title: Merging two dataframes in python pandas
body: <p>I have a dataframe A:</p>
  
  <pre><code>a 1
  a 2
  b 1
  b 2
  </code></pre>
  
  <p>Another dataframe B:</p>
  
  <pre><code>a 3
  a 4 
  b 3
  </code></pre>
  
  <p>I want my result dataframe to be like </p>
  
  <pre><code>a 1 3
  a 1 4 
  a 2 3
  a 2 4 
  b 1 3 
  b 2 3 
  </code></pre>
  
  <p>I wondering how I can get the desired result in python. Thanks a lot!</p>