Do not reinstall in develop mode without setup.py

The detection logic for figuring out whether a editable install needs to
be reinstalled only works for setuptools based builds using setup.py.
If setup.py cannot be found the code will fail with an invocation error.
Currently no standard mechanism exists to query a package installed in
editable mode whether it should be reinstalled or not. Hence, this
change skips reinstallation if setup.py cannot be found.

Fixes tox-dev#2197

CONTRIBUTORS

@@ -114,6 +114,7 @@ Tim Laurence
 Tyagraj Desigar
 Usama Sadiq
 Ville Skyttä
+Vincent Vanlaer
 Vlastimil Zíma
 Xander Johnson
 anatoly techtonik

docs/changelog/2197.bugfix.rst

@@ -0,0 +1 @@
+Fixed an issue where ``usedevelop`` would cause an invocation error if setup.py does not exist. -- by :user:`VincentVanlaer`

docs/config.rst

@@ -596,7 +596,9 @@ Complete list of settings that you can put into ``testenv*`` sections:
     Install the current package in development mode with "setup.py
     develop" instead of installing from the ``sdist`` package. (This
     uses pip's ``-e`` option, so should be avoided if you've specified a
-    custom :conf:`install_command` that does not support ``-e``).
+    custom :conf:`install_command` that does not support ``-e``). Note that
+    changes to the build/install process (including changes in dependencies)
+    are only detected when using setuptools with setup.py.
 
 .. conf:: skip_install ^ true|false ^ false
 

src/tox/venv.py

@@ -325,6 +325,10 @@ def finish(self):
 
     def _needs_reinstall(self, setupdir, action):
         setup_py = setupdir.join("setup.py")
+
+        if not setup_py.exists():
+            return False
+
         setup_cfg = setupdir.join("setup.cfg")
         args = [self.envconfig.envpython, str(setup_py), "--name"]
         env = self._get_os_environ()