Fixed #24963 -- Added File.seekable() on Python 3.

django/core/files/utils.py

+from django.utils import six
+
+
 class FileProxyMixin(object):
     """
     A mixin class used to forward file methods to an underlaying file
@@ -24,6 +27,8 @@ class FileProxyMixin(object):
     write = property(lambda self: self.file.write)
     writelines = property(lambda self: self.file.writelines)
     xreadlines = property(lambda self: self.file.xreadlines)
+    if six.PY3:
+        seekable = property(lambda self: self.file.seekable)
 
     def __iter__(self):
         return iter(self.file)

docs/ref/files/file.txt

@@ -89,7 +89,12 @@ The ``File`` Class
     the following attributes and methods of its ``file`` object:
     ``encoding``, ``fileno``, ``flush``, ``isatty``, ``newlines``,
     ``read``, ``readinto``, ``readlines``, ``seek``, ``softspace``, ``tell``,
-    ``truncate``, ``writelines``, ``xreadlines``.
+    ``truncate``, ``writelines``, ``xreadlines``. If you are using
+    Python 3, the ``seekable`` method is also available.
+
+    .. versionchanged:: 1.9
+
+        The ``seekable`` method was added.
 
 .. currentmodule:: django.core.files.base
 

docs/releases/1.9.txt

@@ -200,6 +200,9 @@ File Storage
   <django.core.files.storage.Storage.get_valid_name>` is now called when
   the :attr:`~django.db.models.FileField.upload_to` is a callable.
 
+* :class:`~django.core.files.File` now has the ``seekable()`` method when using
+  Python 3.
+
 File Uploads
 ^^^^^^^^^^^^
 

tests/files/tests.py

@@ -120,6 +120,19 @@ class FileTests(unittest.TestCase):
         f = File(StringIO('one\ntwo\nthree'))
         self.assertEqual(list(f), ['one\n', 'two\n', 'three'])
 
+    def test_seekable(self):
+        """
+        File.seekable() should be available on Python 3.
+        """
+        with tempfile.TemporaryFile() as temp:
+            temp.write(b"contents\n")
+            test_file = File(temp, name="something.txt")
+            if six.PY2:
+                self.assertFalse(hasattr(test_file, 'seekable'))
+            if six.PY3:
+                self.assertTrue(hasattr(test_file, 'seekable'))
+                self.assertTrue(test_file.seekable())
+
 
 class NoNameFileTestCase(unittest.TestCase):
     """