BREAK: switch arguments of BlattWeisskopfSquared

docs/_extend_docstrings.py

@@ -65,9 +65,9 @@ def extend_docstrings() -> None:
 def extend_BlattWeisskopfSquared() -> None:
     from ampform.dynamics import BlattWeisskopfSquared
 
-    L = sp.Symbol("L", integer=True)
     z = sp.Symbol("z", real=True)
-    expr = BlattWeisskopfSquared(L, z)
+    L = sp.Symbol("L", integer=True)
+    expr = BlattWeisskopfSquared(z, angular_momentum=L)
     _append_latex_doit_definition(expr, deep=True, full_width=True)
 
 

docs/usage/dynamics.ipynb

@@ -159,7 +159,7 @@
     "\n",
     "L = sp.Symbol(\"L\", integer=True)\n",
     "z = sp.Symbol(\"z\", real=True)\n",
-    "ff2 = BlattWeisskopfSquared(L, z)\n",
+    "ff2 = BlattWeisskopfSquared(z, L)\n",
     "Math(sp.multiline_latex(ff2, ff2.doit(), environment=\"eqnarray\"))"
    ]
   },
@@ -183,7 +183,7 @@
     "m, m_a, m_b, d = sp.symbols(\"m, m_a, m_b, d\")\n",
     "s = m**2\n",
     "q_squared = BreakupMomentumSquared(s, m_a, m_b)\n",
-    "ff2 = BlattWeisskopfSquared(L, z=q_squared * d**2)"
+    "ff2 = BlattWeisskopfSquared(q_squared * d**2, angular_momentum=L)"
    ]
   },
   {

src/ampform/dynamics/__init__.py

@@ -39,12 +39,12 @@ class BlattWeisskopfSquared(sp.Expr):
     r"""Blatt-Weisskopf function :math:`B_L^2(z)`, up to :math:`L \leq 8`.
 
     Args:
-        angular_momentum: Angular momentum :math:`L` of the decaying particle.
-
         z: Argument of the Blatt-Weisskopf function :math:`B_L^2(z)`. A usual
             choice is :math:`z = (d q)^2` with :math:`d` the impact parameter and
             :math:`q` the breakup-momentum (see `.BreakupMomentumSquared`).
 
+        angular_momentum: Angular momentum :math:`L` of the decaying particle.
+
     Note that equal powers of :math:`z` appear in the nominator and the denominator,
     while some sources have nominator :math:`1`, instead of :math:`z^L`. Compare for
     instance Equation (50.27) in :pdg-review:`2021; Resonances; p.9`.
@@ -57,8 +57,8 @@ class BlattWeisskopfSquared(sp.Expr):
 
     See also :ref:`usage/dynamics:Form factor`.
     """
-    angular_momentum: Any
     z: Any
+    angular_momentum: Any
     _latex_repr_ = R"B_{{{angular_momentum}}}^2\left({z}\right)"
 
     max_angular_momentum: ClassVar[int | None] = None
@@ -69,8 +69,8 @@ class BlattWeisskopfSquared(sp.Expr):
     """
 
     def evaluate(self) -> sp.Expr:
-        angular_momentum: sp.Expr = self.args[0]  # type: ignore[assignment]
-        z: sp.Expr = self.args[1]  # type: ignore[assignment]
+        z: sp.Expr = self.args[0]  # type: ignore[assignment]
+        angular_momentum: sp.Expr = self.args[1]  # type: ignore[assignment]
         cases: dict[int, sp.Expr] = {
             0: sp.S.One,
             1: 2 * z / (z + 1),
@@ -204,12 +204,12 @@ def evaluate(self) -> sp.Expr:
         q_squared = BreakupMomentumSquared(s, m_a, m_b)
         q0_squared = BreakupMomentumSquared(mass0**2, m_a, m_b)  # type: ignore[operator]
         form_factor_sq = BlattWeisskopfSquared(
+            q_squared * meson_radius**2,  # type: ignore[operator]
             angular_momentum,
-            z=q_squared * meson_radius**2,  # type: ignore[operator]
         )
         form_factor0_sq = BlattWeisskopfSquared(
+            q0_squared * meson_radius**2,  # type: ignore[operator]
             angular_momentum,
-            z=q0_squared * meson_radius**2,  # type: ignore[operator]
         )
         rho = self.phsp_factor(s, m_a, m_b)
         rho0 = self.phsp_factor(mass0**2, m_a, m_b)  # type: ignore[operator]
@@ -299,5 +299,5 @@ def formulate_form_factor(s, m_a, m_b, angular_momentum, meson_radius) -> sp.Exp
     `~sympy.functions.elementary.miscellaneous.sqrt` of a `.BlattWeisskopfSquared`.
     """
     q_squared = BreakupMomentumSquared(s, m_a, m_b)
-    ff_squared = BlattWeisskopfSquared(angular_momentum, z=q_squared * meson_radius**2)
+    ff_squared = BlattWeisskopfSquared(q_squared * meson_radius**2, angular_momentum)
     return sp.sqrt(ff_squared)

tests/dynamics/test_dynamics.py

@@ -24,7 +24,7 @@ class TestBlattWeisskopfSquared:
     def test_max_angular_momentum(self):
         z = sp.Symbol("z")
         angular_momentum = sp.Symbol("L", integer=True)
-        form_factor = BlattWeisskopfSquared(angular_momentum, z=z)
+        form_factor = BlattWeisskopfSquared(z, angular_momentum)
         form_factor_9 = form_factor.subs(angular_momentum, 8).evaluate()
         factor, z_power, _ = form_factor_9.args
         assert factor == 4392846440677

tests/dynamics/test_sympy.py

@@ -28,7 +28,7 @@ def test_pickle():
         assert expr == imported_expr
 
         # Pickle classes derived from UnevaluatedExpression
-        expr = BlattWeisskopfSquared(angular_momentum, z=z)
+        expr = BlattWeisskopfSquared(z, angular_momentum)
         pickled_obj = pickle.dumps(expr)
         imported_expr = pickle.loads(pickled_obj)  # noqa: S301
         assert expr == imported_expr

tests/sympy/test_caching.py

@@ -78,17 +78,17 @@ def test_get_readable_hash_large(amplitude_model: tuple[str, HelicityModel]):
         # https://github.com/ComPWA/ampform/actions/runs/3277058875/jobs/5393849802
         # https://github.com/ComPWA/ampform/actions/runs/3277143883/jobs/5394043014
         expected_hash = {
-            "canonical-helicity": "pythonhashseed-0-2810545974244949713",
-            "helicity": "pythonhashseed-0+7376549944210451653",
+            "canonical-helicity": "pythonhashseed-0-3873186712292274641",
+            "helicity": "pythonhashseed-0-8800154542426799839",
         }[formalism]
     elif sys.version_info >= (3, 11):
         expected_hash = {
-            "canonical-helicity": "pythonhashseed-0-3472750730904901316",
-            "helicity": "pythonhashseed-0-6929663921313890862",
+            "canonical-helicity": "pythonhashseed-0+4035132515642199515",
+            "helicity": "pythonhashseed-0-2843057473565885663",
         }[formalism]
     else:
         expected_hash = {
-            "canonical-helicity": "pythonhashseed-0+3198785896067421186",
-            "helicity": "pythonhashseed-0-2637772876619854778",
+            "canonical-helicity": "pythonhashseed-0+3420919389670627445",
+            "helicity": "pythonhashseed-0-6681863313351758450",
         }[formalism]
     assert get_readable_hash(model.expression) == expected_hash