Add iterator support for AbstractJob (quantumlib#5136)

- Add support for next() in AbstractJob
- Changes __iter__ to return a generator, which
will have support for iteration in for loops, as well as
next().
- Also improve the iteration tests to be more thorough and accurate.

Fixes: quantumlib#5120

cirq-google/cirq_google/engine/abstract_job.py

@@ -181,7 +181,7 @@ def calibration_results(self) -> Sequence['calibration_result.CalibrationResult'
         """
 
     def __iter__(self) -> Iterator[cirq.Result]:
-        return iter(self.results())
+        yield from self.results()
 
     # pylint: disable=function-redefined
     @overload

cirq-google/cirq_google/engine/abstract_job_test.py

@@ -12,6 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from typing import Dict, List, TYPE_CHECKING
+import pytest
+import numpy as np
+import cirq
 from cirq_google.engine.abstract_job import AbstractJob
 
 if TYPE_CHECKING:
@@ -83,17 +86,40 @@ def batched_results(self):
         pass
 
     def results(self):
-        return list(range(5))
+        return list(
+            cirq.ResultDict(params={}, measurements={'a': np.asarray([t])}) for t in range(5)
+        )
 
     def calibration_results(self):
         pass
 
 
 def test_instantiation_and_iteration():
     job = MockJob()
+
+    # Test length
     assert len(job) == 5
-    assert job[3] == 3
+
+    # Test direct indexing
+    assert job[3].measurements['a'][0] == 3
+
+    #  Test iterating through for loop
     count = 0
-    for num in job:
-        assert num == count
+    for result in job:
+        assert result.measurements['a'][0] == count
         count += 1
+
+    # Test iterator using iterator
+    iterator = iter(job)
+    result = next(iterator)
+    assert result.measurements['a'][0] == 0
+    result = next(iterator)
+    assert result.measurements['a'][0] == 1
+    result = next(iterator)
+    assert result.measurements['a'][0] == 2
+    result = next(iterator)
+    assert result.measurements['a'][0] == 3
+    result = next(iterator)
+    assert result.measurements['a'][0] == 4
+    with pytest.raises(StopIteration):
+        next(iterator)