Add hint of Issue #478 to error text

AUTHORS

@@ -44,6 +44,7 @@ David Mohr
 David Vierra
 Diego Russo
 Dmitry Dygalo
+Duncan Betts
 Edison Gustavo Muenz
 Edoardo Batini
 Eduardo Schettino

CHANGELOG.rst

@@ -1,7 +1,7 @@
 3.0.5.dev0
 ==========
 
-*
+* Add hint to error message hinting possible missing __init__.py (`#478`_). Thanks `@DuncanBetts`_.
 
 * An error message is now displayed if ``--confcutdir`` is not a valid directory, avoiding
   subtle bugs (`#2078`_).
@@ -20,8 +20,10 @@
 *
 
 .. _@adler-j: https://github.com/adler-j
+.. _@DuncanBetts: https://github.com/DuncanBetts
 .. _@nedbat: https://github.com/nedbat
 
+.. _#478: https://github.com/pytest-dev/pytest/issues/478
 .. _#2038: https://github.com/pytest-dev/pytest/issues/2038
 .. _#2078: https://github.com/pytest-dev/pytest/issues/2078
 .. _#2082: https://github.com/pytest-dev/pytest/issues/2082

_pytest/main.py

@@ -704,10 +704,9 @@ class Session(FSCollector):
         path = self.config.invocation_dir.join(relpath, abs=True)
         if not path.check():
             if self.config.option.pyargs:
-                msg = "file or package not found: "
+                raise pytest.UsageError("file or package not found: " + arg + " (missing __init__.py?)")
             else:
-                msg = "file not found: "
+                raise pytest.UsageError("file not found: " + arg)
-            raise pytest.UsageError(msg + arg)
         parts[0] = path
         return parts