[Python-Dev] py3k: TypeError: object.init() takes no parameters (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sat Jan 17 01:38:47 CET 2009

Terry Reedy wrote:

Given "The second use case is to support cooperative multiple inheritence in a dynamic execution environment. ... Good design dictates that this method have the same calling signature in every case (because the order of parent calls is determined at runtime and because that order adapts to changes in the class hierarchy)." the change makes object unsuitable as a multiple inheritance base.

I think as a practical matter it is anyway since practical cases have other methods that object does not have that need exist in the pyramid base. This must be why there have not been squawks about the change in 2.6.

I think that sums up the reasoning fairly well - the (IMO, reasonable) expectation is that a CMI hierarchy will look something like:

object | CMIBase |

While the signatures of new and init will remain the same for all classes in the CMI hierarchy, CMIBase will assume the only class after it in the MRO is object, with signatures for new and init that accept no additional arguments beyond the class and instance respectively.

Note that CMIBase serves another purpose in such a hierarchy: isinstance(obj, CMIBase) becomes a concise check to see if an object is a member of the hierarchy or not.

So I wonder whether the proper change might have been to remove object.init.

That would have broken too much code, since a lot of immutable types rely on it (they only override new and leave init alone).

For more info, see Guido's checkin changing the behaviour and the associated tracker issue: http://svn.python.org/view?rev=54539&view=rev http://bugs.python.org/issue1683368

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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