Improvement #2: show all discovered interpreters

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

@@ -27,15 +27,15 @@
 
 # TODO: Move under subsystems/ .
 class PythonInterpreterCache(Subsystem):
-  """Finds python interpreters on the local system."""
+  """Finds Python interpreters on the local system."""
   options_scope = 'python-interpreter-cache'
 
   @classmethod
   def subsystem_dependencies(cls):
     return super(PythonInterpreterCache, cls).subsystem_dependencies() + (PythonSetup,)
 
   class UnsatisfiableInterpreterConstraintsError(TaskError):
-    """Indicates a python interpreter matching given constraints could not be located."""
+    """Indicates a Python interpreter matching given constraints could not be located."""
 
   @staticmethod
   def _matches(interpreter, filters=()):
@@ -89,17 +89,27 @@ def select_interpreter_for_targets(self, targets):
 
     if not allowed_interpreters:
       # Create a helpful error message.
+      all_interpreter_version_strings = sorted({
+        interpreter.version_string
+        # NB: self.setup requires filters to avoid using the global interpreter constraints.
+        # We allow any interpreter other than 3.0-3.3, which is known to choke with Pants.
+        for interpreter in self.setup(filters=("CPython<3", "CPython>=3.3", "PyPy"))
+      })
       unique_compatibilities = {tuple(c) for c in tgts_by_compatibilities.keys()}
       unique_compatibilities_strs = [','.join(x) for x in unique_compatibilities if x]
       tgts_by_compatibilities_strs = [t[0].address.spec for t in tgts_by_compatibilities.values()]
       raise self.UnsatisfiableInterpreterConstraintsError(dedent("""\
         Unable to detect a suitable interpreter for compatibilities: {} (Conflicting targets: {})
 
-        To fix this, either modify your Python interpreter constraints by following
-        https://www.pantsbuild.org/python_readme.html#configure-the-python-version or install the
-        targeted interpreter on your system.""".format(
+        Pants detected these interpreter versions on your system: {}
+
+        Possible ways to fix this:
+        * Modify your Python interpreter constraints by following https://www.pantsbuild.org/python_readme.html#configure-the-python-version.
+        * Ensure the targeted Python version is installed and discoverable.
+        * Modify Pants' interpreter search paths via --pants-setup-interpreter-search-paths.""".format(
           ' && '.join(sorted(unique_compatibilities_strs)),
-          ', '.join(tgts_by_compatibilities_strs)
+          ', '.join(tgts_by_compatibilities_strs),
+          ', '.join(all_interpreter_version_strings)
         )))
     # Return the lowest compatible interpreter.
     return min(allowed_interpreters)