fix Beta from_mean constructor bug (#38)

* return value instead of tuple

* relax type hints

conjugate/distributions.py

@@ -50,12 +50,10 @@
 from conjugate.slice import SliceMixin
 
 
-def get_beta_param_from_mean_and_alpha(
-    mean: NUMERIC, alpha: NUMERIC
-) -> Tuple[NUMERIC, NUMERIC]:
+def get_beta_param_from_mean_and_alpha(mean: NUMERIC, alpha: NUMERIC) -> NUMERIC:
     beta = alpha * ((1 / mean) - 1)
 
-    return alpha, beta
+    return beta
 
 
 @dataclass
@@ -75,13 +73,15 @@ def __post_init__(self) -> None:
         self.max_value = 1.0
 
     @classmethod
-    def from_mean(cls, mean: float, alpha: float) -> "Beta":
+    def from_mean(cls, mean: NUMERIC, alpha: NUMERIC) -> "Beta":
         """Alternative constructor from mean and alpha."""
         beta = get_beta_param_from_mean_and_alpha(mean=mean, alpha=alpha)
         return cls(alpha=alpha, beta=beta)
 
     @classmethod
-    def from_successes_and_failures(cls, successes: int, failures: int) -> "Beta":
+    def from_successes_and_failures(
+        cls, successes: NUMERIC, failures: NUMERIC
+    ) -> "Beta":
         """Alternative constructor based on hyperparameter interpretation."""
         alpha = successes + 1
         beta = failures + 1

tests/test_distributions.py

@@ -5,6 +5,7 @@
 import matplotlib.pyplot as plt
 
 from conjugate.distributions import (
+    get_beta_param_from_mean_and_alpha,
     Beta,
     Dirichlet,
     Gamma,
@@ -32,6 +33,16 @@ def test_beta(alpha, beta) -> None:
     assert isinstance(ax, plt.Axes)
 
 
+@pytest.mark.parametrize("mean", [0.025, 0.5, 0.75])
+@pytest.mark.parametrize("alpha", [1, 10, 100])
+def test_beta_mean_constructor(mean: float, alpha: float) -> None:
+    beta = get_beta_param_from_mean_and_alpha(mean, alpha)
+    dist = Beta.from_mean(mean=mean, alpha=alpha)
+
+    assert beta > 0
+    assert dist.beta == beta
+
+
 @pytest.mark.parametrize(
     "alpha, beta, result_alpha, result_beta",
     [