basic language question

Stephan Diehl stephan.diehl at
Fri Sep 5 10:40:48 CEST 2003

Stephan Diehl wrote:

> Terry Reedy wrote:
>> "Stephan Diehl" <stephan.diehl at> wrote in message
>> news:bj7ots$48j$06$1 at
>>> Once in a while, I get bitten by the fact, that mutating list
>> methods such
>>> as 'append' or 'extend' return None instead of the (mutated) list
>> itself.
> [...]
>> 'Returners' could wrap no-return mutators with functions or
>> derived-class methods that do return the object, but I have never seen
>> anyone post a complete module that does so for, say, all list
>> mutators.
> That's actually a nice idea. I might just do that.

o.k., the following short code would give you a list class, that returns
'self' when invoking any of the mutating methods.
The solution involves a metaclass and I wouldn't consider this code more as
an example than an industrial strength solution (for example, at the
moment, you couldn't overload any of these methods)
def wrapedmeth(classname,meth):
    def _meth(self,*argl,**argd):
        return self

    return _meth

class ReturnMeta(type):
    def __new__(cls,classname,bases,classdict):
        wrap = classdict.get('return_self_super_methods')
        if wrap is not None:
            for method in wrap:
                classdict[method] = wrapedmeth(classname,meth)
        return super(ReturnMeta,cls).__new__(cls,classname,bases,classdict)

class mylist(list):
    __metaclass__ = ReturnMeta
    return_self_super_methods = ['append',

if __name__ == '__main__':
    print 'l = [1,2]'
    print 'mylist: print l.append(3)'
    l = mylist([1,2])
    print l.append(3)
    print 'list: print l.append(3)'
    l = [1,2]
    print l.append(3)

have fun


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