Improve performance for large models (#750)

* Avoid allocating an additional LpAffineExpression when creating a LpConstraint comparing against an int or float

* LpConstraint no longer inherits from LpAffineExpression to avoid unnecessary copying of LpAffineExpression's contents

* Avoid creating tempory lists when not needed

* cplex_api: Avoid creating temporary lists in buildSolverModel

* Format with black

* Avoid allocating an extra list in LpAffineExpression.sorted_keys

* Add missing asCplexLpAffineExpression function to LpConstraint

* Add missing isAtomic and atom functions to LpConstraint

* Fix formatting with black

* fixed type of self.constraints

---------

Co-authored-by: pchtsp <pchtsp@gmail.com>

pulp/apis/cplex_api.py

@@ -339,6 +339,8 @@ def buildSolverModel(self, lp):
                 lp.solverModel.objective.set_sense(
                     lp.solverModel.objective.sense.maximize
                 )
+            if lp.objective is None:
+                raise PulpSolverError("No objective set")
             obj = [float(lp.objective.get(var, 0.0)) for var in model_variables]
 
             def cplex_var_lb(var):
@@ -372,15 +374,16 @@ def cplex_var_types(var):
             rows = []
             senses = []
             rhs = []
-            rownames = []
-            for name, constraint in lp.constraints.items():
+            rownames = list(lp.constraints.keys())
+            for constraint in lp.constraints.values():
                 # build the expression
-                expr = [(var.name, float(coeff)) for var, coeff in constraint.items()]
-                if not expr:
+                if len(constraint) == 0:
                     # if the constraint is empty
                     rows.append(([], []))
                 else:
-                    rows.append(list(zip(*expr)))
+                    expr1 = [var.name for var in constraint.keys()]
+                    expr2 = [float(coeff) for coeff in constraint.values()]
+                    rows.append((expr1, expr2))
                 if constraint.sense == constants.LpConstraintLE:
                     senses.append("L")
                 elif constraint.sense == constants.LpConstraintGE:
@@ -389,7 +392,6 @@ def cplex_var_types(var):
                     senses.append("E")
                 else:
                     raise PulpSolverError("Detected an invalid constraint type")
-                rownames.append(name)
                 rhs.append(float(-constraint.constant))
             lp.solverModel.linear_constraints.add(
                 lin_expr=rows, senses=senses, rhs=rhs, names=rownames