In [2]:
df
Out[2]:
a b c
1 4.0 7 10.0
2 5.0 8 11.0
3 NaN 9 NaN

Selecting a single column yields a Series.

In [3]:
df['a']
Out[3]:
1    4.0
2    5.0
3    NaN
Name: a, dtype: float64

Select a single column to yield a DataFrame.

In [4]:
df[['a']]
Out[4]:
a
1 4.0
2 5.0
3 NaN

Selecting multiple columns yields a DataFrame.

In [5]:
df[['a', 'c']]
Out[5]:
a c
1 4.0 10.0
2 5.0 11.0
3 NaN NaN