time to eat our own dogfood (#1214)

Resolves #1215

Author: gaborbernat

azure-pipelines.yml

@@ -120,8 +120,11 @@ jobs:
           shutil.copy(str(coverage_file), str(destination))'
     displayName: move coverage files into .tox
 
-  - script: 'python -m pip install -U tox --pre'
-    displayName: install tox
+  - script: "python -m pip install -U pip setuptools --user -v"
+      displayName: upgrade pip
+
+  - script: 'python -m pip install . -U --user'
+    displayName: install tox - eat our own dog food
 
   - script: 'python -m tox -e py --sdistonly'
     displayName: generate version.py
@@ -185,7 +188,7 @@ jobs:
     - task: TwineAuthenticate@0
       inputs:
         externalFeeds: 'toxdev'
-    - script: 'python3.7 -m pip install -U twine pip tox'
+    - script: 'python3.7 -m pip install -U twine pip . --user'
       displayName: "acquire build tools"
     - script: 'python3.7 -m pip wheel -w "$(System.DefaultWorkingDirectory)/w" --no-deps .'
       displayName: "build wheel"

azure-run-tox-env.yml

@@ -33,10 +33,10 @@ jobs:
     displayName: show python information
 
   - script: "python -m pip install -U pip setuptools --user -v"
-    displayName: upgrade pip and setuptools
+    displayName: upgrade pip
 
-  - script: "python -m pip install -U tox --pre --user -v"
-    displayName: install tox
+  - script: "python -m pip install -U . --user -v"
+    displayName: install tox - eat our own dog food
 
   - script: ${{ format('python -m tox -e {0} --notest', parameters.tox) }}
     displayName: install test dependencies

docs/changelog/1215.bugfix.rst

@@ -0,0 +1 @@
+Fix tox CI, better error reporting when locating via the py fails - by :user:`gaborbernat`

src/tox/interpreters.py

@@ -9,6 +9,7 @@
 import py
 
 import tox
+from tox import reporter
 from tox.constants import SITE_PACKAGE_QUERY_SCRIPT, VERSION_QUERY_SCRIPT
 
 
@@ -162,13 +163,18 @@ def locate_via_py(*parts):
         ver = "-{}".format(".".join(parts))
         py_exe = distutils.spawn.find_executable("py")
         if py_exe:
+            cmd = py_exe, ver, VERSION_QUERY_SCRIPT
             proc = subprocess.Popen(
-                (py_exe, ver, VERSION_QUERY_SCRIPT),
-                stdout=subprocess.PIPE,
-                stderr=subprocess.PIPE,
-                universal_newlines=True,
+                cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=True
             )
-            out, _ = proc.communicate()
-            result = json.loads(out)
+            out, err = proc.communicate()
             if not proc.returncode:
-                return result["executable"]
+                try:
+                    result = json.loads(out)
+                except ValueError as exception:
+                    failure = exception
+                else:
+                    return result["executable"]
+            else:
+                failure = "exit code {}".format(proc.returncode)
+            reporter.info("{!r} cmd {!r} out {!r} err {!r} ".format(failure, cmd, out, err))