title: Pandas adding percentage column by grouping
body: <p>what would be the syntax for computing the percent of a value over the total by rows?<br>
  For example,  I want to add a percentage column to my df according to its Item and Name1:</p>
  
  <pre><code>Item  Name1   Name2  Value
  111   x       a      1
  111   x       b      2
  111   y       a      1
  222   x       i      1
  222   y       j      1
  222   y       k      1
  </code></pre>
  
  <p>By grouping Item and Name1, the result would be:</p>
  
  <pre><code>Item  Name1   Name2  Value  Percent
  111   x       a      1      .33     because (1/3)
  111   x       b      2      .67     because (2/3)
  111   y       a      1      1       because (1/1)
  222   x       i      1      1       because (1/1)
  222   y       j      1      .5      because (1/2)
  222   y       k      1      .5      because (1/2)
  </code></pre>
  
  <p>Many thanks :)</p>