title: Why is the string split into characters in python
body: <p>The following is the code I wrote:</p>
  <pre class="lang-py prettyprint-override"><code>def comb(self, rows, cols):
      return [s+t for s in a for t in b]
  </code></pre>
  <p>If the values of <code>rows</code> and <code>cols</code> are</p>
  <pre class="lang-py prettyprint-override"><code>rows = ['abc','efg']
  cols = ['123','456']
  </code></pre>
  <p>Expected output: <code>['abc123','abc456,'efg123','efg456']</code></p>
  <p>Program output: <code>['a1', 'a2', 'a3', 'b1', 'b2', 'b3', 'c1', 'c2', 'c3']</code></p>
  <p>I am new to Python programming. Could you help me understand what is happening? I already fixed the output but I would like to understand why this happened.</p>