Raise more specific exception on concat([None])

[jason-curtis]

More specific exception allows for a safer, more explicit catch by a caller.
ConcatenationError follows the pattern of MergeError which is used in the same file when something is wrong with a caller's inputs to merge().

[jreback]

@jorisvandenbossche you think this is an API change? seems completely back-compat

[shoyer]

Maybe this could just be a ValueError?

I'm not opposed to more specific exceptions, but if you are going to define a new exception, it probably should be exposed a higher level of the API than pandas.tools.merge. I'm guessing there is precedent for this somewhere?

[jason-curtis]

The pattern I'm following is from MergeError (declared on line 44). MergeError is raised when something is wrong with a caller's inputs to merge().

ValueError would also be an improvement but I figured I should follow the pattern used in the file.

[shoyer]

OK, fair enough.

[jorisvandenbossche]

Yeah, as far as I understand it correctly, this should be back compat (as if a user is checking for an Exception now, this will still work for ConcatenationError).

However, I do follow @shoyer here in the question if it is not more useful to provide eg ValueError.
I know it would be more consistent with the rest of the file to use a custom error, but is that pattern used a lot elsewhere in pandas? (@jreback?) If we think something like ValueError is better, I think we should do that regardless of consistency with merge.
And I personally find ValueError possibly more informative than ConcatenationError, as there are more things that could go wrong in the concatenation function than wrong input values.

[jreback]

I think MergeError could be changed to ValueError. Which is what this really is, either the arguments are incorrect (e.g. can't specify multiple conflicting options), or your data is not-unique.

So IMHO, I think let's make MergeError inherit from ValueError (I think this is back-compat), but put in a note that we are getting rid of it in 0.16? (or just drop it)?
and make this bare Exception into a ValueError

[jason-curtis]

Updated to remove ConcatenationError and make ValueError more central, as suggested. I'll leave the deprecation of MergeError to you folks if that's OK. Not sure how you'd like to document/go forward with that kind of thing.

[jorisvandenbossche]

Can you sqaush your commits?

[jason-curtis]

@jorisvandenbossche done!

[jreback]

@thatneat can you add a release note to v0.15.0 API changes (reference this issue, saying a MergeError will now specialize as a ValueError or something like that).

squash and I think good to go

[jreback]

merged via ce79c80

thanks!