code: def _handle_function_exists(self, node, scope, ctxt, stream):
          
          res = fields.Int()
          try:
              func = self._handle_node(node.expr, scope, ctxt, stream)
              if isinstance(func, functions.BaseFunction):
                  res._pfp__set_value(1)
              else:
                  res._pfp__set_value(0)
          except errors.UnresolvedID:
              res._pfp__set_value(0)
          return res
docs: Handle the function_exists unary operator
  
          :node: TODO
          :scope: TODO
          :ctxt: TODO
          :stream: TODO
          :returns: TODO