ParentedTree.collapse_unary(collapsePOS=False, collapseRoot=False, joinChar=u'+')

Collapse subtrees with a single child (ie. unary productions) into a new non-terminal (Tree node) joined by ‘joinChar’. This is useful when working with algorithms that do not allow unary productions, and completely removing the unary productions would require loss of useful information. The Tree is modified directly (since it is passed by reference) and no value is returned.

  • collapsePOS (bool) – ‘False’ (default) will not collapse the parent of leaf nodes (ie. Part-of-Speech tags) since they are always unary productions
  • collapseRoot (bool) – ‘False’ (default) will not modify the root production if it is unary. For the Penn WSJ treebank corpus, this corresponds to the TOP -> productions.
  • joinChar (str) – A string used to connect collapsed node values (default = “+”)