API/BUG: Make to_json index= arg consistent with orient arg

pandas/core/generic.py

@@ -2307,7 +2307,7 @@ def to_json(
         default_handler: Callable[[Any], JSONSerializable] | None = None,
         lines: bool_t = False,
         compression: CompressionOptions = "infer",
-        index: bool_t = True,
+        index: bool_t | None = None,
         indent: int | None = None,
         storage_options: StorageOptions = None,
         mode: Literal["a", "w"] = "w",
@@ -2376,10 +2376,11 @@ def to_json(
 
             .. versionchanged:: 1.4.0 Zstandard support.
 
-        index : bool, default True
-            Whether to include the index values in the JSON string. Not
-            including the index (``index=False``) is only supported when
-            orient is 'split' or 'table'.
+        index : bool or None, default None
+            The index is only used when 'orient' is 'split', 'index', 'column',
+            or 'table'. Of these, 'index' and 'column' do not support
+            `index=False`.
+
         indent : int, optional
            Length of whitespace used to indent each record.
 

pandas/io/json/_json.py

@@ -100,7 +100,7 @@ def to_json(
     default_handler: Callable[[Any], JSONSerializable] | None = ...,
     lines: bool = ...,
     compression: CompressionOptions = ...,
-    index: bool = ...,
+    index: bool | None = ...,
     indent: int = ...,
     storage_options: StorageOptions = ...,
     mode: Literal["a", "w"] = ...,
@@ -120,7 +120,7 @@ def to_json(
     default_handler: Callable[[Any], JSONSerializable] | None = ...,
     lines: bool = ...,
     compression: CompressionOptions = ...,
-    index: bool = ...,
+    index: bool | None = ...,
     indent: int = ...,
     storage_options: StorageOptions = ...,
     mode: Literal["a", "w"] = ...,
@@ -139,15 +139,24 @@ def to_json(
     default_handler: Callable[[Any], JSONSerializable] | None = None,
     lines: bool = False,
     compression: CompressionOptions = "infer",
-    index: bool = True,
+    index: bool | None = None,
     indent: int = 0,
     storage_options: StorageOptions = None,
     mode: Literal["a", "w"] = "w",
 ) -> str | None:
-    if not index and orient not in ["split", "table"]:
+    if orient in ["records", "values"] and index is True:
         raise ValueError(
-            "'index=False' is only valid when 'orient' is 'split' or 'table'"
+            "'index=True' is only valid when 'orient' is 'split', 'table', "
+            "'index', or 'columns'. Convert index to column for other orients."
         )
+    elif orient in ["index", "columns"] and index is False:
+        raise ValueError(
+            "'index=False' is only valid when 'orient' is 'split', 'table', "
+            "'records', or 'values'. Convert index to column for other orients."
+        )
+    elif index is None:
+        # will be ignored for orient='records' and 'values'
+        index = True
 
     if lines and orient != "records":
         raise ValueError("'lines' keyword only valid when 'orient' is records")

pandas/tests/io/json/test_pandas.py

@@ -1472,17 +1472,34 @@ def test_index_false_to_json_table(self, data):
 
         assert result == expected
 
-    @pytest.mark.parametrize("orient", ["records", "index", "columns", "values"])
+    @pytest.mark.parametrize("orient", ["index", "columns"])
     def test_index_false_error_to_json(self, orient):
-        # GH 17394
+        # GH 17394, 25513
         # Testing error message from to_json with index=False
 
         df = DataFrame([[1, 2], [4, 5]], columns=["a", "b"])
 
-        msg = "'index=False' is only valid when 'orient' is 'split' or 'table'"
+        msg = (
+            "'index=False' is only valid when 'orient' is 'split', "
+            "'table', 'records', or 'values'"
+        )
         with pytest.raises(ValueError, match=msg):
             df.to_json(orient=orient, index=False)
 
+    @pytest.mark.parametrize("orient", ["records", "values"])
+    def test_index_true_error_to_json(self, orient):
+        # GH 25513
+        # Testing error message from to_json with index=True
+
+        df = DataFrame([[1, 2], [4, 5]], columns=["a", "b"])
+
+        msg = (
+            "'index=True' is only valid when 'orient' is 'split', "
+            "'table', 'index', or 'columns'"
+        )
+        with pytest.raises(ValueError, match=msg):
+            df.to_json(orient=orient, index=True)
+
     @pytest.mark.parametrize("orient", ["split", "table"])
     @pytest.mark.parametrize("index", [True, False])
     def test_index_false_from_json_to_json(self, orient, index):