old_contents: """
  Copyright 2015 Jason M. Sachs
  
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at
  
      http://www.apache.org/licenses/LICENSE-2.0
  
  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.
  """
  
  def walk_subsets(items):
      pool = list(items)
      subst = False
      for k in xrange(len(pool)):
          remaining = pool[(k+1):]
          yield pool[k], subst + (k>0), remaining
          # iterating loop can del remaining[:] to stop recursion
          subst = bool(remaining)
          if subst:
              for out in walk_subsets(remaining):
                  yield out
  
new_contents: """
  Copyright 2015 Jason M. Sachs
  
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at
  
      http://www.apache.org/licenses/LICENSE-2.0
  
  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.
  
  example:
  
      def example(items, skipitems):
          l = []
          for k,remove,remaining in walk_subsets(items):
              for _ in xrange(remove):
                  l.pop()
              l.append(k)
              if l in skipitems:
                  del remaining[:]
              print l
              
      example(range(4), ())
      print ""
      example(range(5), ([1], [0,2]))
      
  output:
  
      [0]
      [0, 1]
      [0, 1, 2]
      [0, 1, 2, 3]
      [0, 1, 3]
      [0, 2]
      [0, 2, 3]
      [0, 3]
      [1]
      [1, 2]
      [1, 2, 3]
      [1, 3]
      [2]
      [2, 3]
      [3]
      
      [0]
      [0, 1]
      [0, 1, 2]
      [0, 1, 2, 3]
      [0, 1, 2, 3, 4]
      [0, 1, 2, 4]
      [0, 1, 3]
      [0, 1, 3, 4]
      [0, 1, 4]
      [0, 2]
      [0, 3]
      [0, 3, 4]
      [0, 4]
      [1]
      [2]
      [2, 3]
      [2, 3, 4]
      [2, 4]
      [3]
      [3, 4]
      [4]
  
  """
  
  def walk_subsets(items):
      pool = list(items)
      subst = False
      for k in xrange(len(pool)):
          remaining = pool[(k+1):]
          yield pool[k], subst + (k>0), remaining
          # iterating loop can del remaining[:] to stop recursion
          subst = bool(remaining)
          if subst:
              for out in walk_subsets(remaining):
                  yield out
  
subject: Update walk_subsets.py