fix(python): support corr() for single-column DataFrames (#13728)

Co-authored-by: Wainberg <m.wainberg@utoronto.ca>

py-polars/polars/dataframe/frame.py

@@ -10220,7 +10220,10 @@ def corr(self, **kwargs: Any) -> DataFrame:
         │ 1.0  ┆ -1.0 ┆ 1.0  │
         └──────┴──────┴──────┘
         """
-        return DataFrame(np.corrcoef(self.to_numpy().T, **kwargs), schema=self.columns)
+        correlation_matrix = np.corrcoef(self.to_numpy(), rowvar=False, **kwargs)
+        if self.width == 1:
+            correlation_matrix = np.array([correlation_matrix])
+        return DataFrame(correlation_matrix, schema=self.columns)
 
     def merge_sorted(self, other: DataFrame, key: str) -> DataFrame:
         """

py-polars/tests/unit/operations/test_statistics.py

@@ -9,6 +9,11 @@
 
 
 def test_corr() -> None:
+    df = pl.DataFrame({"a": [1, 2, 3]})
+    result = df.corr()
+    expected = pl.DataFrame({"a": [1.0]})
+    assert_frame_equal(result, expected)
+
     df = pl.DataFrame(
         {
             "a": [1, 2, 4],