Refactor PythonSetup.interpreter_or_constraints() to take a compatibility value rather than a PythonTarget

src/python/pants/backend/python/interpreter_cache.py

@@ -71,7 +71,7 @@ def partition_targets_by_compatibility(self, targets):
 
     for target in targets:
       if isinstance(target, PythonTarget):
-        c = self.python_setup.compatibility_or_constraints(target)
+        c = self.python_setup.compatibility_or_constraints(target.compatibility)
         tgts_by_compatibilities[c].append(target)
         filters.update(c)
     return tgts_by_compatibilities, filters

src/python/pants/backend/python/subsystems/pex_build_util.py

@@ -360,8 +360,10 @@ def add_interpreter_constraints_from(self, constraint_tgts):
     # TODO this would be a great place to validate the constraints and present a good error message
     # if they are incompatible because all the sources of the constraints are available.
     # See: https://github.com/pantsbuild/pex/blob/584b6e367939d24bc28aa9fa36eb911c8297dac8/pex/interpreter_constraints.py
-    constraint_tuples = {self._python_setup_subsystem.compatibility_or_constraints(tgt)
-                         for tgt in constraint_tgts}
+    constraint_tuples = {
+      self._python_setup_subsystem.compatibility_or_constraints(tgt.compatibility)
+      for tgt in constraint_tgts
+    }
     for constraint_tuple in constraint_tuples:
       for constraint in constraint_tuple:
         self.add_interpreter_constraint(constraint)

src/python/pants/backend/python/subsystems/python_setup.py

@@ -122,14 +122,16 @@ def artifact_cache_dir(self):
   def scratch_dir(self):
     return os.path.join(self.get_options().pants_workdir, *self.options_scope.split('.'))
 
-  def compatibility_or_constraints(self, target):
+  def compatibility_or_constraints(self, compatibility):
     """
-    Return either the compatibility of the given target, or the interpreter constraints.
-    If interpreter constraints are supplied by the CLI flag, return those only.
+    Return either the given compatibility, or the interpreter constraints. If interpreter
+    constraints are supplied by the CLI flag, return those only.
+
+    :param compatibility: Optional[List[str]], e.g. None or ['CPython>3'].
     """
     if self.get_options().is_flagged('interpreter_constraints'):
       return tuple(self.interpreter_constraints)
-    return tuple(target.compatibility or self.interpreter_constraints)
+    return tuple(compatibility or self.interpreter_constraints)
 
   @classmethod
   def expand_interpreter_search_paths(cls, interpreter_search_paths, pyenv_root_func=None):

src/python/pants/backend/python/tasks/python_execution_task_base.py

@@ -139,7 +139,7 @@ def create_pex(self, pex_info=None, pin_selected_interpreter=False):
         else:
           constraints = {
             constraint for rt in relevant_targets if is_python_target(rt)
-            for constraint in PythonSetup.global_instance().compatibility_or_constraints(rt)
+            for constraint in PythonSetup.global_instance().compatibility_or_constraints(rt.compatibility)
           }
           self.context.log.debug('target set {} has constraints: {}'
                                  .format(relevant_targets, constraints))

src/python/pants/backend/python/tasks/select_interpreter.py

@@ -29,7 +29,9 @@ def __init__(self, python_setup):
   def compute_fingerprint(self, python_target):
     # Consider the target's compatibility requirements, and if those are missing then fall back
     # to the global interpreter constraints. Only these two values can affect the selected interpreter.
-    hash_elements_for_target = sorted(self.python_setup.compatibility_or_constraints(python_target))
+    hash_elements_for_target = sorted(
+      self.python_setup.compatibility_or_constraints(python_target.compatibility)
+    )
     if not hash_elements_for_target:
       return None
     hasher = hashlib.sha1()

src/python/pants/backend/python/tasks/unpack_wheels.py

@@ -69,7 +69,7 @@ def _get_matching_wheel(self, pex_path, interpreter, requirements, module_name):
 
   @memoized_method
   def _compatible_interpreter(self, unpacked_whls):
-    constraints = PythonSetup.global_instance().compatibility_or_constraints(unpacked_whls)
+    constraints = PythonSetup.global_instance().compatibility_or_constraints(unpacked_whls.compatibility)
     allowable_interpreters = PythonInterpreterCache.global_instance().setup(filters=constraints)
     return min(allowable_interpreters)