The right way to 'call' a class attribute inside the same class
ben+python at benfinney.id.au
Mon Dec 12 18:17:03 EST 2016
Ned Batchelder <ned at nedbatchelder.com> writes:
> Claiming that __init__ isn't a constructor seems overly pedantic to
Whereas to me, claiming that ‘Foo.__init__’ is a constructor seems
* Classes already have a constructor, ‘Foo.__new__’. If we call
something else the constructor, what do we call ‘__new__’? Are they
* A constructor for Foo should start from “no instance” and result in
“an instance of Foo”. ‘Foo.__init__’ does not do that; ‘Foo.__new__’
* A constructor should return the instance. ‘Foo.__init__’ does not do
that; ‘Foo.__new__’ does.
If the differences didn't matter I would agree that “overly pedantic” is
fair. But those differences trip up newcomers. Thinking of
‘Foo.__init__’ leads people to wonder where the ‘self’ attribute came
from – am I not meant to be constructing it? — and to attempt to return
that instance. And when the time comes to lean about ‘__new__’ the
confusion continues, because the newcomer has been told that something
*else* is the constructor, so what's this?
> What's true is that Python's constructors (__init__) are different than
> C++ constructors. In C++, you don't have an object of type T until the
> constructor has finished. In Python, you have an object of type T before
> __init__ has been entered.
I'm not going to argue that C++ should define terminology for other
languages. But “constructor” should have a close correlation with the
normal English-language meaning of the term.
That meaning matches ‘Foo.__new__’, which makes that method a
constructor. It does not match ‘Foo.__init__’, which makes that method
not a constructor.
> The reason to call __init__ a constructor is because of what is the
> same between C++ and Python: the constructor is where the author of
> the class can initialize instances of the class.
So you've just described what ‘Foo._init__’ does: it initialises the
existing instance. That's why it is better to call it the “initialiser”,
a term we already have and use correctly.
\ “DRM doesn't inconvenience [lawbreakers] — indeed, over time it |
`\ trains law-abiding users to become [lawbreakers] out of sheer |
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