[Sugar-devel] Correct use of leading underscores in Python?

[Tomeu Vizoso]

On Tue, Jul 21, 2009 at 00:52, Martin Dengler<martin at martindengler.com> wrote:
> On Mon, Jul 20, 2009 at 10:59:05AM -0500, Jim Simmons wrote:
>> I don't want to start a religious war here, but I could use some
>> guidance.  In writing my Activities I have often copied and pasted
>> bits of code from other Activities.  For instance, the toolbars from
>> Read and Speak.  As a result of this I have code in which leading
>> underscores are used in both variable names and method names, with no
>> particular rhyme or reason for it.  I want my code to follow good
>> practices, but the only thing I've found which talks about leading
>> underscores in Python is the PEP 8 style guide, which says:
>>
>> "In addition, the following special forms using leading or trailing
>> underscores are recognized (these can generally be combined with any
>> case convention):
>>
>> - _single_leading_underscore: weak "internal use" indicator.  E.g.
>> "from M import *" does not import objects whose name starts with an
>> underscore."
>
> Did you miss the following points on double leading underscores?
>
>    - __double_leading_underscore: when naming a class attribute,
>      invokes name
>      mangling (inside class FooBar, __boo becomes _FooBar__boo; see
>      below).

This is actually important in Sugar, we use double leading underscores
for signal handlers because a subclass that listens to the same signal
could inadvertently override the parent's handler and lead to very
difficult to debug problems, ex.: __button_clicked_cb

Regards,

Tomeu

>    - __double_leading_and_trailing_underscore__: "magic" objects or
>      attributes that live in user-controlled namespaces.
>      E.g. __init__,
>      __import__ or __file__.  Never invent such names; only use them
>      as documented.
>
> IMO:
>
> - one leading underscore is "private" ("weak internal use") or, in
>  the context of inheritance, like java's "protected" (private to a
>  class and its subclasses but can be / is expected to be overridden).
>
> - two leading underscores is "extra private" ("strong internal use")
>  or, in the context of inheritance, like java's "private" (private to
>  a class and not visible to subclasses without good,
>  you-better-know-why-you're-thumbing-your-nose-at-name-mangling
>  reasons).
>
>> So when do you use leading underscores to name something?  Or do you
>> ever use them?
>
> They're used a lot by python code.  One should almost certainly be
> using them in code that's used by other code.
>
>> Thanks,
>>
>> James Simmons
>
> Martin
>
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