old_contents: from spyc.graph import Vertex, find_cycle, topological_sort
  
  
  class Scheduler(object):
  
      def __init__(self):
          self.specs = {}
  
      def ensure(self, spec):
          """Require that ``spec`` is satisfied."""
          if spec.key() in self.specs:
              self.specs[spec.key()].data.merge(spec)
          else:
              self.specs[spec.key()] = Vertex(spec)
  
      def depend(self, first, next):
          """Specify that ``first`` depends on ``next``.
  
          This also has the effect of invoking ``ensure`` on both resources.
          """
          first.schedule(self)
          next.schedule(self)
          self.specs[first.key()].edges.add(self.specs[next.key()])
  
      def apply(self):
          verticies = set(self.specs.values())
          cycle = find_cycle(verticies)
          if cycle is not None:
              assert False  # TODO proper checking
          for v in topological_sort(verticies):
              v.data.apply()
  
new_contents: from spyc.graph import Vertex, find_cycle, topological_sort
  
  
  class CircularDependency(Exception):
      pass
  
  
  class Scheduler(object):
  
      def __init__(self):
          self.specs = {}
  
      def ensure(self, spec):
          """Require that ``spec`` is satisfied."""
          if spec.key() in self.specs:
              self.specs[spec.key()].data.merge(spec)
          else:
              self.specs[spec.key()] = Vertex(spec)
  
      def depend(self, first, next):
          """Specify that ``first`` depends on ``next``.
  
          This also has the effect of invoking ``ensure`` on both resources.
          """
          first.schedule(self)
          next.schedule(self)
          self.specs[first.key()].edges.add(self.specs[next.key()])
  
      def apply(self):
          verticies = set(self.specs.values())
          cycle = find_cycle(verticies)
          if cycle is not None:
              raise CircularDependency(cycle)
          for v in topological_sort(verticies):
              v.data.apply()
  
subject: Raise a more useful error for circular deps.