numpy
diff --git a/‎numpy/compat/py3k.py
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+11Lines changed: 11 additions & 0 deletions b/‎numpy/compat/py3k.py
Copy file name to clipboardExpand all lines: numpy/compat/py3k.py
+11Lines changed: 11 additions & 0 deletions
diff --git a/‎numpy/compat/tests/test_compat.py
Copy file name to clipboardExpand all lines: numpy/compat/tests/test_compat.py
+17-1Lines changed: 17 additions & 1 deletion b/‎numpy/compat/tests/test_compat.py
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+17-1Lines changed: 17 additions & 1 deletion
diff --git a/‎numpy/lib/format.py
Copy file name to clipboardExpand all lines: numpy/lib/format.py
+6-5Lines changed: 6 additions & 5 deletions b/‎numpy/lib/format.py
Copy file name to clipboardExpand all lines: numpy/lib/format.py
+6-5Lines changed: 6 additions & 5 deletions
@@ -49,6 +49,17 @@ def asstr(s):
 def isfileobj(f):
     return isinstance(f, (io.FileIO, io.BufferedReader, io.BufferedWriter))
 
+def _isfileobj(f):
+    if not isinstance(f, (io.FileIO, io.BufferedReader, io.BufferedWriter)):
+        return False
+    try:
+        # BufferedReader/Writer may raise OSError when
+        # fetching `fileno()` (e.g. when wrapping BytesIO).
+        f.fileno()
+        return True
+    except OSError:
+        return False
+
 def open_latin1(filename, mode='r'):
     return open(filename, mode=mode, encoding='iso-8859-1')
 
 
@@ -1,6 +1,7 @@
 from os.path import join
+from io import BufferedReader, BytesIO
 
-from numpy.compat import isfileobj
+from numpy.compat.py3k import isfileobj, _isfileobj
 from numpy.testing import assert_
 from numpy.testing import tempdir
 
@@ -17,3 +18,18 @@ def test_isfileobj():
 
         with open(filename, 'rb') as f:
             assert_(isfileobj(f))
+
+def test__isfileobj():
+    with tempdir(prefix="numpy_test_compat_") as folder:
+        filename = join(folder, 'a.bin')
+
+        with open(filename, 'wb') as f:
+            assert_(_isfileobj(f))
+
+        with open(filename, 'ab') as f:
+            assert_(_isfileobj(f))
+
+        with open(filename, 'rb') as f:
+            assert_(_isfileobj(f))
+        
+        assert_(_isfileobj(BufferedReader(BytesIO())) is False)
@@ -165,8 +165,9 @@
 import warnings
 from numpy.lib.utils import safe_eval
 from numpy.compat import (
-    isfileobj, os_fspath, pickle
+    os_fspath, pickle
     )
+from numpy.compat.py3k import _isfileobj
 
 
 __all__ = []
@@ -710,15 +711,15 @@ def write_array(fp, array, version=None, allow_pickle=True, pickle_kwargs=None):
             pickle_kwargs = {}
         pickle.dump(array, fp, protocol=3, **pickle_kwargs)
     elif array.flags.f_contiguous and not array.flags.c_contiguous:
-        if isfileobj(fp):
+        if _isfileobj(fp):
             array.T.tofile(fp)
         else:
             for chunk in numpy.nditer(
                     array, flags=['external_loop', 'buffered', 'zerosize_ok'],
                     buffersize=buffersize, order='F'):
                 fp.write(chunk.tobytes('C'))
     else:
-        if isfileobj(fp):
+        if _isfileobj(fp):
             array.tofile(fp)
         else:
             for chunk in numpy.nditer(
@@ -796,7 +797,7 @@ def read_array(fp, allow_pickle=False, pickle_kwargs=None, *,
                                "You may need to pass the encoding= option "
                                "to numpy.load" % (err,)) from err
     else:
-        if isfileobj(fp):
+        if _isfileobj(fp):
             # We can use the fast fromfile() function.
             array = numpy.fromfile(fp, dtype=dtype, count=count)
         else:
@@ -888,7 +889,7 @@ def open_memmap(filename, mode='r+', dtype=None, shape=None,
     numpy.memmap
 
     """
-    if isfileobj(filename):
+    if _isfileobj(filename):
         raise ValueError("Filename must be a string or a path-like object."
                          "  Memmap cannot use existing file handles.")