Use "raise from" in except blocks

This is something that pylint should have caught if we were using it (#146). This way, the tracebacks will say > The above exception was the direct cause of the following exception: rather than > During handling of the above exception, another exception occurred:
Qiskit · Sep 12, 2023 · a4f2e11 · a4f2e11
1 parent 48f1e04
commit a4f2e11
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diff --git a/circuit_knitting/cutting/cutting_experiments.py b/circuit_knitting/cutting/cutting_experiments.py
@@ -182,15 +182,15 @@ def _get_mapping_ids_by_partition(
             if isinstance(inst.operation, SingleQubitQPDGate):
                 try:
                     decomp_id = int(inst.operation.label.split("_")[-1])
-                except (AttributeError, ValueError):
+                except (AttributeError, ValueError) as ex:
                     raise ValueError(
                         "SingleQubitQPDGate instances in input circuit(s) must have their "
                         'labels suffixed with "_<id>", where <id> is the index of the cut '
                         "relative to the other cuts in the circuit. For example, all "
                         "SingleQubitQPDGates belonging to the same cut, N, should have labels "
                         ' formatted as "<your_label>_N". This allows SingleQubitQPDGates '
                         "belonging to the same cut to be sampled jointly."
-                    )
+                    ) from ex
                 decomp_ids.add(decomp_id)
                 subcirc_qpd_gate_ids[label].append([i])
                 subcirc_map_ids[label].append(decomp_id)

diff --git a/circuit_knitting/cutting/cutting_reconstruction.py b/circuit_knitting/cutting/cutting_reconstruction.py
@@ -115,11 +115,11 @@ def reconstruct_expectation_values(
                         num_qpd_bits = results_dict[label].metadata[
                             i * len(so.groups) + k
                         ]["num_qpd_bits"]
-                    except KeyError:
+                    except KeyError as ex:
                         raise ValueError(
                             "The num_qpd_bits field must be set in each subexperiment "
                             "result metadata dictionary."
-                        )
+                        ) from ex
                     else:
                         subsystem_expvals[k] += quasi_prob * _process_outcome(
                             num_qpd_bits,
@@ -159,10 +159,10 @@ def _process_outcome(
     outcome = _outcome_to_int(outcome)
     try:
         qpd_outcomes = outcome & ((1 << num_qpd_bits) - 1)
-    except TypeError:
+    except TypeError as ex:
         raise TypeError(
             f"num_qpd_bits must be an integer, but a {type(num_qpd_bits)} was passed."
-        )
+        ) from ex
 
     meas_outcomes = outcome >> num_qpd_bits