title: Set an optional variable in named tuple
body: <pre><code>from collections import namedtuple
  
  FooT = namedtuple('Foo', 'foo bar')
  def Foo(foo=None, bar=None):
    return FooT(foo,bar)
  
  foo = Foo()
  foo.foo = 29
  throws attribute error
  </code></pre>
  
  <p>So, my use case is a datastructure which have optional fields.. but should be able to modify it if desired..</p>