ERR: better concat error messages #9157

[cel4]

closes #9157

This is a first attempt to improve the error message when trying to concat an empty list/set of objects:

pd.concat([]) currently raises following error:

/Users/ch/miniconda/envs/sci33/lib/python3.3/site-packages/pandas/tools/merge.py in __init__(self, objs, axis, join, join_axes, keys, levels, names, ignore_index, verify_integrity, copy)
    765 
    766         if len(objs) == 0:
--> 767             raise ValueError('All objects passed were None')
    768 
    769         # consolidate data & figure out what our result ndim is going to be

ValueError: All objects passed were None

After the tiny fix it is:

/Users/ch/repo/pandas/pandas/tools/merge.py in __init__(self, objs, axis, join, join_axes, keys, levels, names, ignore_index, verify_integrity, copy)
    767 
    768         if len(objs) == 0:
--> 769             raise ValueError('No objects passed')
    770 
    771         if isinstance(objs, dict):

ValueError: No objects passed

[jreback]

doesn't len(objects) accomplish this?

[cel4]

Well, it does look a little hackish. I had issues with objs being a generator. len(list(objs)) would consume the generator, that's why I used the hack with itertools.tee.

[jreback]

needs a test

[cel4]

Yes, still thinking how to design a good test. I still raise a ValueError, but this time with a slightly different error message. Should I test for the wording of the error message? I am not too happy with this solution since it breaks if someone decides to come up with a better message.

[jreback]

@cel4 you can do a assertRegExp. And it should break if something (e.g. the error message is changed), that's the point.

[cel4]

@jreback, sorry I am very busy at the moment. I am planning to continue the work here in a week or two.

[cel4]

I added a draft version how a test could look like. Would appreciate any comments.

[cel4]

AttributeError: 'TestOrderedMerge' object has no attribute 'assertRegexpMatches' - grr... seems like assertRegexpMatches is not part of the unittest module in the 2.6 version...

[jorisvandenbossche]

@cel4 In the pandas testing module, there is a assertRaisesRegexp available for this -> tm.assertRaisesRegexp()

[cel4]

Hmh, tests are green. Yet I am not sure if I like this solution. Wouldn't it be better if we also check inpd.concat for empty sequences, so we can raise a even better (less general) error message about what went wrong exactly.

[jreback]

this needs to address my comments above
you can force materializing a generator and thrn just take the len

[sergeny]

I propose "ValueError: No objects to concatenate"

[cel4]

@sergeny, I like that suggestion. Unfortunately, I do not have much time at the moment and I totally forgot about that PR. I hope I can finish it soon.

[jreback]

closing as stale, but if you would like to reopen and fixup pls do.

[cel4]

Should be ready for a first round of reviews.

[jreback]

add the issue number here as a comment

[jreback]

looks reasonable. pls add a release note. and squash. (you can put in API changes)

[cel4]

@jreback, When travis is happy, this should be ready for review once again.

[jreback]

merged via 58db03e

thanks!