Avoid unconditionally using gast.Ellipsis, which doesn't exist in gas…

…t 0.3.

Also unpin dependency, to allow gast 0.3+
Keeping the pin for the TF1 nightly, because that one is itself broken on gast 0.3+

Fixes tensorflow#552.

PiperOrigin-RevId: 270138305

setup.py

@@ -32,7 +32,7 @@
     'numpy >= 1.13.3',
     'decorator',
     'cloudpickle == 1.1.1',
-    'gast >= 0.2, < 0.3'  # For autobatching
+    'gast >= 0.2'  # For autobatching
 ]
 
 if '--release' in sys.argv:

tensorflow_probability/python/experimental/auto_batching/frontend.py

@@ -130,12 +130,31 @@ def maybe_replace_function_argument(parent, field_name, child):
   return node, ctx
 
 
-_LITERALS = (gast.Num, gast.Str, gast.Bytes, gast.Ellipsis, gast.NameConstant)
+def _is_literal(node):
+  """Detects whether the given node is a literal.
 
+  This is surprisingly difficult to do robustly across versions of Python and
+  gast, as the parsing of constants has changed, if I may, constantly.
 
-def _is_literal(node):
-  if isinstance(node, _LITERALS):
-    return True
+  Args:
+    node: The node whose status to check.
+
+  Returns:
+    literal: A Python `bool` giving whether the node is constant or not.
+  """
+  try:
+    # TODO(b/140808434): Once we or TF decide to roll forward to gast 0.3, we
+    # won't need this clause.
+    # gast pre-0.3
+    literals = (gast.Num, gast.Str, gast.Bytes, gast.Ellipsis,
+                gast.NameConstant)
+    if isinstance(node, literals):
+      return True
+  except AttributeError:
+    # gast 0.3+
+    if isinstance(node, gast.Constant):
+      return True
+  # Python 2
   if isinstance(node, gast.Name) and node.id in ['True', 'False', 'None']:
     return True
   return False

testing/run_tests.sh

@@ -70,7 +70,8 @@ find_version_str() {
 install_python_packages() {
   # NB: tf-nightly pulls in other deps, like numpy, absl, and six, transitively.
   TF_VERSION_STR=$(find_version_str tf-nightly)
-  pip install tf-nightly==$TF_VERSION_STR gast==0.2.2 \
+  pip install tf-nightly==$TF_VERSION_STR \
+    gast==0.2.2 \
     tf-estimator-nightly==1.14.0.dev2019091701
 
   # The following unofficial dependencies are used only by tests.