Align mypy interpreter constraint with reality. (pantsbuild#7754)

The mypy tool actually requires python 3.5+. On a machine with 3.4 and
3.x higher the minimum interpreter selection scheme would pick 3.4 and
fail.

Fixup interpreter constraints to avoid this erroneous error and correct
the corresponding test. Cleanup dead code in
interpreter_selection_utils.py along the way.

contrib/mypy/src/python/pants/contrib/mypy/tasks/mypy_task.py

@@ -27,6 +27,7 @@
 class MypyTask(ResolveRequirementsTaskBase):
   """Invoke the mypy static type analyzer for Python."""
 
+  _MYPY_COMPATIBLE_INTERPETER_CONSTRAINT = '>=3.5'
   _PYTHON_SOURCE_EXTENSION = '.py'
 
   @classmethod
@@ -48,8 +49,10 @@ def supports_passthru_args(cls):
   def subsystem_dependencies(cls):
     return super(MypyTask, cls).subsystem_dependencies() + (PythonInterpreterCache,)
 
-  def find_py3_interpreter(self):
-    interpreters = self._interpreter_cache.setup(filters=['>=3'])
+  def find_mypy_interpreter(self):
+    interpreters = self._interpreter_cache.setup(
+      filters=[self._MYPY_COMPATIBLE_INTERPETER_CONSTRAINT]
+    )
     return min(interpreters) if interpreters else None
 
   @staticmethod
@@ -101,17 +104,20 @@ def _run_mypy(self, py3_interpreter, mypy_args, **kwargs):
     return pex.run(mypy_args, **kwargs)
 
   def execute(self):
-    py3_interpreter = self.find_py3_interpreter()
-    if not py3_interpreter:
-      raise TaskError('Unable to find a Python 3.x interpreter (required for mypy).')
+    mypy_interpreter = self.find_mypy_interpreter()
+    if not mypy_interpreter:
+      raise TaskError('Unable to find a Python {} interpreter (required for mypy).'
+                      .format(self._MYPY_COMPATIBLE_INTERPETER_CONSTRAINT))
 
     sources = self._calculate_python_sources(self.context.target_roots)
     if not sources:
       self.context.log.debug('No Python sources to check.')
       return
 
     # Determine interpreter used by the sources so we can tell mypy.
-    interpreter_for_targets = self._interpreter_cache.select_interpreter_for_targets(self.context.target_roots)
+    interpreter_for_targets = self._interpreter_cache.select_interpreter_for_targets(
+      self.context.target_roots
+    )
     if not interpreter_for_targets:
       raise TaskError('No Python interpreter compatible with specified sources.')
 
@@ -141,7 +147,7 @@ def execute(self):
         log_config=WorkUnit.LogConfig(level=self.get_options().level,
                                       colors=self.get_options().colors),
         cmd=' '.join(cmd)) as workunit:
-        returncode = self._run_mypy(py3_interpreter, cmd,
+        returncode = self._run_mypy(mypy_interpreter, cmd,
           env={'MYPYPATH': mypy_path}, stdout=workunit.output('stdout'), stderr=subprocess.STDOUT)
         if returncode != 0:
           raise TaskError('mypy failed: code={}'.format(returncode))

contrib/mypy/tests/python/pants_test/contrib/mypy/tasks/test_mypy_integration.py

@@ -4,7 +4,7 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-from pants_test.backend.python.interpreter_selection_utils import PY_3, has_python_version
+from pants_test.backend.python.interpreter_selection_utils import has_python_version
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest
 
 
@@ -16,13 +16,14 @@ def test_mypy(self):
       '--',
       '--follow-imports=silent'
     ]
-    if has_python_version(PY_3):
-      # Python 3.x is available. Test that we see an error in this integration test.
+
+    if any((has_python_version('3.{}'.format(v)) for v in (5, 6, 7, 8))):
+      # Python 3.5+ is available. Test that we see an error in this integration test.
       with self.pants_results(cmd) as pants_run:
         self.assert_success(pants_run)
     else:
-      # Python 3.x was not found. Test whether mypy task fails for that reason.
+      # Python 3.5+ was not found. Test whether mypy task fails for that reason.
       with self.pants_results(cmd) as pants_run:
         self.assert_failure(pants_run)
-        self.assertIn('Unable to find a Python 3.x interpreter (required for mypy)',
+        self.assertIn('Unable to find a Python >=3.5 interpreter (required for mypy)',
                       pants_run.stdout_data)

tests/python/pants_test/backend/python/interpreter_selection_utils.py

@@ -15,20 +15,8 @@
 PY_2 = '2'
 PY_3 = '3'
 
-PY_26 = '2.6'
 PY_27 = '2.7'
-PY_34 = '3.4'
-PY_35 = '3.5'
 PY_36 = '3.6'
-PY_37 = '3.7'
-PY_38 = '3.8'
-
-
-def find_all_pythons_present(*versions):
-  """Return sorted list of all Python versions present on the system."""
-  if not versions:
-    versions = {PY_26, PY_27, PY_34, PY_35, PY_36, PY_37, PY_38}
-  return sorted(version for version in versions if has_python_version(version))
 
 
 def has_python_version(version):
@@ -58,16 +46,6 @@ def python_interpreter_path(version):
     return None
 
 
-def skip_unless_any_pythons_present(*versions):
-  """A decorator that only runs the decorated test method if any of the specified pythons are present.
-
-  :param string *versions: Python version strings, such as 2.7, 3.
-  """
-  if any(v for v in versions if has_python_version(v)):
-    return skipIf(False, 'At least one of the expected python versions found.')
-  return skipIf(True, 'Could not find at least one of the required pythons from {} on the system. Skipping.'.format(versions))
-
-
 def skip_unless_all_pythons_present(*versions):
   """A decorator that only runs the decorated test method if all of the specified pythons are present.