User-facing AssertionError with groupby on list of empty list.

[jtratner]

I hesitate to post this, but grouping on a list of empty list yields an AssertionError. It's so edge case I'm not sure it's worth bring up though:

df = pd.DataFrame(zip(range(10), range(10)), columns=['apple', 'b'])
df.groupby([[]])
# Long traceback yields: AssertionError: Grouper and axis must be same length

Came from this SO issue (but it's really only related because of the error message) - http://stackoverflow.com/questions/19483991/how-to-make-grouper-and-axis-the-same-length

What's the problem there? that seems like a reasonable assertion and a reasonable error message.

[jreback]

I think it should just be a ValueError, maybe a tad bit more informative..

[jtratner]

Oh I just closed it because I had opened it and it didn't seem particularly
important.

On Mon, Nov 17, 2014, 3:06 AM jreback notifications@github.com wrote:

Reopened #5289 #5289.

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#5289 (comment).

[simonjayhawkins]

this issue can be closed.

original code sample raises ValueError on 0.25.0.dev0+208.g74a9ae309

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\simon\OneDrive\code\pandas-simonjayhawkins\pandas\core\generic.py", line 7672, in groupby
    observed=observed, **kwargs)
  File "C:\Users\simon\OneDrive\code\pandas-simonjayhawkins\pandas\core\groupby\groupby.py", line 2206, in groupby
    return klass(obj, by, **kwds)
  File "C:\Users\simon\OneDrive\code\pandas-simonjayhawkins\pandas\core\groupby\groupby.py", line 363, in __init__
    mutated=self.mutated)
  File "C:\Users\simon\OneDrive\code\pandas-simonjayhawkins\pandas\core\groupby\grouper.py", line 605, in _get_grouper
    if not isinstance(gpr, Grouping) else gpr)
  File "C:\Users\simon\OneDrive\code\pandas-simonjayhawkins\pandas\core\groupby\grouper.py", line 242, in __init__
    self.grouper = _convert_grouper(index, grouper)
  File "C:\Users\simon\OneDrive\code\pandas-simonjayhawkins\pandas\core\groupby\grouper.py", line 632, in _convert_grouper
    raise ValueError('Grouper and axis must be same length')
ValueError: Grouper and axis must be same length