Changing data in a dataframe with hierarchical indexing

How can I change every element in a DataFrame with hierarchical indexing? For example, maybe I want to convert strings into floats:

from pandas import DataFrame
f = DataFrame({'a': ['1,000','2,000','3,000'], 'b': ['2,000','3,000','4,000']})
f.columns = [['level1', 'level1'],['item1', 'item2']]
f
Out[152]:
        level1
     item1   item2
0    1,000   2,000
1    2,000   3,000
2    3,000   4,000

I tried this:

def clean(group):
    group = group.map(lambda x: x.replace(',', ''))
    return group
f.apply(clean)
Out[153]:
(level1, item1) (level1, item2)
0    1000    2000
1    2000    3000
2    3000    4000

As you can see, it changes the hierarchical indexing quite a bit. How can I avoid this? Or maybe there is a better way.

Thanks

---

Pass the axis option to the apply function:

In [265]: f.apply(clean, axis=1)
Out[265]:
  level1
   item1 item2
0   1000  2000
1   2000  3000
2   3000  4000

When both axes have hierarchical indices here's a workaround:

In [316]: f.index = [[1,2,3],[1,2,3]]

In [317]: f
Out[317]:
    level1
     item1  item2
1 1  1,000  2,000
2 2  2,000  3,000
3 3  3,000  4,000

In [314]: f.apply(clean, axis=1).reindex(f.index)
Out[314]:
    level1
     item1 item2
1 1   1000  2000
2 2   2000  3000
3 3   3000  4000

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