Refactor expression classes to Python classes

Converted several expression-related classes from Cython cdef classes to standard Python classes and updated their _evaluate methods to be regular Python methods. This change improves compatibility and maintainability by removing Cython-specific constructs.

Author: Zeroto521

src/pyscipopt/expr.pxi

@@ -15,7 +15,7 @@ def _is_number(e):
         return False
 
 
-cdef class Term:
+class Term:
     """A monomial term consisting of one or more variables."""
 
     __slots__ = ("vars", "ptrs")
@@ -46,7 +46,7 @@ cdef class Term:
     def __repr__(self):
         return f"Term({', '.join(map(str, self.vars))})"
 
-    cdef float _evaluate(self, SCIP* scip, SCIP_SOL* sol):
+    def _evaluate(self, scip, sol):
         if self.vars:
             return math.prod(SCIPgetSolVal(scip, sol, ptr) for ptr in self.ptrs)
         return 1.0  # constant term
@@ -226,7 +226,7 @@ class Expr:
         return nodes
 
 
-cdef class SumExpr(Expr):
+class SumExpr(Expr):
     """Expression like `expression1 + expression2 + constant`."""
 
     def __add__(self, other):
@@ -246,7 +246,7 @@ cdef class SumExpr(Expr):
     def degree(self):
         return float("inf")
 
-    cdef float _evaluate(self, SCIP* scip, SCIP_SOL* sol):
+    def _evaluate(self, scip, sol):
         return _evaluate(self.children, scip, sol)
 
 
@@ -370,7 +370,7 @@ class FuncExpr(Expr):
         return float("inf")
 
 
-cdef class ProdExpr(FuncExpr):
+class ProdExpr(FuncExpr):
     """Expression like `coefficient * expression`."""
 
     def __init__(self, *children, coef: float = 1.0):
@@ -402,11 +402,11 @@ cdef class ProdExpr(FuncExpr):
             return ConstExpr(0.0)
         return self
 
-    cdef float _evaluate(self, SCIP* scip, SCIP_SOL* sol):
+    def _evaluate(self, scip, sol):
         return self.coef * _evaluate(self.children, scip, sol)
 
 
-cdef class PowExpr(FuncExpr):
+class PowExpr(FuncExpr):
     """Expression like `pow(expression, exponent)`."""
 
     def __init__(self, base, expo: float = 1.0):
@@ -426,11 +426,11 @@ cdef class PowExpr(FuncExpr):
             return tuple(self)[0]
         return self
 
-    cdef float _evaluate(self, SCIP* scip, SCIP_SOL* sol):
+    def _evaluate(self, scip, sol):
         return pow(_evaluate(self.children, scip, sol), self.expo)
 
 
-cdef class UnaryExpr(FuncExpr):
+class UnaryExpr(FuncExpr):
     """Expression like `f(expression)`."""
 
     def __init__(self, expr: Expr):
@@ -451,7 +451,7 @@ cdef class UnaryExpr(FuncExpr):
         nodes.append((type(self), start + len(nodes) - 1))
         return nodes
 
-    cdef float _evaluate(self, SCIP* scip, SCIP_SOL* sol):
+    def _evaluate(self, scip, sol):
         return self.op(_evaluate(self.children, scip, sol))