title: Python igraph: TypeError: iterable must return pairs of integers or strings
body: <p>I was wondering what is wring with this code:</p>
  
  <pre><code>import igraph as ig
  test = ig.Graph()
  test.add_vertices(0)
  test.add_vertices(1)
  test.add_vertices(2)
  test.add_edges((1,2))`
  </code></pre>
  
  <p>When I run it, I receive the following error:</p>
  
  <pre><code>TypeError: iterable must return pairs of integers or strings
  </code></pre>