[Python-ideas] Required to call superclass __init__
ntoronto at cs.byu.edu
Tue Nov 13 08:23:40 CET 2007
I'm not talking about having the runtime call the superclass __init__
for you, as I am aware of the arguments over it and I am against it
myself. I'm talking about checking whether it's been called within a
subclass's own __init__.
There are many kinds of objects with such complex underpinnings or
initialization that leaving out the call to superclass __init__ would be
disastrous. There are two situations I can think of where enforcing its
invocation could be useful: a corporate environment and a teaching
environment. (I've done the former and I'm working in the latter.)
If someone forgets to call a superclass __init__, problems may not show
up until much later. Even if they do show up immediately, it's almost
never obvious what the real problem is, especially to someone who is new
to programming or is working on someone else's code.
I've got a working prototype metaclass and class instance
(require_super) and decorator (super_required). Decorating a
require_super method with @super_required will require any subclass
override to call its superclass method, or it throws a TypeError upon
exiting the subclass method. Here's how it works on the __init__ problem:
a = A() # No problem
b = B() # No problem
pass # this could be a problem
c = C() # TypeError: C.__init__: no super call
d = D() # TypeError: C.__init__: no super call
As long as A.__init__ is eventually called, it doesn't raise a TypeError.
There's not much magic involved (as metaclasses go), just explicit and
implicit method wrappers, and no crufty-looking magic words in the
subclasses. Not calling the superclass method results in immediate
runtime feedback. I've tested this on a medium-small, real-life
single-inheritance hierarchy and it seems to work just fine. (I *think*
it should work with multiple inheritance.)
1. Is the original problem (missed superclass method calls) big enough
to warrant language, runtime, or library support for a similar solution?
2. Does anybody but me think this is a great idea?
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