Allow empty BackendResult (#974)

pytket/docs/changelog.rst

@@ -13,6 +13,10 @@ Minor new features:
   be overridden using the ``always_squash_symbols`` parameter to
   ``SquashCustom``.
 
+Fixes:
+
+* Allow ``BackendResult`` objects containing no results.
+
 1.18.0 (August 2023)
 --------------------
 

pytket/pytket/backends/backendresult.py

@@ -138,7 +138,10 @@ def _process_unitids(
                     raise ValueError(
                         "Provide either counts or shots, both is not valid."
                     )
-                _bitlength = next(self._counts.elements()).width
+                try:
+                    _bitlength = next(self._counts.elements()).width
+                except StopIteration:
+                    _bitlength = len(c_bits)
 
             if self._shots is not None:
                 _bitlength = self._shots.width
@@ -252,6 +255,7 @@ def _get_measured_res(
                     Counter({outcome.choose_indices(chosen_readouts): count})
                     for outcome, count in new_counts.items()
                 ),
+                Counter(),
             )
         if self._shots is not None:
             if ppcirc is not None:

pytket/tests/backend_test.py

@@ -589,6 +589,12 @@ def test_postprocess_4() -> None:
     assert all(readout[0] == 1 for readout in counts.keys())
 
 
+def test_empty_backenresult() -> None:
+    # https://github.com/CQCL/tket/issues/961
+    r = BackendResult(counts=Counter(), c_bits=[Bit(name="A", index=0)])
+    assert r.get_counts() == Counter()
+
+
 if __name__ == "__main__":
     # test_resulthandle()
     # test_bell()