[Stable] Fix fitter selection logic in process tomography (#431)

qiskit/ignis/verification/tomography/fitters/process_fitter.py

@@ -21,7 +21,7 @@
 from qiskit import QiskitError
 from qiskit.quantum_info.operators import Choi
 from .base_fitter import TomographyFitter
-from .cvx_fit import cvxpy, cvx_fit
+from .cvx_fit import cvx_fit
 from .lstsq_fit import lstsq_fit
 
 
@@ -137,10 +137,11 @@ def fit(self,  # pylint: disable=arguments-differ
                              "to a process matrix.")
         # Choose automatic method
         if method == 'auto':
-            if cvxpy is None:
-                method = 'lstsq'
-            else:
+            self._check_for_sdp_solver()
+            if self._HAS_SDP_SOLVER:
                 method = 'cvx'
+            else:
+                method = 'lstsq'
         if method == 'lstsq':
             return Choi(lstsq_fit(data, basis_matrix, weights=weights,
                                   trace=dim, **kwargs))

requirements-dev.txt

@@ -1,4 +1,3 @@
-cvxpy>=1.0.15
 pylint==2.4.4
 pycodestyle
 qiskit-aer>=0.3.0

tox.ini

@@ -13,6 +13,7 @@ setenv =
 deps = numpy>=1.13
        Cython>=0.27.1
        setuptools>=40.1.0
+       cvxpy>=1.0.15
 commands =
     pip install -U -c constraints.txt qiskit-terra
     pip install -U -c constraints.txt -r{toxinidir}/requirements-dev.txt