[Python-Dev] Replacing self.__dict__ in __init__

[Tin Tvrtković]

Hi Python-dev,

I'm one of the core attrs contributors, and I'm contemplating applying an
optimization to our generated __init__s. Before someone warns me python-dev
is for the development of the language itself, there are two reasons I'm
posting this here:

1) it's a very low level question that I'd really like the input of the
core devs on, and
2) maybe this will find its way into dataclasses if it works out.

I've found that, if a class has more than one attribute, instead of
creating an init like this:

    self.a = a
    self.b = b
    self.c = c

it's faster to do this:

    self.__dict__ = {'a': a, 'b': b, 'c': c}

i.e. to replace the instance dictionary altogether. On PyPy, their core
devs inform me this is a bad idea because the instance dictionary is
special there, so we won't be doing this on PyPy.

But is it safe to do on CPython?

To make the question simpler, disregard the possibility of custom setters
on the attributes.

Thanks in advance!
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