fix(rust): properly display other length in simple project and displa…

…y more columns up to a maximum length (#16464)

Co-authored-by: Stijn de Gooijer <[email protected]>

crates/polars-plan/src/logical_plan/alp/format.rs

@@ -623,14 +623,34 @@ pub(crate) struct ColumnsDisplay<'a>(pub(crate) &'a Schema);
 impl fmt::Display for ColumnsDisplay<'_> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let len = self.0.len();
-        let mut iter_names = self.0.iter_names();
+        let mut iter_names = self.0.iter_names().enumerate();
 
-        if let Some(fst) = iter_names.next() {
+        const MAX_LEN: usize = 32;
+        const ADD_PER_ITEM: usize = 4;
+
+        let mut current_len = 0;
+
+        if let Some((_, fst)) = iter_names.next() {
             write!(f, "\"{fst}\"")?;
 
-            if len > 0 {
-                write!(f, ", ... {len} other columns")?;
+            current_len += fst.len() + ADD_PER_ITEM;
+        }
+
+        for (i, col) in iter_names {
+            current_len += col.len() + ADD_PER_ITEM;
+
+            if current_len > MAX_LEN {
+                write!(f, ", ... {} other ", len - i)?;
+                if len - i == 1 {
+                    f.write_str("column")?;
+                } else {
+                    f.write_str("columns")?;
+                }
+
+                break;
             }
+
+            write!(f, ", \"{col}\"")?;
         }
 
         Ok(())

py-polars/tests/unit/test_format.py

@@ -417,3 +417,55 @@ def test_fmt_decimal_max_scale() -> None:
 	0.14282911023321884847623576259639164703
 ]"""
     assert result == expected
+
+
+@pytest.mark.parametrize(
+    ("lf", "expected"),
+    [
+        (
+            (
+                pl.LazyFrame({"a": [1]})
+                .with_columns(b=pl.col("a"))
+                .with_columns(c=pl.col("b"), d=pl.col("a"))
+            ),
+            'simple π 4/4 ["a", "b", "c", "d"]',
+        ),
+        (
+            (
+                pl.LazyFrame({"a_very_very_long_string": [1], "a": [1]})
+                .with_columns(b=pl.col("a"))
+                .with_columns(c=pl.col("b"), d=pl.col("a"))
+            ),
+            'simple π 5/5 ["a_very_very_long_string", "a", ... 3 other columns]',
+        ),
+        (
+            (
+                pl.LazyFrame({"an_even_longer_very_very_long_string": [1], "a": [1]})
+                .with_columns(b=pl.col("a"))
+                .with_columns(c=pl.col("b"), d=pl.col("a"))
+            ),
+            'simple π 5/5 ["an_even_longer_very_very_long_string", ... 4 other columns]',
+        ),
+        (
+            (
+                pl.LazyFrame({"a": [1]})
+                .with_columns(b=pl.col("a"))
+                .with_columns(c=pl.col("b"), a_very_long_string_at_the_end=pl.col("a"))
+            ),
+            'simple π 4/4 ["a", "b", "c", ... 1 other column]',
+        ),
+        (
+            (
+                pl.LazyFrame({"a": [1]})
+                .with_columns(b=pl.col("a"))
+                .with_columns(
+                    a_very_long_string_in_the_middle=pl.col("b"), d=pl.col("a")
+                )
+            ),
+            'simple π 4/4 ["a", "b", ... 2 other columns]',
+        ),
+    ],
+)
+def test_simple_project_format(lf: pl.LazyFrame, expected: str) -> None:
+    result = lf.explain()
+    assert expected in result