title: python-igraph: cannot decompose directed graph (neither STRONG nor WEAK)
body: <p>Using python-igraph 0.7.1 (Python 3.4.8), I want to <strong>decompose a directed graph into all unconnected subgraphs</strong>. However, <code>g.decompose(mode=WEAK)</code> fails:</p>
  
  <pre><code>igraph._igraph.InternalError: Error at components.c:469: only 'IGRAPH_WEAK' is implemented, Invalid value
  </code></pre>
  
  <p>As the error message and google (e.g. <a href="http://igraph-help.nongnu.narkive.com/JUljwz0R/extracting-sub-graph" rel="nofollow noreferrer">click</a>) tells me, <code>STRONG</code> decomposition is not implemented and thus expected to fail, however <strong><code>WEAK</code> decomposition SHOULD work</strong>. Well, it doesn't for me!</p>
  
  <p>Here's a minimal example:</p>
  
  <pre><code>from igraph import Graph, WEAK
  g = Graph(directed=True)
  g.add_vertices(4)
  g.add_edges([(0, 1), (2, 3)])
  g.decompose(mode=WEAK) # &lt;= FAILS!
  </code></pre>
  
  <p>I've tried <code>mode="weak"</code>, <code>mode="WEAK"</code>, <code>mode=WEAK</code>, and <code>mode=1</code>, and always get the same error.</p>
  
  <p>Is there maybe a <strong>workaround</strong>, i.e. a <strong>set of some other commands that lead to the same result?</strong> (Note that I'm not really familiar with graph theory, so I might miss something obvious in this regard.)</p>