title: How to check if a string only contains letters?
body: <p>I'm trying to check if a string only contains letters, not digits or symbols.</p>
  <p>For example:</p>
  <pre><code>&gt;&gt;&gt; only_letters(&quot;hello&quot;)
  True
  &gt;&gt;&gt; only_letters(&quot;he7lo&quot;)
  False
  </code></pre>