title: Why round(4.5) == 4 and round(5.5) == 6 in Python 3.5?
body: <p>Looks like both 4.5 and 5.5 have exact float representations in Python 3.5:</p>
  
  <pre><code>&gt;&gt;&gt; from decimal import Decimal
  &gt;&gt;&gt; Decimal(4.5)
  Decimal('4.5')
  &gt;&gt;&gt; Decimal(5.5)
  Decimal('5.5')
  </code></pre>
  
  <p>If this is the case, then why</p>
  
  <pre><code>&gt;&gt;&gt; round(4.5)
  4
  &gt;&gt;&gt; round(5.5)
  6
  </code></pre>
  
  <p>?</p>