Fix hyperelliptic curve dynamic class construction to allow proper method inheritance

src/sage/schemes/hyperelliptic_curves/constructor.py

@@ -92,11 +92,11 @@
         sage: HyperellipticCurve(x^8 + 1, x)
         Traceback (most recent call last):
         ...
-        ValueError: Not a hyperelliptic curve: highly singular at infinity.
+        ValueError: not a hyperelliptic curve: highly singular at infinity
         sage: HyperellipticCurve(x^8 + x^7 + 1, x^4)
         Traceback (most recent call last):
         ...
-        ValueError: Not a hyperelliptic curve: singularity in the provided affine patch.
+        ValueError: not a hyperelliptic curve: singularity in the provided affine patch
 
         sage: F.<t> = PowerSeriesRing(FiniteField(2))
         sage: P.<x> = PolynomialRing(FractionField(F))
@@ -128,7 +128,7 @@
         sage: HyperellipticCurve((x^3-x+2)^2*(x^6-1))
         Traceback (most recent call last):
         ...
-        ValueError: Not a hyperelliptic curve: singularity in the provided affine patch.
+        ValueError: not a hyperelliptic curve: singularity in the provided affine patch
 
         sage: HyperellipticCurve((x^3-x+2)^2*(x^6-1), check_squarefree=False)
         Hyperelliptic Curve over Finite Field of size 7 defined by
@@ -147,7 +147,7 @@
         sage: HyperellipticCurve(f, h)
         Traceback (most recent call last):
         ...
-        ValueError: Not a hyperelliptic curve: highly singular at infinity.
+        ValueError: not a hyperelliptic curve: highly singular at infinity
 
         sage: HyperellipticCurve(F)
         Hyperelliptic Curve over Rational Field defined by y^2 = x^6 + 1
@@ -160,7 +160,7 @@
         sage: HyperellipticCurve(x^5 + t)
         Traceback (most recent call last):
         ...
-        ValueError: Not a hyperelliptic curve: singularity in the provided affine patch.
+        ValueError: not a hyperelliptic curve: singularity in the provided affine patch
 
     Input with integer coefficients creates objects with the integers
     as base ring, but only checks smoothness over `\QQ`, not over Spec(`\ZZ`).
@@ -203,59 +203,72 @@
     """
     # F is the discriminant; use this for the type check
     # rather than f and h, one of which might be constant.
-    F = h**2 + 4*f
+    F = h**2 + 4 * f
     if not isinstance(F, Polynomial):
-        raise TypeError("Arguments f (= %s) and h (= %s) must be polynomials" % (f, h))
+        raise TypeError(f"arguments {f = } and {h = } must be polynomials")
     P = F.parent()
     f = P(f)
     h = P(h)
     df = f.degree()
-    dh_2 = 2*h.degree()
+    dh_2 = 2 * h.degree()
     if dh_2 < df:
-        g = (df-1)//2
+        g = (df - 1) // 2
     else:
-        g = (dh_2-1)//2
+        g = (dh_2 - 1) // 2
     if check_squarefree:
         # Assuming we are working over a field, this checks that after
         # resolving the singularity at infinity, we get a smooth double cover
         # of P^1.
         if P(2) == 0:
             # characteristic 2
             if h == 0:
-                raise ValueError("In characteristic 2, argument h (= %s) must be non-zero." % h)
-            if h[g+1] == 0 and f[2*g+1]**2 == f[2*g+2]*h[g]**2:
-                raise ValueError("Not a hyperelliptic curve: "
-                                 "highly singular at infinity.")
-            should_be_coprime = [h, f*h.derivative()**2+f.derivative()**2]
+                raise ValueError(
+                    f"for characteristic 2, argument {h = } must be non-zero"
+                )
+            if h[g + 1] == 0 and f[2 * g + 1] ** 2 == f[2 * g + 2] * h[g] ** 2:
+                raise ValueError(
+                    "not a hyperelliptic curve: highly singular at infinity"
+                )
+            should_be_coprime = [h, f * h.derivative() ** 2 + f.derivative() ** 2]
         else:
             # characteristic not 2
-            if F.degree() not in [2*g+1, 2*g+2]:
-                raise ValueError("Not a hyperelliptic curve: "
-                                 "highly singular at infinity.")
+            if F.degree() not in [2 * g + 1, 2 * g + 2]:
+                raise ValueError(
+                    "not a hyperelliptic curve: highly singular at infinity"
+                )
             should_be_coprime = [F, F.derivative()]
         try:
             smooth = should_be_coprime[0].gcd(should_be_coprime[1]).degree() == 0
         except (AttributeError, NotImplementedError, TypeError):
             try:
                 smooth = should_be_coprime[0].resultant(should_be_coprime[1]) != 0
             except (AttributeError, NotImplementedError, TypeError):
-                raise NotImplementedError("Cannot determine whether "
-                      "polynomials %s have a common root. Use "
-                      "check_squarefree=False to skip this check." %
-                      should_be_coprime)
+                raise NotImplementedError(
+                    "cannot determine whether "
+                    f"polynomials {should_be_coprime} have a common root, use "
+                    "check_squarefree=False to skip this check"
+                )
         if not smooth:
-            raise ValueError("Not a hyperelliptic curve: "
-                             "singularity in the provided affine patch.")
+            raise ValueError(
+                "not a hyperelliptic curve: singularity in the provided affine patch"
+            )
     R = P.base_ring()
     PP = ProjectiveSpace(2, R)
     if names is None:
         names = ["x", "y"]
 
-    superclass = []
+    bases = []
     cls_name = ["HyperellipticCurve"]
 
+    # For certain genus we specialise to child classes with
+    # optimised methods
     genus_classes = {2: HyperellipticCurve_g2}
+    if g in genus_classes:
+        bases.append(genus_classes[g])
+        cls_name.append(f"g{g}")
 
+    # For certain base fields, we specialise to subclasses
+    # with special case methods
     def is_FiniteField(x):
         return isinstance(x, FiniteField)
 
@@ -265,19 +278,22 @@
     fields = [
         ("FiniteField", is_FiniteField, HyperellipticCurve_finite_field),
         ("RationalField", is_RationalField, HyperellipticCurve_rational_field),
-        ("pAdicField", is_pAdicField, HyperellipticCurve_padic_field)]
-
-    if g in genus_classes:
-        superclass.append(genus_classes[g])
-        cls_name.append("g%s" % g)
+        ("pAdicField", is_pAdicField, HyperellipticCurve_padic_field),
+    ]
 
     for name, test, cls in fields:
         if test(R):
-            superclass.append(cls)
+            bases.append(cls)
             cls_name.append(name)
             break
 
+    # If no specialised child class was found, we simply use the
+    # generic class in the class construction
+    if not bases:
+        bases = [HyperellipticCurve_generic]
+
+    # Dynamically build a class from multiple inheritance. Note that
+    # all classes we slect from are children of HyperellipticCurve_generic
     class_name = "_".join(cls_name)
-    cls = dynamic_class(class_name, tuple(superclass),
-                        HyperellipticCurve_generic, doccls=HyperellipticCurve)
+    cls = dynamic_class(class_name, tuple(bases), doccls=HyperellipticCurve)
     return cls(PP, f, h, names=names, genus=g)

src/sage/schemes/hyperelliptic_curves/hyperelliptic_g2.py

@@ -45,6 +45,15 @@ def jacobian(self):
             sage: f = x^5 - x^4 + 3
             sage: HyperellipticCurve(f).jacobian()
             Jacobian of Hyperelliptic Curve over Rational Field defined by y^2 = x^5 - x^4 + 3
+
+        TESTS:
+
+        Ensure that :issue:`37612` is fixed::
+
+            sage: R.<x> = QQ[]
+            sage: f = x^5 - x^4 + 3
+            sage: type(HyperellipticCurve(f).jacobian())
+            <class 'sage.schemes.hyperelliptic_curves.jacobian_g2.HyperellipticJacobian_g2_with_category'>
         """
         return jacobian_g2.HyperellipticJacobian_g2(self)