csv don't skip delimiter in whitespace trimming (#3796)

polars/polars-io/src/csv_core/csv.rs

@@ -285,7 +285,7 @@ impl<'a> CoreReader<'a> {
         let starting_point_offset = bytes.as_ptr() as usize;
 
         // Skip all leading white space and the occasional utf8-bom
-        bytes = skip_whitespace(skip_bom(bytes));
+        bytes = skip_whitespace_exclude(skip_bom(bytes), self.delimiter);
         // \n\n can be a empty string row of a single column
         // in other cases we skip it.
         if self.schema.len() > 1 {

py-polars/tests/io/test_csv.py

@@ -437,3 +437,15 @@ def test_glob_csv(io_test_dir: str) -> None:
     path = os.path.join(io_test_dir, "small*.csv")
     assert pl.scan_csv(path).collect().shape == (3, 11)
     assert pl.read_csv(path).shape == (3, 11)
+
+
+def test_csv_whitepsace_do_not_skip() -> None:
+    csv = "\t\t\t\t0\t1"
+    assert pl.read_csv(csv.encode(), sep="\t", has_header=False).to_dict(False) == {
+        "column_1": [None],
+        "column_2": [None],
+        "column_3": [None],
+        "column_4": [None],
+        "column_5": [0],
+        "column_6": [1],
+    }