)) { /* and has __init__.py */
+				Py_XDECREF(copy);
+				return &fd_package;
+			}
+			else {
+				char warnstr[MAXPATHLEN+80];
+				sprintf(warnstr, "Not importing directory "
+					"'%.*s': missing __init__.py", 
+					MAXPATHLEN, buf);
+				if (PyErr_Warn(PyExc_ImportWarning,
+					       warnstr)) {
+					Py_XDECREF(copy);
+					return NULL;
+				}
+			}
 		}
 #else
 		/* XXX How are you going to test for directories? */
 #ifdef RISCOS
 		if (isdir(buf) &&
-		    find_init_module(buf) &&
 		    case_ok(buf, len, namelen, name)) {
-			Py_XDECREF(copy);
-			return &fd_package;
+			if (find_init_module(buf)) {
+				Py_XDECREF(copy);
+				return &fd_package;
+			}
+			else {
+				char warnstr[MAXPATHLEN+80];
+				sprintf(warnstr, "Not importing directory "
+					"'%.*s': missing __init__.py", 
+					MAXPATHLEN, buf);
+				if (PyErr_Warn(PyExc_ImportWarning,
+					       warnstr)) {
+					Py_XDECREF(copy);
+					return NULL;
+				}
 		}
 #endif
 #endif