BUG: break cyclic refs in recursive closures

Fixes #10620

3 files changed, 32 insertions, 9 deletions

diff --git a/numpy/core/arrayprint.py b/numpy/core/arrayprint.py
index 84943cafc..cbe95f51b 100644
--- a/numpy/core/arrayprint.py
+++ b/numpy/core/arrayprint.py
@@ -772,7 +772,8 @@ def _formatArray(a, format_function, line_width, next_line_prefix,
                     s += hanging_indent + summary_insert + line_sep
 
             for i in range(trailing_items, 1, -1):
-                nested = recurser(index + (-i,), next_hanging_indent, next_width)
+                nested = recurser(index + (-i,), next_hanging_indent,
+                                  next_width)
                 s += hanging_indent + nested + line_sep
 
             nested = recurser(index + (-1,), next_hanging_indent, next_width)
@@ -782,12 +783,16 @@ def _formatArray(a, format_function, line_width, next_line_prefix,
         s = '[' + s[len(hanging_indent):] + ']'
         return s
 
-    # invoke the recursive part with an initial index and prefix
-    return recurser(
-        index=(),
-        hanging_indent=next_line_prefix,
-        curr_width=line_width)
-
+    try:
+        # invoke the recursive part with an initial index and prefix
+        return recurser(index=(),
+                        hanging_indent=next_line_prefix,
+                        curr_width=line_width)
+    finally:
+        # recursive closures have a cyclic reference to themselves, which
+        # requires gc to collect (gh-10620). To avoid this problem, for
+        # performance and PyPy friendliness, we break the cycle:
+        recurser = None
 
 def _none_or_positive_arg(x, name):
     if x is None:
diff --git a/numpy/core/shape_base.py b/numpy/core/shape_base.py
index 65c3ed00d..319c25088 100644
--- a/numpy/core/shape_base.py
+++ b/numpy/core/shape_base.py
@@ -446,7 +446,13 @@ def _block(arrays, max_depth, result_ndim):
             # type(arrays) is not list
             return atleast_nd(arrays, result_ndim)
 
-    return block_recursion(arrays)
+    try:
+        return block_recursion(arrays)
+    finally:
+        # recursive closures have a cyclic reference to themselves, which
+        # requires gc to collect (gh-10620). To avoid this problem, for
+        # performance and PyPy friendliness, we break the cycle:
+        block_recursion = None
 
 
 def block(arrays):
diff --git a/numpy/core/tests/test_arrayprint.py b/numpy/core/tests/test_arrayprint.py
index d4c565c57..88aaa3403 100644
--- a/numpy/core/tests/test_arrayprint.py
+++ b/numpy/core/tests/test_arrayprint.py
@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import division, absolute_import, print_function
 
-import sys
+import sys, gc
 
 import numpy as np
 from numpy.testing import (
@@ -355,6 +355,18 @@ class TestArray2String(object):
             "[ 'xxxxx']"
         )
 
+    def test_refcount(self):
+        # make sure we do not hold references to the array due to a recursive
+        # closure (gh-10620)
+        gc.disable()
+        a = np.arange(2)
+        r1 = sys.getrefcount(a)
+        np.array2string(a)
+        np.array2string(a)
+        r2 = sys.getrefcount(a)
+        gc.collect()
+        gc.enable()
+        assert_(r1 == r2)
 
 class TestPrintOptions(object):
     """Test getting and setting global print options."""