Index pandas DataFrame by column numbers, when column names are integers

Question

I am trying to keep just certain columns of a DataFrame, and it works fine when column names are strings:

In [2]: import numpy as np

In [3]: import pandas as pd

In [4]: a = np.arange(35).reshape(5,7)

In [5]: df = pd.DataFrame(a, ['x', 'y', 'u', 'z', 'w'], ['a', 'b', 'c', 'd', 'e', 'f', 'g'])

In [6]: df
Out[6]: 
    a   b   c   d   e   f   g
x   0   1   2   3   4   5   6
y   7   8   9  10  11  12  13
u  14  15  16  17  18  19  20
z  21  22  23  24  25  26  27
w  28  29  30  31  32  33  34

[5 rows x 7 columns]

In [7]: df[[1,3]] #No problem
Out[7]: 
    b   d
x   1   3
y   8  10
u  15  17
z  22  24
w  29  31

However, when column names are integers, I am getting a key error:

In [8]: df = pd.DataFrame(a, ['x', 'y', 'u', 'z', 'w'], range(10, 17))

In [9]: df
Out[9]: 
   10  11  12  13  14  15  16
x   0   1   2   3   4   5   6
y   7   8   9  10  11  12  13
u  14  15  16  17  18  19  20
z  21  22  23  24  25  26  27
w  28  29  30  31  32  33  34

[5 rows x 7 columns]

In [10]: df[[1,3]]

Results in:

KeyError: '[1 3] not in index'

I can see why pandas does not allow that -> to avoid mix up between indexing by column names and column numbers. However, is there a way to tell pandas that I want to index by column numbers? Of course, one solution is to convert column names to strings, but I am wondering if there is a better solution.

Answer 1

This is exactly the purpose of iloc, see here

In [37]: df
Out[37]: 
   10  11  12  13  14  15  16
x   0   1   2   3   4   5   6
y   7   8   9  10  11  12  13
u  14  15  16  17  18  19  20
z  21  22  23  24  25  26  27
w  28  29  30  31  32  33  34

In [38]: df.iloc[:,[1,3]]
Out[38]: 
   11  13
x   1   3
y   8  10
u  15  17
z  22  24
w  29  31

Answer 2

Just convert the headers from integer to string. This should be done almost always as a best practice when working with pandas datasets to avoid surprise

df.columns = df.columns.map(str)

Answer 3

This is certainly one of those things that feels like a bug but is really a design decision (I think).

A few work around options:

rename the columns with their positions as their name:

 df.columns = arange(0,len(df.columns))

Another way is to get names from df.columns:

print df[ df.columns[[1,3]] ]
   11  13
x   1   3
y   8  10
u  15  17
z  22  24
w  29  31

I suspect this is the most appealing as it just requires adding a wee bit of code and not changing any column names.

Answer 4

import pandas as pd
df = pd.DataFrame(a, ['x', 'y', 'u', 'z', 'w'], range(10, 17))

#Let say you want to keep only columns 1,2 (these are locations not names)
needed_columns = [1,2]

df = df[df.columns[needed_columns]

print(df)

11  12
x   1   2
y   8   9
u   15  16
z   22  23
w   29  30