now that we are going to have wheels, py source code might be not be installed

and the resulting genscript is useless

--HG--
branch : pluggy1

_pytest/genscript.py

@@ -34,6 +34,9 @@ def pkg_to_mapping(name):
         for pyfile in toplevel.visit('*.py'):
             pkg = pkgname(name, toplevel, pyfile)
             name2src[pkg] = pyfile.read()
+        # with wheels py source code might be not be installed
+        # and the resulting genscript is useless, just bail out.
+        assert name2src, "no source code found for %r at %r" %(name, toplevel)
     return name2src
 
 def compress_mapping(mapping):

doc/en/goodpractises.txt

@@ -169,6 +169,14 @@ You can tell people to download the script and then e.g.  run it like this::
 
     python runtests.py
 
+.. note::
+
+   You must have pytest and its dependencies installed as an sdist, not
+   as wheels because genscript need the source code for generating a
+   standalone script.
+
+
+
 
 Integrating with distutils / ``python setup.py test``
 --------------------------------------------------------