pandas (original) (raw)

Related #7770

Using the example of the docs (http://pandas.pydata.org/pandas-docs/stable/reshaping.html#multiple-levels):

columns = MultiIndex.from_tuples([('A', 'cat', 'long'), ('B', 'cat', 'long'), ('A', 'dog', 'short'), ('B', 'dog', 'short')], 
                                 names=['exp', 'animal', 'hair_length'])
df = DataFrame(randn(4, 4), columns=columns)

CONTEXT: df.stack(level=['animal', 'hair_length']) and df.stack(level=[1, 2]) are equivalent (feature introduced in #7770). Mixing integers location and string names (eg df.stack(level=['animal', 2])) gives a ValueError.

But if you have level names of mixed types, some different (and wrong things) happen:

With a total different number, it still works as it should:

df.columns.names = ['exp', 'animal', 10]  
df.stack(level=['animal', 10])

With the number 1, it treats the 1 as a level number instead of the level name, leading to a wrong result (two times the same level unstacked):

In [42]: df.columns.names = ['exp', 'animal', 1]  
In [43]: df.stack(level=['animal', 1])  
Out[43]:  
exp                     A         B  
  animal animal  
0 cat    cat    -1.006065  0.401136  
  dog    dog     0.526734 -1.753478  
1 cat    cat    -0.718401 -0.400386  
  dog    dog    -0.951336 -1.074323  
2 cat    cat     1.119843 -0.606982  
  dog    dog     0.371467 -1.837341  
3 cat    cat    -1.467968  1.114524  
  dog    dog    -0.040112  0.240026

With the number 0, it gives a strange error:

In [46]: df.columns.names = ['exp', 'animal', 0]  
In [47]: df.stack(level=['animal', 0])  
---------------------------------------------------------------------------  
KeyError                                  Traceback (most recent call last)  
<ipython-input-47-4e9507e0708f> in <module>()  
----> 1 df.stack(level=['animal', 0])  
/home/joris/scipy/pandas/pandas/core/frame.pyc in stack(self, level, dropna)  
3390  
3391         if isinstance(level, (tuple, list)):  
-> 3392             return stack_multiple(self, level, dropna=dropna)  
3393         else:  
3394             return stack(self, level, dropna=dropna)  
....  
/home/joris/scipy/pandas/pandas/core/index.pyc in _partial_tup_index(self, tup, side)  
3820             raise KeyError('Key length (%d) was greater than MultiIndex'  
3821                            ' lexsort depth (%d)' %  
-> 3822                            (len(tup), self.lexsort_depth))  
3823  
3824         n = len(tup)  
KeyError: 'Key length (2) was greater than MultiIndex lexsort depth (0)'