feat: add DataFrame.to_string

Return every row as a pretty-printed table, without the row limit that
repr() and show() apply. Wraps upstream DataFrame::to_string.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: timsaucer

crates/core/src/dataframe.rs

@@ -709,6 +709,12 @@ impl PyDataFrame {
             .to_pyarrow(py)
     }
 
+    /// Execute the DataFrame and render every row as a pretty-printed table
+    fn to_string(&self, py: Python) -> PyDataFusionResult<String> {
+        let df = self.df.as_ref().clone();
+        Ok(wait_for_future(py, df.to_string())??)
+    }
+
     /// Print the result, 20 lines by default
     #[pyo3(signature = (num=20))]
     fn show(&self, py: Python, num: usize) -> PyDataFusionResult<()> {

python/datafusion/dataframe.py

@@ -1018,6 +1018,26 @@ def collect_partitioned(self) -> list[list[pa.RecordBatch]]:
         """
         return self.df.collect_partitioned()
 
+    def to_string(self) -> str:
+        """Execute the DataFrame and return every row as a text table.
+
+        Unlike ``repr(df)`` and :py:meth:`show`, the result is not truncated, so
+        this collects the whole DataFrame into memory. A DataFrame with no rows
+        renders as an empty border with no header.
+
+        Examples:
+            >>> ctx = dfn.SessionContext()
+            >>> df = ctx.from_pydict({"a": [1, 2], "b": ["x", None]})
+            >>> print(df.to_string())
+            +---+---+
+            | a | b |
+            +---+---+
+            | 1 | x |
+            | 2 |   |
+            +---+---+
+        """
+        return self.df.to_string()
+
     def show(self, num: int = 20) -> None:
         """Execute the DataFrame and print the result to the console.
 

python/tests/test_dataframe.py

@@ -3999,3 +3999,20 @@ def test_unnest_columns_with_recursions(input_data, recursions, expected_a):
         kwargs["recursions"] = recursions
     result = df.unnest_columns("a", **kwargs).collect()[0]
     assert result.column(0).to_pylist() == expected_a
+
+
+def test_to_string_includes_every_row(ctx):
+    n = 500
+    df = ctx.from_pydict({"a": list(range(n))})
+    text = df.to_string()
+    # One row per line between the header and footer borders.
+    body = [line for line in text.splitlines() if line.startswith("| ")]
+    assert len(body) == n + 1  # header + rows
+    assert f"| {n - 1}" in text
+    assert "Data truncated" not in text
+
+
+def test_to_string_empty(ctx):
+    # Upstream renders zero rows as a bare border, without the header.
+    df = ctx.from_pydict({"a": [1]}).filter(column("a") > literal(5))
+    assert df.to_string() == "++\n++"