GH-39961: [C++][Python] Propagate CSV parse delimiter to write options

cpp/src/arrow/dataset/file_csv.cc

@@ -474,6 +474,7 @@ std::shared_ptr<FileWriteOptions> CsvFileFormat::DefaultWriteOptions() {
       new CsvFileWriteOptions(shared_from_this()));
   csv_options->write_options =
       std::make_shared<csv::WriteOptions>(csv::WriteOptions::Defaults());
+  csv_options->write_options->delimiter = parse_options.delimiter;
   return csv_options;
 }
 

cpp/src/arrow/dataset/file_csv_test.cc

@@ -449,6 +449,22 @@ TEST_P(TestCsvFileFormat, WriteRecordBatchReaderCustomOptions) {
   ASSERT_EQ("0\n0\n0\n0\n0\n", written->ToString());
 }
 
+TEST_P(TestCsvFileFormat, WriteRecordBatchReaderDelimiterPropagation) {
+  // Verify that CsvFileFormat::DefaultWriteOptions() propagates the parse
+  // delimiter to write options
+  auto csv_format = std::make_shared<CsvFileFormat>();
+  csv_format->parse_options.delimiter = '|';
+  auto write_options =
+      checked_pointer_cast<CsvFileWriteOptions>(csv_format->DefaultWriteOptions());
+  ASSERT_EQ(write_options->write_options->delimiter, '|');
+
+  // Verify that the default delimiter is ',' when no parse options are set
+  auto default_format = std::make_shared<CsvFileFormat>();
+  auto default_write_options =
+      checked_pointer_cast<CsvFileWriteOptions>(default_format->DefaultWriteOptions());
+  ASSERT_EQ(default_write_options->write_options->delimiter, ',');
+}
+
 TEST_P(TestCsvFileFormat, CountRows) { TestCountRows(); }
 
 TEST_P(TestCsvFileFormat, FragmentEquals) { TestFragmentEquals(); }

python/pyarrow/_dataset.pyx

@@ -2255,7 +2255,17 @@ cdef class CsvFileFormat(FileFormat):
         """
         cdef CsvFileWriteOptions opts = \
             <CsvFileWriteOptions> FileFormat.make_write_options(self)
-        opts.write_options = WriteOptions(**kwargs)
+        # Start from the C++ defaults, which carry over fields from the
+        # format's parse_options (e.g. the delimiter), and apply caller
+        # overrides on top instead of replacing the WriteOptions object
+        # and discarding those defaults. None is treated as "unspecified"
+        # to match the previous WriteOptions(**kwargs) semantics.
+        write_options = opts.write_options
+        for key, value in kwargs.items():
+            if value is None:
+                continue
+            setattr(write_options, key, value)
+        opts.write_options = write_options
         return opts
 
     @property

python/pyarrow/tests/test_dataset.py

@@ -4987,6 +4987,47 @@ def test_write_dataset_csv(tempdir):
     assert result.equals(table)
 
 
+def test_csv_make_write_options_uses_parse_delimiter():
+    # CsvFileFormat.make_write_options propagates the parse delimiter
+    # to write options when no explicit delimiter is given
+    csv_format = ds.CsvFileFormat(pa.csv.ParseOptions(delimiter="|"))
+    write_opts = csv_format.make_write_options()
+    assert write_opts.write_options.delimiter == "|"
+
+    # delimiter=None is treated as "unspecified" and the propagated
+    # value is preserved (matches WriteOptions(**kwargs) semantics)
+    write_opts = csv_format.make_write_options(delimiter=None)
+    assert write_opts.write_options.delimiter == "|"
+
+    # An explicitly passed delimiter takes precedence
+    csv_format = ds.CsvFileFormat(pa.csv.ParseOptions(delimiter=">"))
+    write_opts = csv_format.make_write_options(delimiter="|")
+    assert write_opts.write_options.delimiter == "|"
+
+    # The default delimiter is still "," when no parse options are given
+    csv_format = ds.CsvFileFormat()
+    write_opts = csv_format.make_write_options()
+    assert write_opts.write_options.delimiter == ","
+
+
+def test_write_dataset_uses_csv_parse_delimiter(tempdir):
+    table = pa.table({
+        "B": ["B1", "B2"],
+        "C": ["C1", "C2"],
+    })
+
+    csv_format = ds.CsvFileFormat(pa.csv.ParseOptions(delimiter=">"))
+    ds.write_dataset(table, tempdir, format=csv_format)
+
+    with open(tempdir / "part-0.csv") as fh:
+        content = fh.read()
+    assert content == '"B">"C"\n"B1">"C1"\n"B2">"C2"\n'
+
+    # Roundtrip: reading back with the same delimiter recovers the table
+    result = ds.dataset(tempdir, format=csv_format).to_table()
+    assert result.equals(table)
+
+
 @pytest.mark.parquet
 def test_write_dataset_parquet_file_visitor(tempdir):
     table = pa.table([