feat: improve error handling of column.stability when given a colum…

…n that contains only None

Co-authored-by: patrikguempel <128832338+patrikguempel@users.noreply.github.com>

src/safeds/data/tabular/containers/_column.py

@@ -504,6 +504,8 @@ def stability(self) -> float:
         \frac{\text{number of occurrences of most common non-null value}}{\text{number of non-null values}}
         $$
 
+        The stability cannot be calculated for a column with only null values.
+
         Returns
         -------
         stability : float
@@ -516,6 +518,10 @@ def stability(self) -> float:
         """
         if self._data.size == 0:
             raise ColumnSizeError("> 0", "0")
+
+        if self.all(lambda x: x is None):
+            raise ValueError("Stability cannot be calculated for a column with only null values.")
+
         return self._data.value_counts()[self.mode()[0]] / self._data.count()
 
     def standard_deviation(self) -> float:

tests/safeds/data/tabular/containers/_column/test_stability.py

@@ -25,7 +25,13 @@ def test_should_return_stability_of_column(values: list[Any], expected: float) -
     assert column.stability() == expected
 
 
-def test_should_raise_if_column_is_empty() -> None:
+def test_should_raise_column_size_error_if_column_is_empty() -> None:
     column: Column[Any] = Column("A", [])
-    with pytest.raises(ColumnSizeError):
+    with pytest.raises(ColumnSizeError, match="Expected a column of size > 0 but got column of size 0."):
+        column.stability()
+
+
+def test_should_raise_value_error_if_column_contains_only_none() -> None:
+    column: Column[Any] = Column("A", [None, None])
+    with pytest.raises(ValueError, match="Stability cannot be calculated for a column with only null values."):
         column.stability()