[Biopython-dev] GSoC python variant update

[Lenna Peterson]

On Wed, Aug 8, 2012 at 12:37 PM, Laurent Gautier <lgautier at gmail.com> wrote:
> On 2012-08-08 18:00, biopython-dev-request at lists.open-bio.org wro
>
>>
>>> >* In order to customize the display of positions (e.g. 0-based or
>>> >1-based), I'm using a class as a configuration container. I've read on
>>> >StackOverflow that attempts to use globals or a singleton class are
>>> >discouraged in Python, but I have not found practical suggestions for
>>> >how to implement module-wide configurations. Suggestions are welcome.
>
>
> Module-wide configuration can be implemented as variables, as long as they
> are declared before the functions using them.
> If considering a package rather than a single module, options can be stored
> in a module dedicated to options (since Python modules are singletons).
>

Hi Laurent,

I really like the idea of a configuration module. I will definitely
move in that direction.

>
>> With configuration items like this, you have two choices:
>>
>> - A global variable.
>> - Pass the configuration to every function that needs it.
>>
>> There are tradeoffs with both approaches, but for this case I agree with
>> your decision to use globals. Most people will want 0-based/Biopython
>> style but it gives those who don't a knob to switch over.
>
>
> I'd argue that allowing to switch is an invitation to spectacular issues
> down the road.
> An easy, yet frightening, example would be the case where using third-party
> code (such a module) changes this without you knowing.
>
> An other scary thought is that this would amount to bringing the infamous
> Perl variable "$[" to Python. Go explain again that folks should Python for
> its elegance and simplicity after that.
>
>

Yikes. My approach will not be comparable to $[. For starters, it
wouldn't modify the behavior of every sequence-like object.

My current thought would be to store the 0-based position in an
attribute `pos`, have a property `pos_str` that returns `pos` +
`Config.index`. For representations, `__str__` will return `pos_str`,
and `__repr__` will return `pos` (always 0-based). Math would always
use the 0-based position.

I intend to keep the influence of the hypothetical mapping Config
module limited to Biopython Seq* objects. It should also be possible
to make a kill switch, namely, a version of the Config module where
all of the settings are neutral to adding (i.e. `def __add__(self,
other): return other`).

Please let me know if this would not fully address your concerns.

Cheers,

Lenna