Please return leading . in extension

[yarikoptic]

See bids-standard/bids-specification#152 (review) for more info on bringing more consistency to bids spec and tooling

[tyarkoni]

Sounds good, but do we want to issue a deprecation warning for a couple of months before releasing that change? It will definitely break a bunch of pipelines.

[effigies]

The parsing is easily switched from accommodating one to the other, but it's the values we return that are going to cause pain.

Maybe we can place the warning any place that extensions are generated or used, like:

>>> import bids
>>> layout = BIDSLayout(...)
FutureWarning: The 'extension' entity currently excludes the leading '.'. As of version
X.Y.Z, it will include the leading dot. To suppress this warning and include the leading
'.', use `bids.config.set_extension_mode("dot")`.
>>>

>>> import bids
>>> bids.config.set_extension_mode("dot")
>>> layout = BIDSLayout(...)
>>>

[yarikoptic]

oh, like from __future__ import print_function ;) cute idea!
I would make "extension_mode" name a bit more verbose like "dot_extension" - "dots" of various kinds could come up multiple times