Make StudentT/AsymmetricLaplace .dist() signatures consistent (#5628)

* make dist signatures consistent
* update docstring and release notes

RELEASE-NOTES.md

@@ -26,6 +26,11 @@ Also check out the [milestones](https://github.com/pymc-devs/pymc/milestones) fo
 
 All of the above apply to:
 
+Signature and default parameters changed for several distributions (see [#5628](https://github.com/pymc-devs/pymc/pull/5628)):
+    - `pm.StudentT` now requires either `sigma` or `lam` as kwarg
+    - `pm.StudentT` now requires `nu` to be specified (no longer defaults to 1)
+    - `pm.AsymmetricLaplace` positional arguments re-ordered
+    - `pm.AsymmetricLaplace` now requires `mu` to be specified (no longer defaults to 0)
 - BART was removed [#5566](https://github.com/pymc-devs/pymc/pull/5566). It is now available from [pymc-experimental](https://github.com/pymc-devs/pymc-experimental)
 - `BaseStochasticGradient` was removed (see [#5630](https://github.com/pymc-devs/pymc/pull/5630))
 - ⚠ The library is now named, installed and imported as "pymc". For example: `pip install pymc`.

pymc/distributions/continuous.py

@@ -1646,12 +1646,12 @@ class AsymmetricLaplace(Continuous):
 
     Parameters
     ----------
-    b : tensor_like of float
-        Scale parameter (b > 0).
     kappa : tensor_like of float
         Symmetry parameter (kappa > 0).
-    mu : tensor_like of float, default 0
+    mu : tensor_like of float
         Location parameter.
+    b : tensor_like of float
+        Scale parameter (b > 0).
 
     See Also:
     --------
@@ -1660,7 +1660,7 @@ class AsymmetricLaplace(Continuous):
     rv_op = asymmetriclaplace
 
     @classmethod
-    def dist(cls, b, kappa, mu=0, *args, **kwargs):
+    def dist(cls, kappa, mu, b, *args, **kwargs):
         b = at.as_tensor_variable(floatX(b))
         kappa = at.as_tensor_variable(floatX(kappa))
         mu = mu = at.as_tensor_variable(floatX(mu))
@@ -1899,7 +1899,7 @@ class StudentT(Continuous):
     rv_op = studentt
 
     @classmethod
-    def dist(cls, nu, mu=0, lam=None, sigma=None, *args, **kwargs):
+    def dist(cls, nu, mu=0, *, sigma=None, lam=None, **kwargs):
         nu = at.as_tensor_variable(floatX(nu))
         lam, sigma = get_tau_sigma(tau=lam, sigma=sigma)
         sigma = at.as_tensor_variable(sigma)
@@ -2712,7 +2712,7 @@ class HalfStudentT(PositiveContinuous):
 
     Parameters
     ----------
-    nu : tensor_like of float, default 1
+    nu : tensor_like of float
         Degrees of freedom, also known as normality parameter (nu > 0).
     sigma : tensor_like of float, optional
         Scale parameter (sigma > 0). Converges to the standard deviation as nu
@@ -2735,7 +2735,7 @@ class HalfStudentT(PositiveContinuous):
     rv_op = halfstudentt
 
     @classmethod
-    def dist(cls, nu=1, sigma=None, lam=None, *args, **kwargs):
+    def dist(cls, nu, sigma=None, lam=None, *args, **kwargs):
         nu = at.as_tensor_variable(floatX(nu))
         lam, sigma = get_tau_sigma(lam, sigma)
         sigma = at.as_tensor_variable(sigma)

pymc/tests/test_distributions.py

@@ -1612,6 +1612,7 @@ def test_half_studentt(self):
             Rplus,
             {"sigma": Rplus},
             lambda value, sigma: sp.halfcauchy.logpdf(value, 0, sigma),
+            extra_args={"nu": 1},
         )
 
     def test_skew_normal(self):