JoseleMG JoseleMG - 1 month ago 18
Python Question

Python pandas Multindex column

First of all, I'm using python 3.50 in a jupyter notebook.

I want to create a DataFrame for showing some data in a report. I want it to have two index column (Excuse me if the term to refer it is not the correct. I'm not use to work with pandas).

I have this example code that works:

frame = pd.DataFrame(np.arange(12).reshape(( 4, 3)),
index =[['a', 'a', 'b', 'b'], [1, 2, 1, 2]],
columns =[['Ohio', 'Ohio', 'Ohio'], ['Green', 'Red', 'Green']])


But when I try to take it to my case, it gives me an error:

cell_rise_Inv= pd.DataFrame([[0.00483211, 0.00511619, 0.00891821, 0.0449637, 0.205753],
[0.00520049, 0.00561577, 0.010993, 0.0468998, 0.207461],
[0.00357213, 0.00429087, 0.0132186, 0.0536389, 0.21384],
[-0.0021868, -0.0011312, 0.0120546, 0.0647213, 0.224749],
[-0.0725403, -0.0700884, -0.0382486, 0.0899121, 0.313639]],
index =[['transition [ns]','transition [ns]','transition [ns]','transition [ns]','transition [ns]'],
[0.0005, 0.001, 0.01, 0.1, 0.5]],
columns =[[0.01, 0.02, 0.05, 0.1, 0.5],['capacitance [pF]','capacitance [pF]','capacitance [pF]','capacitance [pF]','capacitance [pF]']])
cell_rise_Inv

---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-89-180a1ad88403> in <module>()
6 index =[['transition [ns]','transition [ns]','transition [ns]','transition [ns]','transition [ns]'],
7 [0.0005, 0.001, 0.01, 0.1, 0.5]],
----> 8 columns =[[0.01, 0.02, 0.05, 0.1, 0.5],['capacitance [pF]','capacitance [pF]','capacitance [pF]','capacitance [pF]','capacitance [pF]']])
9 cell_rise_Inv

C:\Users\Josele\Anaconda3\lib\site-packages\pandas\core\frame.py in __init__(self, data, index, columns, dtype, copy)
261 if com.is_named_tuple(data[0]) and columns is None:
262 columns = data[0]._fields
--> 263 arrays, columns = _to_arrays(data, columns, dtype=dtype)
264 columns = _ensure_index(columns)
265

C:\Users\Josele\Anaconda3\lib\site-packages\pandas\core\frame.py in _to_arrays(data, columns, coerce_float, dtype)
5350 if isinstance(data[0], (list, tuple)):
5351 return _list_to_arrays(data, columns, coerce_float=coerce_float,
-> 5352 dtype=dtype)
5353 elif isinstance(data[0], collections.Mapping):
5354 return _list_of_dict_to_arrays(data, columns,

C:\Users\Josele\Anaconda3\lib\site-packages\pandas\core\frame.py in _list_to_arrays(data, columns, coerce_float, dtype)
5429 content = list(lib.to_object_array(data).T)
5430 return _convert_object_array(content, columns, dtype=dtype,
-> 5431 coerce_float=coerce_float)
5432
5433

C:\Users\Josele\Anaconda3\lib\site-packages\pandas\core\frame.py in _convert_object_array(content, columns, coerce_float, dtype)
5487 # caller's responsibility to check for this...
5488 raise AssertionError('%d columns passed, passed data had %s '
-> 5489 'columns' % (len(columns), len(content)))
5490
5491 # provide soft conversion of object dtypes

AssertionError: 2 columns passed, passed data had 5 columns


Any ideas? I can't understand why the example works and mine don't do it. :S

Thank you in advance :).

Answer

There is one major difference between your code and the example: the example passes a numpy array as the input rather than a nested list. In fact, adding np.array(...) around your list works just fine:

cell_rise_Inv= pd.DataFrame(
        np.array([[0.00483211, 0.00511619, 0.00891821, 0.0449637, 0.205753], 
                  [0.00520049, 0.00561577, 0.010993, 0.0468998, 0.207461],
                  [0.00357213, 0.00429087, 0.0132186, 0.0536389, 0.21384],
                  [-0.0021868, -0.0011312, 0.0120546, 0.0647213, 0.224749],
                  [-0.0725403, -0.0700884, -0.0382486, 0.0899121, 0.313639]]), 
        index=[['transition [ns]'] * 5,
               [0.0005, 0.001, 0.01, 0.1, 0.5]],
        columns=[['capacitance [pF]'] * 5,
                 [0.01, 0.02, 0.05, 0.1, 0.5]])

I shortened the repeated strings in the index and swapped the order of the index levels, but those are not significant changes.

EDIT Did a little investigating. If you pass in a nested list (without the np.array call), the call will work without columns and even if columns is a 1D list. For some reason the nested list of two elements is not being interpreted as a multiindex unless the input is an ndarray.

I filed issue #14467 with pandas based on this question.