title: Apply Pandas series string function to the whole dataframe
body: <p>I want to apply the method <code>pd.Series.str.join()</code> to my whole dataframe</p>
  <pre><code>A         B
  [foo,bar] [1,2]
  [bar,foo] [3,4]
  </code></pre>
  <p>Desired output:</p>
  <pre><code>A      B
  foobar 12
  barfoo 34
  </code></pre>
  <p>For now I used a quite slow method:
  <code>a = [df[x].str.join('') for x in df.columns]</code></p>
  <p>I tried</p>
  <pre><code>df.apply(pd.Series.str.join)
  </code></pre>
  <p>and</p>
  <pre><code>df.agg(pd.Series.str.join)
  </code></pre>
  <p>and</p>
  <pre><code>df.applymap(str.join)
  </code></pre>
  <p>but none of them seem to work. For extension of the question, how can I efficiently apply series method to the whole dataframe?</p>
  <p>Thank you.</p>