If newer versions are using Jedi (e.g. at the moment Python 3.7), it shouldn't just result in a grammar issue, just because that grammar doesn't not exist. Just take the Python 3.6 grammar instead.

jedi/_compatibility.py

@@ -233,6 +233,11 @@ try:
 except ImportError:
     from itertools import izip_longest as zip_longest  # Python 2
 
+try:
+    FileNotFoundError = FileNotFoundError
+except NameError:
+    FileNotFoundError = IOError
+
 
 def no_unicode_pprint(dct):
     """

jedi/parser/__init__.py

@@ -18,6 +18,7 @@ complexity of the ``Parser`` (there's another parser sitting inside
 import os
 import re
 
+from jedi._compatibility import FileNotFoundError
 from jedi.parser import tree as pt
 from jedi.parser import tokenize
 from jedi.parser.token import (DEDENT, INDENT, ENDMARKER, NEWLINE, NUMBER,
@@ -40,7 +41,7 @@ class ParseError(Exception):
     """
 
 
-def load_grammar(version='3.4'):
+def load_grammar(version='3.6'):
     # For now we only support two different Python syntax versions: The latest
     # Python 3 and Python 2. This may change.
     if version in ('3.2', '3.3'):
@@ -55,7 +56,11 @@ def load_grammar(version='3.4'):
     try:
         return _loaded_grammars[path]
     except KeyError:
+        try:
             return _loaded_grammars.setdefault(path, generate_grammar(path))
+        except FileNotFoundError:
+            # Just load the default if the file does not exist.
+            return load_grammar()
 
 
 class ParserSyntaxError(object):

test/test_parser/test_parser.py

@@ -234,3 +234,12 @@ def test_python3_octal():
         assert module.children[0].children[0].type == 'number'
     else:
         assert module.children[0].type == 'error_node'
+
+
+def test_load_newer_grammar():
+    # This version shouldn't be out for a while, but if we somehow get this it
+    # should just take the latest Python grammar.
+    load_grammar('15.8')
+    # The same is true for very old grammars (even though this is probably not
+    # going to be an issue.
+    load_grammar('1.5')