add sign (#2977)

py-polars/docs/source/reference/expression.rst

@@ -162,6 +162,7 @@ Computations
     Expr.log
     Expr.log10
     Expr.exp
+    Expr.sign
     Expr.unique_counts
     Expr.value_counts
 

py-polars/docs/source/reference/series.rst

@@ -140,6 +140,7 @@ Computations
     Series.log
     Series.log10
     Series.exp
+    Series.sign
 
 Manipulation/ selection
 -----------------------

py-polars/polars/internals/expr.py

@@ -2446,6 +2446,12 @@ def upper_bound(self) -> "Expr":
         """
         return wrap_expr(self._pyexpr.upper_bound())
 
+    def sign(self) -> "Expr":
+        """
+        Returns an element-wise indication of the sign of a number.
+        """
+        return np.sign(self)  # type: ignore
+
     def sin(self) -> "Expr":
         """
         Compute the element-wise value for Trigonometric sine on an array

py-polars/polars/internals/series.py

@@ -2355,6 +2355,12 @@ def mode(self) -> "Series":
         """
         return wrap_s(self._s.mode())
 
+    def sign(self) -> "Series":
+        """
+        Returns an element-wise indication of the sign of a number.
+        """
+        return np.sign(self)  # type: ignore
+
     def sin(self) -> "Series":
         """
         Compute the element-wise value for Trigonometric sine.

py-polars/tests/test_series.py

@@ -1494,3 +1494,9 @@ def test_str_split() -> None:
         assert out[0].to_list() == ["a,", " b"]
         assert out[1].to_list() == ["a"]
         assert out[2].to_list() == ["ab,", "c,", "de"]
+
+
+def test_sign() -> None:
+    a = pl.Series("a", [10, -20, None])
+    expected = pl.Series("a", [1, -1, None])
+    verify_series_and_expr_api(a, expected, "sign")