DOCplex has removed one_letter_symbol() from VarType causing problems…

… in Aqua optimization module (qiskit-community#1420)

* fix

* better fix

* added reno

* fix lint

qiskit/optimization/problems/quadratic_program.py

@@ -29,6 +29,7 @@
 from docplex.mp.model import Model
 from docplex.mp.model_reader import ModelReader
 from docplex.mp.quad import QuadExpr
+from docplex.mp.vartype import ContinuousVarType, BinaryVarType, IntegerVarType
 
 from qiskit.aqua import MissingOptionalLibraryError
 from qiskit.aqua.operators import I, OperatorBase, PauliOp, WeightedPauliOperator, SummedOp, ListOp
@@ -560,11 +561,11 @@ def from_docplex(self, model: Model) -> None:
         # keep track of names separately, since docplex allows to have None names.
         var_names = {}
         for x in model.iter_variables():
-            if x.get_vartype().one_letter_symbol() == 'C':
+            if isinstance(x.get_vartype(), ContinuousVarType):
                 x_new = self.continuous_var(x.lb, x.ub, x.name)
-            elif x.get_vartype().one_letter_symbol() == 'B':
+            elif isinstance(x.get_vartype(), BinaryVarType):
                 x_new = self.binary_var(x.name)
-            elif x.get_vartype().one_letter_symbol() == 'I':
+            elif isinstance(x.get_vartype(), IntegerVarType):
                 x_new = self.integer_var(x.lb, x.ub, x.name)
             else:
                 raise QiskitOptimizationError(

releasenotes/notes/docplex-fix-e401d4b511ca5244.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    A method ``one_letter_symbol`` has been removed from the ``VarType`` in the latest
+    build of DOCplex making Aqua incompatible with this version. So instead of using this method
+    an explicit type check of variable types has been introduced in the Aqua optimization module.