Adding functionality for mode='a' when saving DataFrame.to_json. Only supported when lines=True and orient='records'.

Author: SFuller4

pandas/core/generic.py

@@ -2400,6 +2400,7 @@ def to_json(
         index: bool_t = True,
         indent: int | None = None,
         storage_options: StorageOptions = None,
+        mode: str = 'w',
     ) -> str | None:
         """
         Convert the object to a JSON string.
@@ -2478,6 +2479,11 @@ def to_json(
 
             .. versionadded:: 1.2.0
 
+        mode : str, default 'w' (writing)
+            Specify the IO mode for output when supplying a path_or_buf. 
+            Accepted args are 'w' (writing) and 'a' (append) only. 
+            Supplying mode='a' is only supported when lines is True and orient is 'records'.
+
         Returns
         -------
         None or str
@@ -2661,6 +2667,7 @@ def to_json(
             index=index,
             indent=indent,
             storage_options=storage_options,
+            mode=mode,
         )
 
     @final

pandas/io/json/_json.py

@@ -136,6 +136,7 @@ def to_json(
     index: bool = True,
     indent: int = 0,
     storage_options: StorageOptions = None,
+    mode: str = 'w',
 ) -> str | None:
 
     if not index and orient not in ["split", "table"]:
@@ -146,6 +147,17 @@ def to_json(
     if lines and orient != "records":
         raise ValueError("'lines' keyword only valid when 'orient' is records")
 
+    if mode not in ['a', 'w']:
+        raise ValueError(
+            f"mode={mode!r} is not a valid option. Only 'w' and 'a' are currently supported."
+        )
+
+    if mode == 'a' and (not lines or orient != 'records'):
+        raise ValueError(
+            "mode='a' (append) is only supported when lines is True and orient is 'records'"
+        )
+
+
     if orient == "table" and isinstance(obj, Series):
         obj = obj.to_frame(name=obj.name or "values")
 
@@ -177,7 +189,7 @@ def to_json(
     if path_or_buf is not None:
         # apply compression and byte/text conversion
         with get_handle(
-            path_or_buf, "w", compression=compression, storage_options=storage_options
+            path_or_buf, mode, compression=compression, storage_options=storage_options
         ) as handles:
             handles.handle.write(s)
     else: