feat[rust, python]: allow min/max on utf8 dtype

polars/polars-core/src/chunked_array/ops/aggregate.rs

@@ -813,23 +813,27 @@ impl ChunkAggSeries for BooleanChunked {
     }
 }
 
-macro_rules! one_null_utf8 {
-    ($self:ident) => {{
-        let mut builder = Utf8ChunkedBuilder::new($self.name(), 1, 0);
-        builder.append_null();
-        builder.finish().into_series()
-    }};
-}
-
 impl ChunkAggSeries for Utf8Chunked {
     fn sum_as_series(&self) -> Series {
-        one_null_utf8!(self)
+        Utf8Chunked::full_null(self.name(), 1).into_series()
     }
     fn max_as_series(&self) -> Series {
-        one_null_utf8!(self)
+        Series::new(
+            self.name(),
+            &[self
+                .downcast_iter()
+                .filter_map(compute::aggregate::max_string)
+                .fold_first_(|acc, v| if acc > v { acc } else { v })],
+        )
     }
     fn min_as_series(&self) -> Series {
-        one_null_utf8!(self)
+        Series::new(
+            self.name(),
+            &[self
+                .downcast_iter()
+                .filter_map(compute::aggregate::min_string)
+                .fold_first_(|acc, v| if acc < v { acc } else { v })],
+        )
     }
 }
 

py-polars/polars/internals/series/series.py

@@ -911,7 +911,7 @@ def product(self) -> int | float:
         """Reduce this Series to the product value."""
         return self.to_frame().select(pli.col(self.name).product()).to_series()[0]
 
-    def min(self) -> int | float | date | datetime | timedelta:
+    def min(self) -> int | float | date | datetime | timedelta | str:
         """
         Get the minimal value in this Series.
 
@@ -924,7 +924,7 @@ def min(self) -> int | float | date | datetime | timedelta:
         """
         return self._s.min()
 
-    def max(self) -> int | float | date | datetime | timedelta:
+    def max(self) -> int | float | date | datetime | timedelta | str:
         """
         Get the maximum value in this Series.
 

py-polars/tests/test_utf8.py

@@ -0,0 +1,7 @@
+import polars as pl
+
+
+def test_min_max_agg_on_str() -> None:
+    strings = ["b", "a", "x"]
+    s = pl.Series(strings)
+    assert (s.min(), s.max()) == ("a", "x")