Fix UnsatisfiableInterpreterConstraintsError. (#1024)

Handle the case when there are no candidates and add a test for this.

pex/interpreter_constraints.py

@@ -44,14 +44,17 @@ def create_message(self, preamble=None):
         :return: A descriptive message useable for display to an end user.
         :rtype: str
         """
+        preamble = "{}\n\n".format(preamble) if preamble else ""
+        if not self.candidates:
+            return "{}No interpreters could be found on the system.".format(preamble)
         binary_column_width = max(len(candidate.binary) for candidate in self.candidates)
         interpreters_format = "{{binary: >{}}} {{requirement}}".format(binary_column_width)
         return (
             "{preamble}"
             "Examined the following interpreters:\n  {interpreters}\n\n"
             "None were compatible with the requested constraints:\n  {constraints}"
         ).format(
-            preamble="{}\n\n".format(preamble) if preamble else "",
+            preamble=preamble,
             interpreters="\n  ".join(
                 interpreters_format.format(
                     binary=candidate.binary, requirement=candidate.identity.requirement

tests/test_pex_bootstrapper.py

@@ -64,6 +64,11 @@ def test_find_compatible_interpreters():
     )
 
 
+def test_find_compatible_interpreters_none():
+    with pytest.raises(UnsatisfiableInterpreterConstraintsError):
+        find_interpreters([os.path.devnull], ">2")
+
+
 def test_find_compatible_interpreters_bias_current():
     py36 = ensure_python_interpreter(PY36)
     assert [os.path.realpath(sys.executable), py36] == find_interpreters([py36, sys.executable])