BUG: infinite recursion in str of 0d subclasses

Fixes #10360

2 files changed, 63 insertions, 6 deletions

diff --git a/numpy/core/arrayprint.py b/numpy/core/arrayprint.py
index 472318098..0fdd6fc7f 100644
--- a/numpy/core/arrayprint.py
+++ b/numpy/core/arrayprint.py
@@ -468,14 +468,17 @@ def _recursive_guard(fillvalue='...'):
 # gracefully handle recursive calls, when object arrays contain themselves
 @_recursive_guard()
 def _array2string(a, options, separator=' ', prefix=""):
-    # The formatter __init__s cannot deal with subclasses yet
-    data = asarray(a)
+    # The formatter __init__s in _get_format_function cannot deal with
+    # subclasses yet, and we also need to avoid recursion issues in
+    # _formatArray with subclasses which return 0d arrays in place of scalars
+    a = asarray(a)
 
     if a.size > options['threshold']:
         summary_insert = "..."
-        data = _leading_trailing(data, options['edgeitems'])
+        data = _leading_trailing(a, options['edgeitems'])
     else:
         summary_insert = ""
+        data = a
 
     # find the right formatting function for the array
     format_function = _get_format_function(data, **options)
@@ -501,7 +504,7 @@ def array2string(a, max_line_width=None, precision=None,
 
     Parameters
     ----------
-    a : ndarray
+    a : array_like
         Input array.
     max_line_width : int, optional
         The maximum number of columns the string should span. Newline
@@ -763,7 +766,7 @@ def _formatArray(a, format_function, line_width, next_line_prefix,
 
             if show_summary:
                 if legacy == '1.13':
-                    # trailing space, fixed number of newlines, and fixed separator
+                    # trailing space, fixed nbr of newlines, and fixed separator
                     s += hanging_indent + summary_insert + ", \n"
                 else:
                     s += hanging_indent + summary_insert + line_sep
@@ -1413,6 +1416,8 @@ def array_repr(arr, max_line_width=None, precision=None, suppress_small=None):
 
     return arr_str + spacer + dtype_str
 
+_guarded_str = _recursive_guard()(str)
+
 def array_str(a, max_line_width=None, precision=None, suppress_small=None):
     """
     Return a string representation of the data in an array.
@@ -1455,7 +1460,10 @@ def array_str(a, max_line_width=None, precision=None, suppress_small=None):
     # so floats are not truncated by `precision`, and strings are not wrapped
     # in quotes. So we return the str of the scalar value.
     if a.shape == ():
-        return str(a[()])
+        # obtain a scalar and call str on it, avoiding problems for subclasses
+        # for which indexing with () returns a 0d instead of a scalar by using
+        # ndarray's getindex. Also guard against recursive 0d object arrays.
+        return _guarded_str(np.ndarray.__getitem__(a, ()))
 
     return array2string(a, max_line_width, precision, suppress_small, ' ', "")
 
diff --git a/numpy/core/tests/test_arrayprint.py b/numpy/core/tests/test_arrayprint.py
index faf0a3272..600a566db 100644
--- a/numpy/core/tests/test_arrayprint.py
+++ b/numpy/core/tests/test_arrayprint.py
@@ -34,6 +34,55 @@ class TestArrayRepr(object):
             "     [(1,), (1,)]], dtype=[('a', '<i4')])"
         )
 
+    def test_0d_object_subclass(self):
+        # make sure that subclasses which return 0ds instead
+        # of scalars don't cause infinite recursion in str
+        class sub(np.ndarray):
+            def __new__(cls, inp):
+                obj = np.asarray(inp).view(cls)
+                return obj
+
+            def __getitem__(self, ind):
+                ret = super(sub, self).__getitem__(ind)
+                return sub(ret)
+
+        x = sub(1)
+        assert_equal(repr(x), 'sub(1)')
+        assert_equal(str(x), '1')
+
+        x = sub([1, 1])
+        assert_equal(repr(x), 'sub([1, 1])')
+        assert_equal(str(x), '[1 1]')
+
+        # check it works properly with object arrays too
+        x = sub(None)
+        assert_equal(repr(x), 'sub(None, dtype=object)')
+        assert_equal(str(x), 'None')
+
+        # plus recursive object arrays (even depth > 1)
+        y = sub(None)
+        x[()] = y
+        y[()] = x
+        assert_equal(repr(x),
+            'sub(sub(sub(..., dtype=object), dtype=object), dtype=object)')
+        assert_equal(str(x), '...')
+
+        # nested 0d-subclass-object
+        x = sub(None)
+        x[()] = sub(None)
+        assert_equal(repr(x), 'sub(sub(None, dtype=object), dtype=object)')
+        assert_equal(str(x), 'None')
+
+        # test that object + subclass is OK:
+        x = sub([None, None])
+        assert_equal(repr(x), 'sub([None, None], dtype=object)')
+        assert_equal(str(x), '[None None]')
+
+        x = sub([None, sub([None, None])])
+        assert_equal(repr(x),
+            'sub([None, sub([None, None], dtype=object)], dtype=object)')
+        assert_equal(str(x), '[None sub([None, None], dtype=object)]')
+
     def test_self_containing(self):
         arr0d = np.array(None)
         arr0d[()] = arr0d