title: python3.6 variable substitution in jupyter notebook
body: <p>I have the following syntax that should be correct in Python3.6</p>
  
  <pre><code>for k in current:
          new_key = f'{key}.{k}' if len(key) &gt; 0 else k
          flatten(current[k], new_key, result)
  </code></pre>
  
  <p>the "f" before the string in new_key makes all the following code colored in red.
  Is there any mistake in the code? I'm able to run it without errors?</p>