diff --git a/pysoundfile.py b/pysoundfile.py
index dc330b8..1be4bce 100644
--- a/pysoundfile.py
+++ b/pysoundfile.py
@@ -356,21 +356,14 @@ def __init__(self, file, mode='r', sample_rate=None, channels=None,
             self._file = _snd.sf_open(file, mode_int, self._info)
         elif isinstance(file, int):
             self._file = _snd.sf_open_fd(file, mode_int, self._info, closefd)
-        else:
-            # Note: readinto() is not checked
-            for attr in ('seek', 'read', 'write', 'tell'):
-                if not hasattr(file, attr):
-                    raise RuntimeError(
-                        "file must be a filename, a file descriptor or "
-                        "a file-like object with the methods "
-                        "'seek()', 'read()', 'write()' and 'tell()'")
-            # Note: the callback functions in _vio must be kept alive!
-            self._vio = self._init_vio(file)
-            vio = _ffi.new("SF_VIRTUAL_IO*", self._vio)
-            self._file = _snd.sf_open_virtual(vio, mode_int, self._info,
-                                              _ffi.NULL)
+        elif all(hasattr(file, a) for a in ('seek', 'read', 'write', 'tell')):
+            self._file = _snd.sf_open_virtual(
+                self._init_virtual_io(file), mode_int, self._info, _ffi.NULL)
             self._name = str(file)
-
+        else:
+            raise RuntimeError("file must be a filename, a file descriptor or "
+                               "a file-like object with the methods "
+                               "'seek()', 'read()', 'write()' and 'tell()'")
         self._handle_error()
 
     name = property(lambda self: self._name)
@@ -397,26 +390,23 @@ def __init__(self, file, mode='r', sample_rate=None, channels=None,
     # avoid confusion if something goes wrong before assigning self._file:
     _file = None
 
-    def _init_vio(self, file):
+    def _init_virtual_io(self, file):
         @_ffi.callback("sf_vio_get_filelen")
         def vio_get_filelen(user_data):
-            # Streams must set _length or implement __len__
-            if hasattr(file, '_length'):
-                size = file._length
-            elif not hasattr(file, '__len__'):
-                old_file_position = file.tell()
+            # first try __len__(), if not available fall back to seek()/tell()
+            try:
+                size = len(file)
+            except TypeError:
+                curr = file.tell()
                 file.seek(0, SEEK_END)
                 size = file.tell()
-                file.seek(old_file_position, SEEK_SET)
-            else:
-                size = len(file)
+                file.seek(curr, SEEK_SET)
             return size
 
         @_ffi.callback("sf_vio_seek")
         def vio_seek(offset, whence, user_data):
             file.seek(offset, whence)
-            curr = file.tell()
-            return curr
+            return file.tell()
 
         @_ffi.callback("sf_vio_read")
         def vio_read(ptr, count, user_data):
@@ -435,18 +425,20 @@ def vio_read(ptr, count, user_data):
         def vio_write(ptr, count, user_data):
             buf = _ffi.buffer(ptr, count)
             data = buf[:]
-            length = file.write(data)
-            return length
+            return file.write(data)
 
         @_ffi.callback("sf_vio_tell")
         def vio_tell(user_data):
             return file.tell()
 
-        return {'get_filelen': vio_get_filelen,
-                'seek': vio_seek,
-                'read': vio_read,
-                'write': vio_write,
-                'tell': vio_tell}
+        # Note: the callback functions must be kept alive!
+        self._virtual_io = {'get_filelen': vio_get_filelen,
+                            'seek': vio_seek,
+                            'read': vio_read,
+                            'write': vio_write,
+                            'tell': vio_tell}
+
+        return _ffi.new("SF_VIRTUAL_IO*", self._virtual_io)
 
     def __del__(self):
         self.close()