fix(scan_csv): handle empty csv file exception (#2934)

polars/polars-lazy/src/logical_plan/builder.rs

@@ -127,7 +127,9 @@ impl LogicalPlanBuilder {
         let path = path.into();
         let mut file = std::fs::File::open(&path)?;
         let mut magic_nr = [0u8; 2];
-        file.read_exact(&mut magic_nr)?;
+        file.read_exact(&mut magic_nr)
+            .map_err(|_| PolarsError::NoData("empty csv".into()))?;
+
         if is_compressed(&magic_nr) {
             return Err(PolarsError::ComputeError(
                 "cannot scan compressed csv; use read_csv for compressed data".into(),

py-polars/tests/io/test_lazy_csv.py

@@ -1,8 +1,10 @@
 # flake8: noqa: W191,E101
+import io
 from os import path
 from pathlib import Path
 
 import numpy as np
+import pytest
 
 import polars as pl
 
@@ -12,6 +14,12 @@ def test_scan_csv() -> None:
     assert df.collect().shape == (4, 3)
 
 
+def test_scan_empty_csv() -> None:
+    with pytest.raises(Exception) as excinfo:
+        pl.scan_csv(Path(__file__).parent.parent / "files" / "empty.csv").collect()
+    assert str(excinfo.value) == "empty csv"
+
+
 def test_invalid_utf8() -> None:
     np.random.seed(1)
     bts = bytes(np.random.randint(0, 255, 200))