[Python-Dev] Problems with the new super()

Guido van Rossum guido at python.org
Wed Apr 30 23:57:32 CEST 2008

The staticmethod thing isn't new; that's also the case in 2.x.

The super() thing is a case of practicality beats purity. Note that
you pay a small but measurable cost for the implicit __class__ (it's
implemented as a "cell variable", the same mechanism used for nested
scopes) so we wouldn't want to introduce it unless it is used.

On Wed, Apr 30, 2008 at 2:46 PM, Armin Ronacher
<armin.ronacher at active-4.com> wrote:
> Hi all,
>  I blogged about that topic today which turned out to be a very bad idea,
>  so I summarize it for the mailinglist here to hopefully start a discussion
>  about the topic, which I think is rather important.
>  In the last weeks something remarkable happened in the Python3 sources:
>  self kinda became implicit.  Not in function definitions, but in super
>  calls.  But not only self: also the class passed to `super`.  That's
>  interesting because it means that the language shifts into a completely
>  different direction.
>  `super` was rarely used in the past, mainly because it was weird to use.
>  In the most common use case the current class and the current instance
>  where passed to it, and the super typed returned looked up the parent
>  methods on the MRO for you.  It was useful for multiple inheritance and
>  mixin classes that don't know their parent but confusing for many.  I can
>  see that a replacement is a good idea, but I'm not so sure if the current
>  implementation is the way to go.
>  The main problem with replacing `super(Foo, self).bar()` with something
>  like `super.bar()` is obviously that self is explicit and the class (in
>  that case Foo) can't be determined by the caller.  Furthermore the Python
>  principle was always against functions doing stack introspection to find
>  the caller.  There are few examples in the stdlib or builtins that do
>  some sort of caller introspection.  Those are the special functions
>  `vars`, `locals`, `gloabal`, `vars` and some functions in the inspect
>  module.  And all of them do nothing more than getting the current frame
>  and accessing the dict of locals or globals.  What super in current
>  Python 3 builds does goes way beyond that.
>  The implementation of the automatic super currently has two ugly details
>  that I think violate the Python Zen:  The bytecode generated is differently
>  if the name "super" is used in the function.  `__class__` is only added as
>  cell to the code if `super` or `__class__` is referenced.  That and the fact
>  that `f_localsplus` is completely unavailable from within python makes the
>  whole process appear magical.
>  This is way more magical than anything we've had in Python in the past and
>  just doesn't fit into the language in my opinion.  We do have an explicit
>  self in methods and methods are more or less just functions.  Python's
>  methods are functions, just that a descriptor puts a method object around
>  it to pass the self as first arguments.  That's an incredible cool thing
>  to have and makes things very simple and non-magical.  Breaking that
>  principle by introducing an automatic super seems to harm the concept.
>  Another odd thing is that Python 3 starts keeping information on the C
>  layer we can't access from within Python.  Super is one example, another
>  good one are methods.  They don't have a descriptor that wraps them if
>  they are accessed via their classes.  This as such is not a problem as you
>  can call them the same (just that you can call them with completely
>  different receivers now) but it becomes a problem if some of the functions
>  are marked as staticmethods.  Then they look completely the same when
>  looking at them from a classes perspective:
>  |   >>> class C:
>  |   ...  normal = lambda x: None
>  |   ...  static = staticmethod(lambda x: None)
>  |   ...
>  |   >>> type(C.normal) is type(C.static)
>  |   True
>  |   >>> C.normal
>  |   <function <lambda> at 0x4da150>
>  As far as I can see a documentation tool has no chance to keep them apart
>  even though they are completely different on an instance:
>  |   >>> type(C().normal) is type(C().static)
>  |   False
>  |   >>> C().normal
>  |   <bound method C.<lambda> of <__main__.C object at 0x4dbcf0>>
>  |   >>> C().static
>  |   <function <lambda> at 0x4da198>
>  While I don't knwo about the method thing, I think an automatic super should
>  at least be implementable from within Python.  I could imagine that by adding
>  __class__ and __self__ to scopes automatically a lot of that magic could be
>  removed.
>  Regards,
>  Armin
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--Guido van Rossum (home page: http://www.python.org/~guido/)

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