Fix RuntimeError when trying to collect package with "__init__.py" only

Fixes pytest-dev#4344.

changelog/4344.bugfix.rst

@@ -0,0 +1 @@
+Fix RuntimeError/StopIteration when trying to collect package with "__init__.py" only.

src/_pytest/main.py

@@ -584,7 +584,13 @@ def _collect(self, arg):
             # Module itself, so just use that. If this special case isn't taken, then all
             # the files in the package will be yielded.
             if argpath.basename == "__init__.py":
-                yield next(m[0].collect())
+                try:
+                    yield next(m[0].collect())
+                except StopIteration:
+                    # The package collects nothing with only an __init__.py
+                    # file in it, which gets ignored by the default
+                    # "python_files" option.
+                    pass
                 return
             yield from m
 

testing/test_collection.py

@@ -1203,6 +1203,18 @@ def test_collect_pkg_init_and_file_in_args(testdir):
     )
 
 
+def test_collect_pkg_init_only(testdir):
+    subdir = testdir.mkdir("sub")
+    init = subdir.ensure("__init__.py")
+    init.write("def test_init(): pass")
+
+    result = testdir.runpytest(str(init))
+    result.stdout.fnmatch_lines(["*no tests ran in*"])
+
+    result = testdir.runpytest("-v", "-o", "python_files=*.py", str(init))
+    result.stdout.fnmatch_lines(["sub/__init__.py::test_init PASSED*", "*1 passed in*"])
+
+
 @pytest.mark.skipif(
     not hasattr(py.path.local, "mksymlinkto"),
     reason="symlink not available on this platform",