Fix printing of results object when it contains no state (#367)

* Fix bug affecting printing of results class

* fixed

* fixed

* Update .github/CHANGELOG.md

Co-Authored-By: antalszava <antalszava@gmail.com>

* Update strawberryfields/api/connection.py

Co-authored-by: antalszava <antalszava@gmail.com>

.github/CHANGELOG.md

@@ -190,6 +190,9 @@
   due to floating point precision error.
   [(#364)](https://github.com/XanaduAI/strawberryfields/pull/364)
 
+* Fixed a bug that caused an exception when printing results with no state.
+  [(#367)](https://github.com/XanaduAI/strawberryfields/pull/367)
+
 * Improves the Takagi decomposition, by making explicit use of the eigendecomposition of real symmetric matrices. [(#352)](https://github.com/XanaduAI/strawberryfields/pull/352)
 
 <h3>Contributors</h3>

strawberryfields/api/connection.py

@@ -227,7 +227,15 @@ def get_job_result(self, job_id: str) -> Result:
             with io.BytesIO() as buf:
                 buf.write(response.content)
                 buf.seek(0)
+
                 samples = np.load(buf, allow_pickle=False)
+
+                if np.issubdtype(samples.dtype, np.integer):
+                    # Samples represent photon numbers.
+                    # Convert to int64, to avoid unexpected behaviour
+                    # when users postprocess these samples.
+                    samples = samples.astype(np.int64)
+
             return Result(samples, is_stateful=False)
         raise RequestFailedError(
             "Failed to get job result: {}".format(self._format_error_message(response))

strawberryfields/api/result.py

@@ -107,8 +107,9 @@ def state(self):
             raise AttributeError("The state is undefined for a stateless computation.")
         return self._state
 
-    def __str__(self):
+    def __repr__(self):
         """String representation."""
-        return "Result: {} subsystems, state: {}\n samples: {}".format(
-            len(self.samples), self.state, self.samples
+        shots, modes = self.samples.shape
+        return "<Result: num_modes={}, shots={}, contains state={}>".format(
+            modes, shots, self._is_stateful
         )

tests/api/test_result.py

@@ -14,6 +14,7 @@
 """
 Unit tests for strawberryfields.api.result
 """
+import numpy as np
 import pytest
 
 from strawberryfields.api import Result
@@ -29,9 +30,27 @@ class TestResult:
     def test_stateless_result_raises_on_state_access(self):
         """Tests that `result.state` raises an error for a stateless result.
         """
-        result = Result([[1, 2], [3, 4]], is_stateful=False)
+        result = Result(np.array([[1, 2], [3, 4]]), is_stateful=False)
 
         with pytest.raises(
             AttributeError, match="The state is undefined for a stateless computation."
         ):
             result.state
+
+    def test_stateless_print(self, capfd):
+        """Test that printing a result object with no state provides the correct output."""
+        result = Result(np.array([[1, 2], [3, 4], [5, 6]]), is_stateful=False)
+        print(result)
+        out, err = capfd.readouterr()
+        assert "modes=2" in out
+        assert "shots=3" in out
+        assert "contains state=False" in out
+
+    def test_state_print(self, capfd):
+        """Test that printing a result object with a state provides the correct output."""
+        result = Result(np.array([[1, 2], [3, 4], [5, 6]]), is_stateful=True)
+        print(result)
+        out, err = capfd.readouterr()
+        assert "modes=2" in out
+        assert "shots=3" in out
+        assert "contains state=True" in out