Python API - delta.appendOnly enforcement

.gitignore

@@ -12,4 +12,5 @@ tlaplus/*.toolbox/*/[0-9]*-[0-9]*-[0-9]*-[0-9]*-[0-9]*-[0-9]*/
 .vscode
 .env
 **/.DS_Store
-**/.python-version
+**/.python-version
+.coverage

python/deltalake/writer.py

@@ -66,13 +66,18 @@ def write_deltalake(
     :param filesystem: Optional filesystem to pass to PyArrow. If not provided will
         be inferred from uri.
     :param mode: How to handle existing data. Default is to error if table
-        already exists. If 'append', will add new data. If 'overwrite', will
-        replace table with new data. If 'ignore', will not write anything if
-        table already exists.
+        already exists. 
+        If 'append', will add new data.
+        If 'overwrite', will replace table with new data. 
+        If 'ignore', will not write anything if table already exists.
     :param name: User-provided identifier for this table.
     :param description: User-provided description for this table.
     :param configuration: A map containing configuration options for the metadata action.
     """
+    config_delta_append_only = configuration and configuration.get("delta.appendOnly", "false") == "true"
+    if config_delta_append_only and mode != "append":
+        raise ValueError("If configuration has delta.appendOnly = 'true', mode must be 'append'")
+
     if isinstance(data, pd.DataFrame):
         data = pa.Table.from_pandas(data)
 

python/tests/test_writer.py

@@ -172,6 +172,24 @@ def test_write_modes(tmp_path: pathlib.Path, sample_data: pa.Table):
     assert DeltaTable(path).to_pyarrow_table() == sample_data
 
 
+def test_append_only(tmp_path: pathlib.Path, sample_data: pa.Table):
+    path = str(tmp_path)
+
+    config = {"delta.appendOnly": "true"}
+    write_deltalake(path, sample_data, mode="append", configuration=config)
+
+    with pytest.raises(ValueError):
+        write_deltalake(path, sample_data, configuration=config)
+
+    with pytest.raises(ValueError):
+        write_deltalake(path, sample_data, configuration=config, mode="overwrite")
+
+    with pytest.raises(ValueError):
+        write_deltalake(path, sample_data, configuration=config, mode="ignore")
+
+    assert DeltaTable(path).to_pyarrow_table() == sample_data
+
+
 def test_writer_with_table(existing_table: DeltaTable, sample_data: pa.Table):
     write_deltalake(existing_table, sample_data, mode="overwrite")
     existing_table.update_incremental()