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Re: What use of these _ prefix members?

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Someone else posting as "me" wrote:

> On 2016-01-10, Peter Otten <__peter__@web.de> wrote:
>>>>> class Derived(Base):
>> ...     def _init(self, x):
>> ...         super()._init(x)
>> ...         print("do something else with", x)
>> ...
>>>>> Derived(42)
>> do something with 42
>> do something else with 42
>><__main__.Derived object at 0x7f8e6b3e9b70>
>>
> 
> I think you are doing inheritance wrong.

If by "you" you mean "me" -- the above sample is an illustration of the 
pattern I expected to see elsewhere in the software Robert was quoting, not 
an endorsement. I have now looked into the hmmlearn source, and it turns out 
that _init() is invoked by the fit() method rather than the initializer. 
But...
 
> AFAIK you should call directly the __init__() of the parent class, and
> pass *args and **kwargs instead.

you sometimes want to break initialisation or any other method into distinct 
steps that don't make sense stand-alone:

class Foo:
    def method(...)
        self._one(...)
        self._two(...)
        self._three(...)

That way subclasses have the option to override only _two() instead of 
method() and users of Foo won't try to invoke _two(...) on its own.

I think this is a good approach.

What arguments you need to accept and/or pass on is a question that you can 
decide depending on the actual use-case. I use

*args, **kwargs

only if function is very generic because it makes code hard to follow.

> Except for that, yes, the _init would be conventionally private. Not
> enforced by name mangling though.

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Thread

What use of these _ prefix members? Robert <rxjwg98@gmail.com> - 2016-01-10 09:55 -0800
  Re: What use of these _ prefix members? Steven D'Aprano <steve@pearwood.info> - 2016-01-11 05:03 +1100
  Re: What use of these _ prefix members? Peter Otten <__peter__@web.de> - 2016-01-10 19:34 +0100
    Re: What use of these _ prefix members? me <self@example.org> - 2016-01-12 14:12 +0000
      Re: What use of these _ prefix members? Peter Otten <__peter__@web.de> - 2016-01-12 15:52 +0100
      Re: What use of these _ prefix members? Jason Swails <jason.swails@gmail.com> - 2016-01-12 10:01 -0500

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