Let us sum up a little bit:

1. access name:
   meta or attrs?

2. passing metadata at array creation:

• solution a: by passing an (ordered) dict

     >>> arr = LArray(data, axes, {'title': 'my title'})

• solution b: via kwargs

     >>> arr = LArray(data, axes, title='my title')

3. What to do with current argument title?

• solution a: implement "blessed" metadata. Question is "which ones?"
• solution b: passing metadata via kwargs.
• solution c: if we pass metadata via a dict and we do not implement blessed metadata, we could keep current title argument for some releases and display a FuturWarning

4. Usage:
   Current proposal is:

   # add new metadata
   >>> arr.attrs.creation_date = datetime(2017, 2, 10)
   # access metadata
   >>> arr.attrs.creation_date
   datetime.datetime(2017, 2, 10, 0, 0)
   # modify metadata
   >>> arr.attrs.creation_date = datetime(2017, 2, 16)
   # delete metadata
   >>> del arr.attrs.creation_date

I mostly have still the same opinion.

1. either is fine by me. I have a slight personal preference for .attrs but we need to poll our users to see what is most intuitive for them.
2. a) must be implemented in any case to disambiguate metadata keywords which collide with other arguments.
   b) could be done in addition, but I am reluctant to do it because that means that all array creation functions should treat extra kwargs like metadata and it means the list of "forbidden" metadata keywords is different depending on the function. It is also problematic for stack() which already uses kwargs for something else. Also, providing a way to set metadata could be useful in fact for any method returning an LArray (and some of them already uses **kwargs for other purposes e.g. aggregates) and loosing argument names validation (which is the flip side of the coin when you use **kwargs) for all methods is a heavy price to pay IMO.
3. given the above, either solution a or c. We could postpone the decision by starting with only "title" as a blessed argument, and later either deprecate it or add a few other "blessed" arguments depending on how it goes.
4. ok

Specifications for metadata:

1. Usage:

   # add new metadata
   >>> arr.attrs.creation_date = datetime(2017, 2, 10)
   # access metadata
   >>> arr.attrs.creation_date
   datetime.datetime(2017, 2, 10, 0, 0)
   # modify metadata
   >>> arr.attrs.creation_date = datetime(2017, 2, 16)
   # delete metadata
   >>> del arr.attrs.creation_date

2. Passing metadata at array creation:
   Users do it by passing an (ordered) dict or a Metadata instance

   >>> arr = LArray(data, axes, attrs=OrderedDict([('title', 'my title')]))

3. Backward compatibility with argument title of LArray.init:
   We implement a "blessed" metadata title.

4. Copying metadata:
   a) When to copy them:

• arr.copy()
• arr.set_axes()
• arr.rename()
• arr.set_labels()
• arr.drop_labels()
• arr.combine_axes()
• arr.split_axes()
• arr.sort_axes()
• arr.sort_values()
• arr.reshape()
• arr.reshape_like()
• arr.compact()
• arr.transpose()
• arr.expand()
• arr.broadcast_with()
• arr.reindex()
• arr.append()
• arr.prepend()
• arr.extend()
• arr.insert()
• arr.align()
• arr.filter()
• arr.with_total()
• arr.__setitem__()
• arr.set()

c) When NOT to copy them:

• all aggregation functions
• all ufuncs
• all unary and binary ops
• to_frame, to_series
• arr.nonzero()
• arr.describe() and describe_by()
• all, any, min, max, labelofmin, ...
• arr.ratio() and arr.rationot0()
• arr.percent()
• arr.divnot0()
• arr.shift() and arr.diff()
• arr.growth_rate()

@gdementen feel free to edit this comment. I will use it as specifications for implementing metadata.

1. I would still like to poll our users for deciding attrs vs meta (unless you did already). Maybe they don't care, maybe they do.
2. This is also possible:

>>> arr = LArray(data, axes, Attrs(title='my title'))

and I think it would be useful to implement this in addition to the dict syntax, in the same vein that we support an AxisCollection object or a string or xyz... We do not necessarily need to advertise this though.

How would like to make the poll? Via the Google group larray-users? Mail?

Result of the poll is meta

For the record: after some discussion, we decided to NOT propagate metadata. Propagating metadata through the code may lead to final arrays with irrelevant metadata.

Please keep a backup of your branch somewhere so that if we change our minds later (or are pressured to do so) you do not need to redo everything...