trimming column named is generating ValueError - pandas

Question

I have a table which I run through a function to trim its columns down to length 128 (I know it's really long, there isn't anything I can do about that) characters so it can use to_sql to create a database from it.
def truncate_column_names(df, length):
rename = {}
for col in df.columns:
if len(col) > length:
new_col = col[:length-3]+"..."
rename[col] = new_col
result = df.rename(columns=rename)
return result
This function works fine and I get a table out just fine but the problem comes when I tried to save the file I get the error
ValueError: Buffer has wrong number of dimensions (expected 1, got 2)
The method I have doing some housekeeping before saving to a file included dropping duplicates and that is where this error is being spit out. I tested this by saving the original dataFrame and then just loading it, running the truncate function, and then trying drop_duplicates on the result and I get the same error.
The headers for the file before I try truncating looks like this:
http://pastebin.com/WXmvwHDg
I trimmed the file down to 1 record and still have the problem.

Answer 1

This was a result of the truncating causing some columns to have non-unique names.
To confirm this was an issue I did a short test:
In [113]: df = pd.DataFrame(columns=["ab", "ac", "ad"])
In [114]: df
Out[114]:
Empty DataFrame
Columns: [ab, ac, ad]
Index: []
In [115]: df.drop_duplicates()
Out[115]:
Empty DataFrame
Columns: [ab, ac, ad]
Index: []
In [116]: df.columns
Out[116]: Index([u'ab', u'ac', u'ad'], dtype='object')
In [117]: df.columns = df.columns.str[:1]
In [118]: df
Out[118]:
Empty DataFrame
Columns: [a, a, a]
Index: []
In [119]: df.drop_duplicates()
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-119-daf275b6788b> in <module>()
----> 1 df.drop_duplicates()
C:\Miniconda\lib\site-packages\pandas\util\decorators.pyc in wrapper(*args, **kw
args)
86 else:
87 kwargs[new_arg_name] = new_arg_value
---> 88 return func(*args, **kwargs)
89 return wrapper
90 return _deprecate_kwarg
C:\Miniconda\lib\site-packages\pandas\core\frame.pyc in drop_duplicates(self, su
bset, take_last, inplace)
2826 deduplicated : DataFrame
2827 """
-> 2828 duplicated = self.duplicated(subset, take_last=take_last)
2829
2830 if inplace:
C:\Miniconda\lib\site-packages\pandas\util\decorators.pyc in wrapper(*args, **kw
args)
86 else:
87 kwargs[new_arg_name] = new_arg_value
---> 88 return func(*args, **kwargs)
89 return wrapper
90 return _deprecate_kwarg
C:\Miniconda\lib\site-packages\pandas\core\frame.pyc in duplicated(self, subset,
take_last)
2871
2872 vals = (self[col].values for col in subset)
-> 2873 labels, shape = map(list, zip( * map(f, vals)))
2874
2875 ids = get_group_index(labels, shape, sort=False, xnull=False)
C:\Miniconda\lib\site-packages\pandas\core\frame.pyc in f(vals)
2860
2861 def f(vals):
-> 2862 labels, shape = factorize(vals, size_hint=min(len(self), _SI
ZE_HINT_LIMIT))
2863 return labels.astype('i8',copy=False), len(shape)
2864
C:\Miniconda\lib\site-packages\pandas\core\algorithms.pyc in factorize(values, s
ort, order, na_sentinel, size_hint)
133 table = hash_klass(size_hint or len(vals))
134 uniques = vec_klass()
--> 135 labels = table.get_labels(vals, uniques, 0, na_sentinel)
136
137 labels = com._ensure_platform_int(labels)
pandas\hashtable.pyx in pandas.hashtable.PyObjectHashTable.get_labels (pandas\ha
shtable.c:13946)()
ValueError: Buffer has wrong number of dimensions (expected 1, got 2)
and got the same result. using df.columns.unique() after the truncation i had ~200 duplicate columns after the truncation