super with only one argument

Michele Simionato michele.simionato at gmail.com
Thu Mar 17 16:47:17 CET 2005


``super`` with only one argument ("bound" super) is a mess.

AFAICT ``super(C)`` is intended to be used as an attribute in
other classes. Then the descriptor magic will automatically convert the

unbound syntax in the bound syntax. For instance:

>>> class B(object):
...     a = 1
>>> class C(B):
...     pass
>>> class D(C):
...     sup = super(C)
>>> d = D()
>>> d.sup.a
1

This works since ``d.sup.a`` calls ``super(C).__get__(d,D).a`` which is
converted to ``super(C, d).a`` and retrieves ``B.a``.

There is a single use case for the single argument
syntax of ``super`` that I am aware of, but I think it gives more
troubles
than advantages. The use case is the implementation of "autosuper" made

by Guido on his essay about new-style classes (the one Grep Chapman is
citing).

The idea there is to use the unbound super objects as private
attributes. For instance, in our example, we could define the
private attribute ``__sup`` in the class ``C`` as the unbound
super object ``super(C)``:

>>> C._C__sup = super(C)

With this definition inside the methods the syntax
``self.__sup.meth(arg)`` can be used
as an alternative to ``super(C, self).meth(arg)``, and the advantage is

that you avoid to repeat the name of the class in the calling
syntax, since that name is hidden in the mangling mechanism of
private names. The creation of the ``__sup`` attributes can be hidden
in a metaclass and made automatic. So, all this seems to work: but
actually this is *not* the case.

Things may wrong in various case, for instance for classmethods,
as in this example::

  #<ex1.py>

  class B(object):
      def __repr__(self):
          return '<instance of %s>' % self.__class__.__name__
      @classmethod
      def meth(cls):
          print "B.meth(%s)" % cls

  class C(B):
      @classmethod
      def meth(cls):
          print "C.meth(%s)" % cls
          cls.__super.meth()

  C._C__super = super(C)

  class D(C):
      pass

  D._D__super = super(D)


  d=D()

  d.meth()

  #</ex1.py>

The last line raises an ``AttributeError: 'super' object has no
attribute
'meth'.``

So, using a ``__super`` unbound super object is not a robust solution
(notice that everything would work by substituting
``self.__super.meth()``
with ``super(C,self).meth()``. There are other ways to avoid repeating
the class name, see for instance my cookbook recipe, which will also be
in the printed version.

If it was me, I would just remove the single argument syntax of
``super``,
making it illegal. But this would probably break someone code, so
I don't think it will ever happen. Another solution would be just to
deprecate it. There is no need for this syntax, one can always
circumvent
it. Also, notice that the unbound form of ``super`` does
not play well with pydoc.
The problems is still there in Python 2.4 (see bug report SF729103)

>>> class B(object): pass
...
>>> class C(B):
...     s=super(B)
...
>>> help(C)
Traceback (most recent call last):
  ...
  ... lots of stuff here
  ...
  File "/usr/lib/python2.4/pydoc.py", line 1290, in docother
    chop = maxlen - len(line)
TypeError: unsupported operand type(s) for -: 'type' and 'int'

           Michele Simionato




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