chore(deps)(deps): bump the python-dependencies group with 4 updates (#896)

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Thomas Schmelzer <thomas.schmelzer@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 3 people

pyproject.toml

@@ -11,7 +11,7 @@ dependencies = [
     "narwhals>=2.0.0",
     "numpy>=2.0.0",
     "plotly>=6.1.1",
-    "polars>=1.42.1",
+    "polars>=1.43.0",
     "scipy>=1.14.1"
 ]
 license = "MIT"

src/jquantstats/_utils/_data.py

@@ -284,7 +284,9 @@ def exponential_stdev(self, window: int = 30, is_halflife: bool = False) -> pl.D
 
         Returns:
             DataFrame of rolling EWMA standard deviations with the same
-            columns as the input returns.
+            columns as the input returns.  The first row is ``null``: an
+            unbiased standard deviation of a single observation is
+            undefined, as in ``pandas.Series.ewm(...).std()``.
 
         """
         asset_cols = self._asset_cols()

tests/test_jquantstats/test__utils/test_data_mutation_kills.py

@@ -292,16 +292,21 @@ def test_exponential_stdev_defaults_span_30(varied_data):
     assert default.equals(explicit)
 
 
-def test_exponential_stdev_halflife_first_row_defined(varied_data):
-    """min_samples=1 must yield a value already at the first row (half-life mode)."""
+def test_exponential_stdev_halflife_second_row_defined(varied_data):
+    """min_samples=1 must yield a value from the second row on (half-life mode).
+
+    Row 0 is ``None`` by construction: an unbiased standard deviation of a
+    single observation is undefined (this matches ``pandas.ewm(...).std()``).
+    A larger ``min_samples`` would leave row 1 null too.
+    """
     result = varied_data.utils.exponential_stdev(window=5, is_halflife=True)
-    assert result["A"][0] is not None
+    assert result["A"][1] is not None
 
 
-def test_exponential_stdev_span_first_row_defined(varied_data):
-    """min_samples=1 must yield a value already at the first row (span mode)."""
+def test_exponential_stdev_span_second_row_defined(varied_data):
+    """min_samples=1 must yield a value from the second row on (span mode)."""
     result = varied_data.utils.exponential_stdev(window=5, is_halflife=False)
-    assert result["A"][0] is not None
+    assert result["A"][1] is not None
 
 
 def test_exponential_stdev_halflife_constant_returns_zero():
@@ -310,7 +315,8 @@ def test_exponential_stdev_halflife_constant_returns_zero():
     result = data.utils.exponential_stdev(window=5, is_halflife=True)
     values = result["A"].to_list()
     assert len(values) == 30
-    for v in values:
+    assert values[0] is None
+    for v in values[1:]:
         assert v is not None
         assert abs(v) <= TOL_FLOAT64