Cannot get right slice bound for non-unique label when indexing data frame with python-pandas

Question

I have such a data frame df:

a         b
10        2
3         1
0         0
0         4
....
# about 50,000+ rows

I wish to choose the df[:5, 'a']. But When I call df.loc[:5, 'a'], I got an error: KeyError: 'Cannot get right slice bound for non-unique label: 5. When I call df.loc[5], the result contains 250 rows while there is just one when I use df.iloc[5]. Why does this thing happen and how can I index it properly? Thank you in advance!

Answer 1

The error message is explained here: if the index is not monotonic, then both slice bounds must be unique members of the index.

The difference between .loc and .iloc is label vs integer position based indexing - see docs. .loc is intended to select individual labels or slices of labels. That's why .loc[5] selects all rows where the index has the value 250 (and the error is about a non-unique index). iloc, in contrast, select row number 5 (0-indexed). That's why you only get a single row, and the index value may or may not be 5. Hope this helps!

Answer 2

To filter with non-unique indexs try something like this: df.loc[(df.index>0)&(df.index<2)]