title: What is an expression in Python?
body: <p>I have some confusion about its meaning or definition. </p>
  
  <p>Isn't that some code that produce or calculate new data values? (Says Zelle in his book)</p>
  
  <p>Then I wonder if a string data type is an expression.</p>
  
  <p>If it is, then what does <code>eval()</code> do when its argument is a string?</p>
  
  <p>The book by Zelle says<code>eval(&lt;string&gt;)</code> evaluates <code>string</code> as an expression, what does that exactly mean if <code>string</code> is already an expression? </p>
  
  <p>And if <code>string</code> is not an expression, then how come it can occur after <code>print</code>?</p>