#1227 fix crash when proposed base python has no version data

docs/changelog/1227.bugfix.rst

@@ -0,0 +1 @@
+don't crash when version information is not available for a proposed base python - by :user:`gaborbernat`

src/tox/config/__init__.py

@@ -561,21 +561,23 @@ def basepython_default(testenv_config, value):
         proposed_python = (implied_python or sys.executable) if value is None else str(value)
         if implied_python is not None and implied_python != proposed_python:
             testenv_config.basepython = proposed_python
-            implied_version = tox.PYTHON.PY_FACTORS_RE.match(factor).group(2)
-            python_info_for_proposed = testenv_config.python_info
-            if not isinstance(python_info_for_proposed, NoInterpreterInfo):
-                proposed_version = "".join(
-                    str(i) for i in python_info_for_proposed.version_info[0:2]
-                )
-                # '27'.startswith('2') or '27'.startswith('27')
-                if not proposed_version.startswith(implied_version):
-                    # TODO(stephenfin): Raise an exception here in tox 4.0
-                    warnings.warn(
-                        "conflicting basepython version (set {}, should be {}) for env '{}';"
-                        "resolve conflict or set ignore_basepython_conflict".format(
-                            proposed_version, implied_version, testenv_config.envname
-                        )
+            match = tox.PYTHON.PY_FACTORS_RE.match(factor)
+            implied_version = match.group(2) if match else None
+            if implied_version is not None:
+                python_info_for_proposed = testenv_config.python_info
+                if not isinstance(python_info_for_proposed, NoInterpreterInfo):
+                    proposed_version = "".join(
+                        str(i) for i in python_info_for_proposed.version_info[0:2]
                     )
+                    # '27'.startswith('2') or '27'.startswith('27')
+                    if not proposed_version.startswith(implied_version):
+                        # TODO(stephenfin): Raise an exception here in tox 4.0
+                        warnings.warn(
+                            "conflicting basepython version (set {}, should be {}) for env '{}';"
+                            "resolve conflict or set ignore_basepython_conflict".format(
+                                proposed_version, implied_version, testenv_config.envname
+                            )
+                        )
         return proposed_python
 
     parser.add_testenv_attribute(

tests/unit/config/test_config.py

@@ -3061,3 +3061,17 @@ def test_posargs_relative_changedir(newconfig, tmpdir):
             dir1.strpath,
             "dir3",
         ]
+
+
+def test_config_no_version_data_in__name(newconfig, capsys):
+    newconfig(
+        """
+        [tox]
+        envlist = py, pypy, jython
+        [testenv]
+        basepython = python
+        """
+    )
+    out, err = capsys.readouterr()
+    assert not out
+    assert not err