title: List of list if statement for Python
body: <p>I have a list that looks like: </p>
  
  <pre><code>[['a', 'b', 'null', 'c'], ['d', 'e', '5', 'f'], ['g' ,'h', 'null' ,'I'] ]
  </code></pre>
  
  <p>I want to uppercase all strings, but when I try:</p>
  
  <pre><code>x = 0
  for row in range(1, sheet.nrows):
      listx.append(sheet.cell(row, x))
      [x.upper() for x in x in listx]
  x += 1
  </code></pre>
  
  <p>I got:</p>
  
  <pre><code>TypeError: 'bool' object is not iterable
  </code></pre>
  
  <p>How can I make a statement for it?</p>