Fix single-return stateful printing

Zac-HD · Zac-HD · commit 4d00351dbdbc · 2022-02-21T13:36:22.000-08:00
diff --git a/hypothesis-python/RELEASE.rst b/hypothesis-python/RELEASE.rst
@@ -0,0 +1,6 @@
+RELEASE_TYPE: patch
+
+This patch fixes a bug in stateful testing, where returning a single value
+wrapped in :func:`~hypothesis.stateful.multiple` would be printed such that
+the assigned variable was a tuple rather than the single element
+(:issue:`3236`).
diff --git a/hypothesis-python/src/hypothesis/stateful.py b/hypothesis-python/src/hypothesis/stateful.py
@@ -333,16 +333,16 @@ def invariants(cls):
 
     def _print_step(self, rule, data, result):
         self.step_count = getattr(self, "step_count", 0) + 1
-        # If the step has target bundles, and the result is a MultipleResults
-        # then we want to assign to multiple variables.
-        if isinstance(result, MultipleResults):
-            n_output_vars = len(result.values)
-        else:
-            n_output_vars = 1
-        if rule.targets and n_output_vars >= 1:
-            output_assignment = ", ".join(self._last_names(n_output_vars)) + " = "
-        else:
-            output_assignment = ""
+        output_assignment = ""
+        if rule.targets:
+            if isinstance(result, MultipleResults):
+                if len(result.values) == 1:
+                    output_assignment = f"({self._last_names(1)[0]},) = "
+                elif result.values:
+                    output_names = self._last_names(len(result.values))
+                    output_assignment = ", ".join(output_names) + " = "
+            else:
+                output_assignment = self._last_names(1)[0] + " = "
         report(
             "{}state.{}({})".format(
                 output_assignment,
diff --git a/hypothesis-python/tests/cover/test_stateful.py b/hypothesis-python/tests/cover/test_stateful.py
@@ -228,6 +228,32 @@ def fail_fast(self):
         state.fail_fast()
 
 
+def test_multiple_variables_printed_single_element():
+    # https://github.com/HypothesisWorks/hypothesis/issues/3236
+    class ProducesMultiple(RuleBasedStateMachine):
+        b = Bundle("b")
+
+        @initialize(target=b)
+        def populate_bundle(self):
+            return multiple(1)
+
+        @rule(b=b)
+        def fail_fast(self, b):
+            assert b != 1
+
+    with capture_out() as o, raises(AssertionError):
+        run_state_machine_as_test(ProducesMultiple)
+
+    assignment_line = o.getvalue().split("\n")[2]
+    assert assignment_line == "(v1,) = state.populate_bundle()"
+
+    state = ProducesMultiple()
+    (v1,) = state.populate_bundle()
+    state.fail_fast((v1,))  # passes if tuple not unpacked
+    with raises(AssertionError):
+        state.fail_fast(v1)
+
+
 def test_no_variables_printed():
     class ProducesNoVariables(RuleBasedStateMachine):
         b = Bundle("b")
