Assert that tox uses the appropriate Python version

smoke/venv.sh

@@ -86,6 +86,23 @@ set -u
 
 # use it with tox
 [[ "$TOX" != "true" ]] && exit 0
+if [[ $PYTHON == python* ]]; then
+    export TOXENV=py${VERSION/./}
+else
+    export TOXENV=$PYTHON
+fi
+
+cat > tox.ini << EOF
+[tox]
+skipsdist = True
+[testenv]
+commands = python -c 'import sys; print(sys.version)'
+EOF
+
+# Tests that the Python version used in tox is really the Python version we want
+tox | tee toxlog
+grep "^$VERSION\.[0-9]" toxlog
+
 cat > tox.ini << EOF
 [tox]
 skipsdist = True
@@ -94,8 +111,5 @@ deps = pytest
 commands = python -m pytest -v test_foo.py
 EOF
 
-if [[ $PYTHON == python* ]]; then
-    tox -e py${VERSION/./}
-else
-    tox -e $PYTHON
-fi
+# A more complex example with pytest
+tox