feat: Add config setting to control how many List items are printed (#…

…11409)

crates/polars-core/src/config.rs

@@ -13,6 +13,7 @@ pub(crate) const FMT_TABLE_HIDE_DATAFRAME_SHAPE_INFORMATION: &str =
 pub(crate) const FMT_TABLE_INLINE_COLUMN_DATA_TYPE: &str =
     "POLARS_FMT_TABLE_INLINE_COLUMN_DATA_TYPE";
 pub(crate) const FMT_TABLE_ROUNDED_CORNERS: &str = "POLARS_FMT_TABLE_ROUNDED_CORNERS";
+pub(crate) const FMT_TABLE_CELL_LIST_LEN: &str = "POLARS_FMT_TABLE_CELL_LIST_LEN";
 
 // Other env vars
 #[cfg(feature = "dtype-decimal")]

crates/polars-core/src/fmt.rs

@@ -1,6 +1,6 @@
 #[cfg(any(feature = "fmt", feature = "fmt_no_tty"))]
 use std::borrow::Cow;
-use std::fmt::{Debug, Display, Formatter};
+use std::fmt::{Debug, Display, Formatter, Write};
 use std::sync::atomic::{AtomicU8, Ordering};
 use std::{fmt, str};
 
@@ -949,12 +949,32 @@ impl Series {
                 self.get(1).unwrap(),
                 self.get(2).unwrap()
             ),
-            _ => format!(
-                "[{}, {}, … {}]",
-                self.get(0).unwrap(),
-                self.get(1).unwrap(),
-                self.get(self.len() - 1).unwrap()
-            ),
+            _ => {
+                let max_items = std::env::var(FMT_TABLE_CELL_LIST_LEN)
+                    .as_deref()
+                    .unwrap_or("")
+                    .parse()
+                    .map_or(3, |n: i64| if n < 0 { self.len() } else { n as usize });
+
+                let mut result = "[".to_owned();
+
+                for (i, item) in self.iter().enumerate() {
+                    write!(result, "{item}").unwrap();
+
+                    if i != self.len() - 1 {
+                        result.push_str(", ");
+                    }
+
+                    if i == max_items - 2 {
+                        result.push_str("… ");
+                        write!(result, "{}", self.get(self.len() - 1).unwrap()).unwrap();
+                        break;
+                    }
+                }
+                result.push(']');
+
+                result
+            },
         }
     }
 }
@@ -1089,19 +1109,31 @@ mod test {
     fn test_fmt_list() {
         let mut builder =
             ListPrimitiveChunkedBuilder::<Int32Type>::new("a", 10, 10, DataType::Int32);
-        builder.append_opt_slice(Some(&[1, 2, 3]));
+        builder.append_opt_slice(Some(&[1, 2, 3, 4, 5, 6]));
         builder.append_opt_slice(None);
         let list = builder.finish().into_series();
 
         assert_eq!(
             r#"shape: (2,)
 Series: 'a' [list[i32]]
 [
-	[1, 2, 3]
+	[1, 2, … 6]
 	null
 ]"#,
             format!("{:?}", list)
         );
+
+        std::env::set_var("POLARS_FMT_TABLE_CELL_LIST_LEN", "10");
+
+        assert_eq!(
+            r#"shape: (2,)
+Series: 'a' [list[i32]]
+[
+	[1, 2, 3, 4, 5, 6]
+	null
+]"#,
+            format!("{:?}", list)
+        )
     }
 
     #[test]

py-polars/polars/config.py

@@ -69,6 +69,7 @@ def _get_float_fmt() -> str:  # pragma: no cover
     "POLARS_FMT_TABLE_HIDE_DATAFRAME_SHAPE_INFORMATION",
     "POLARS_FMT_TABLE_INLINE_COLUMN_DATA_TYPE",
     "POLARS_FMT_TABLE_ROUNDED_CORNERS",
+    "POLARS_FMT_TABLE_CELL_LIST_LEN",
     "POLARS_STREAMING_CHUNK_SIZE",
     "POLARS_TABLE_WIDTH",
     "POLARS_VERBOSE",
@@ -527,6 +528,54 @@ def set_fmt_str_lengths(cls, n: int | None) -> type[Config]:
             os.environ["POLARS_FMT_STR_LEN"] = str(n)
         return cls
 
+    @classmethod
+    def set_fmt_table_cell_list_len(cls, n: int | None) -> type[Config]:
+        """
+        Set the number of elements to display for List values.
+
+        Values less than 0 will result in all values being printed.
+
+        Parameters
+        ----------
+        n : int
+            Number of values to display.
+
+        Examples
+        --------
+        >>> df = pl.DataFrame(
+        ...     {
+        ...         "nums": [
+        ...             [1, 2, 3, 4, 5, 6],
+        ...         ]
+        ...     }
+        ... )
+        >>> df
+        shape: (1, 1)
+        ┌─────────────┐
+        │ nums        │
+        │ ---         │
+        │ list[i64]   │
+        ╞═════════════╡
+        │ [1, 2, … 6] │
+        └─────────────┘
+        >>> with pl.Config(fmt_table_cell_list_len=10):
+        ...     print(df)
+        ...
+        shape: (1, 1)
+        ┌────────────────────┐
+        │ nums               │
+        │ ---                │
+        │ list[i64]          │
+        ╞════════════════════╡
+        │ [1, 2, 3, 4, 5, 6] │
+        └────────────────────┘
+        """
+        if n is None:
+            os.environ.pop("POLARS_FMT_TABLE_CELL_LIST_LEN", None)
+        else:
+            os.environ["POLARS_FMT_TABLE_CELL_LIST_LEN"] = str(n)
+        return cls
+
     @classmethod
     def set_streaming_chunk_size(cls, size: int | None) -> type[Config]:
         """