title: String concatenation of two pandas columns
body: <p>I have a following <code>DataFrame</code>:</p>
  
  <pre><code>from pandas import *
  df = DataFrame({'foo':['a','b','c'], 'bar':[1, 2, 3]})
  </code></pre>
  
  <p>It looks like this:</p>
  
  <pre><code>    bar foo
  0    1   a
  1    2   b
  2    3   c
  </code></pre>
  
  <p>Now I want to have something like:</p>
  
  <pre><code>     bar
  0    1 is a
  1    2 is b
  2    3 is c
  </code></pre>
  
  <p>How can I achieve this?
  I tried the following:</p>
  
  <pre><code>df['foo'] = '%s is %s' % (df['bar'], df['foo'])
  </code></pre>
  
  <p>but it gives me a wrong result:</p>
  
  <pre><code>&gt;&gt;&gt;print df.ix[0]
  
  bar                                                    a
  foo    0    a
  1    b
  2    c
  Name: bar is 0    1
  1    2
  2
  Name: 0
  </code></pre>
  
  <p>Sorry for a dumb question, but this one <a href="https://stackoverflow.com/questions/10972410/pandas-combine-two-columns-in-a-dataframe">pandas: combine two columns in a DataFrame</a> wasn't helpful for me.</p>