From e262502135e69b6fc3de23e67ecee2a914ab1be7 Mon Sep 17 00:00:00 2001
From: Olivier Delalleau <507137+odelalleau@users.noreply.github.com>
Date: Thu, 25 Feb 2021 20:17:23 -0500
Subject: [PATCH] Support and deprecate empty args

---
 omegaconf/grammar/OmegaConfGrammarParser.g4 |  2 +-
 omegaconf/grammar_visitor.py                | 28 ++++++++++++++++++---
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/omegaconf/grammar/OmegaConfGrammarParser.g4 b/omegaconf/grammar/OmegaConfGrammarParser.g4
index eaa496d8f..58000e2b6 100644
--- a/omegaconf/grammar/OmegaConfGrammarParser.g4
+++ b/omegaconf/grammar/OmegaConfGrammarParser.g4
@@ -42,7 +42,7 @@ element:
 listContainer: BRACKET_OPEN sequence? BRACKET_CLOSE;                         // [], [1,2,3], [a,b,[1,2]]
 dictContainer: BRACE_OPEN (dictKeyValuePair (COMMA dictKeyValuePair)*)? BRACE_CLOSE;  // {}, {a:10,b:20}
 dictKeyValuePair: dictKey COLON element;
-sequence: element (COMMA element)*;
+sequence: (element (COMMA element?)*) | (COMMA element?)+;
 
 // Interpolations.
 
diff --git a/omegaconf/grammar_visitor.py b/omegaconf/grammar_visitor.py
index debfa7f52..8f8b87863 100644
--- a/omegaconf/grammar_visitor.py
+++ b/omegaconf/grammar_visitor.py
@@ -1,4 +1,5 @@
 import sys
+import warnings
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -246,20 +247,41 @@ def visitSequence(
     ) -> Generator[Any, None, None]:
         from ._utils import _get_value
 
-        assert ctx.getChildCount() >= 1  # element (COMMA element)*
+        # (element (COMMA element?)*) | (COMMA element?)+
+        assert ctx.getChildCount() >= 1
+
+        def empty_str_warning() -> None:
+            txt = ctx.getText()
+            warnings.warn(
+                f"In the sequence `{txt}` some elements are missing: please replace "
+                f"them with empty strings (\"\" or '') since this will not be supported "
+                f"anymore in the future.",
+                category=UserWarning,
+            )
+
+        is_previous_comma = True  # whether previous child was a comma (init to True)
         for i, child in enumerate(ctx.getChildren()):
-            if i % 2 == 0:
-                assert isinstance(child, OmegaConfGrammarParser.ElementContext)
+            if isinstance(child, OmegaConfGrammarParser.ElementContext):
                 # Also preserve the original text representation of `child` so
                 # as to allow backward compatibility with old resolvers (registered
                 # with `legacy_register_resolver()`). Note that we cannot just cast
                 # the value to string later as for instance `null` would become "None".
                 yield _get_value(self.visitElement(child)), child.getText()
+                is_previous_comma = False
             else:
                 assert (
                     isinstance(child, TerminalNode)
                     and child.symbol.type == OmegaConfGrammarLexer.COMMA
                 )
+                if is_previous_comma:
+                    empty_str_warning()
+                    yield "", ""
+                else:
+                    is_previous_comma = True
+        if is_previous_comma:
+            # Trailing comma.
+            empty_str_warning()
+            yield "", ""
 
     def visitSingleElement(
         self, ctx: OmegaConfGrammarParser.SingleElementContext