feat[rust]: show problematic values in failed cast (#4803)

pola-rs · Sep 12, 2022 · e0167a4 · e0167a4
1 parent af2d161
commit e0167a4
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Showing 2 changed files with 12 additions and 4 deletions.
diff --git a/polars/polars-core/src/series/mod.rs b/polars/polars-core/src/series/mod.rs
@@ -673,14 +673,18 @@ impl Series {
     pub fn strict_cast(&self, data_type: &DataType) -> Result<Series> {
         let s = self.cast(data_type)?;
         if self.null_count() != s.null_count() {
+            let failure_mask = !self.is_null() & s.is_null();
+            let failures = self.filter_threaded(&failure_mask, false)?.unique()?;
             Err(PolarsError::ComputeError(
                 format!(
-                    "strict conversion of cast from {:?} to {:?} failed. consider non-strict cast.\n
-                    If you were trying to cast Utf8 to Date,Time,Datetime, consider using `strptime`",
+                    "Strict conversion from {:?} to {:?} failed for values {}. \
+                    If you were trying to cast Utf8 to Date, Time, or Datetime, \
+                    consider using `strptime`.",
                     self.dtype(),
-                    data_type
+                    data_type,
+                    failures.fmt_list(),
                 )
-                    .into(),
+                .into(),
             ))
         } else {
             Ok(s)

diff --git a/py-polars/tests/unit/test_series.py b/py-polars/tests/unit/test_series.py
@@ -331,6 +331,10 @@ def test_cast() -> None:
     assert a.cast(pl.Datetime).dtype == pl.Datetime
     assert a.cast(pl.Date).dtype == pl.Date
 
+    # display failed values, GH#4706
+    with pytest.raises(pl.ComputeError, match="foobar"):
+        pl.Series(["1", "2", "3", "4", "foobar"]).cast(int)
+
 
 def test_to_python() -> None:
     a = pl.Series("a", range(20))