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

[Martin Dengler]

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).

    - __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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