String concatenation of two pandas columns

I have a following DataFrame:

from pandas import *
df = DataFrame({'foo':['a','b','c'], 'bar':[1, 2, 3]})

It looks like this:

    bar foo
0    1   a
1    2   b
2    3   c

Now I want to have something like:

     bar
0    1 is a
1    2 is b
2    3 is c

How can I achieve this? I tried the following:

df['foo'] = '%s is %s' % (df['bar'], df['foo'])

but it gives me a wrong result:

>>>print df.ix[0]

bar                                                    a
foo    0    a
1    b
2    c
Name: bar is 0    1
1    2
2
Name: 0

Sorry for a dumb question, but this one pandas: combine two columns in a DataFrame wasn't helpful for me.