Merge pull request #10891 from eric-wieser/assert-no-cycles

TST: Extract a helper function to test for reference cycles

1 files changed, 75 insertions, 1 deletions

diff --git a/numpy/testing/tests/test_utils.py b/numpy/testing/tests/test_utils.py
index 35f81d8a7..0592e62f8 100644
--- a/numpy/testing/tests/test_utils.py
+++ b/numpy/testing/tests/test_utils.py
@@ -6,6 +6,7 @@ import os
 import itertools
 import textwrap
 import pytest
+import weakref
 
 import numpy as np
 from numpy.testing import (
@@ -14,7 +15,7 @@ from numpy.testing import (
     assert_raises, assert_warns, assert_no_warnings, assert_allclose,
     assert_approx_equal, assert_array_almost_equal_nulp, assert_array_max_ulp,
     clear_and_catch_warnings, suppress_warnings, assert_string_equal, assert_,
-    tempdir, temppath,
+    tempdir, temppath, assert_no_gc_cycles, HAS_REFCOUNT
     )
 
 
@@ -1360,3 +1361,76 @@ def test_clear_and_catch_warnings_inherit():
         warnings.simplefilter('ignore')
         warnings.warn('Some warning')
     assert_equal(my_mod.__warningregistry__, {})
+
+
+@pytest.mark.skipif(not HAS_REFCOUNT, reason="Python lacks refcounts")
+class TestAssertNoGcCycles(object):
+    """ Test assert_no_gc_cycles """
+    def test_passes(self):
+        def no_cycle():
+            b = []
+            b.append([])
+            return b
+
+        with assert_no_gc_cycles():
+            no_cycle()
+
+        assert_no_gc_cycles(no_cycle)
+
+
+    def test_asserts(self):
+        def make_cycle():
+            a = []
+            a.append(a)
+            a.append(a)
+            return a
+
+        with assert_raises(AssertionError):
+            with assert_no_gc_cycles():
+                make_cycle()
+
+        with assert_raises(AssertionError):
+            assert_no_gc_cycles(make_cycle)
+
+
+    def test_fails(self):
+        """
+        Test that in cases where the garbage cannot be collected, we raise an
+        error, instead of hanging forever trying to clear it.
+        """
+
+        class ReferenceCycleInDel(object):
+            """
+            An object that not only contains a reference cycle, but creates new
+            cycles whenever it's garbage-collected and its __del__ runs
+            """
+            make_cycle = True
+
+            def __init__(self):
+                self.cycle = self
+
+            def __del__(self):
+                # break the current cycle so that `self` can be freed
+                self.cycle = None
+
+                if ReferenceCycleInDel.make_cycle:
+                    # but create a new one so that the garbage collector has more
+                    # work to do.
+                    ReferenceCycleInDel()
+
+        try:
+            w = weakref.ref(ReferenceCycleInDel())
+            try:
+                with assert_raises(RuntimeError):
+                    # this will be unable to get a baseline empty garbage
+                    assert_no_gc_cycles(lambda: None)
+            except AssertionError:
+                # the above test is only necessary if the GC actually tried to free
+                # our object anyway, which python 2.7 does not.
+                if w() is not None:
+                    pytest.skip("GC does not call __del__ on cyclic objects")
+                    raise
+
+        finally:
+            # make sure that we stop creating reference cycles
+            ReferenceCycleInDel.make_cycle = False