Two permutation iterators for classes in Python with and without a generator

Question

I am just testing out a couple of small classes for learning purposes. Here is my current working code:

class Permutation:
 '''An int list a permutation if starting at any index, one can reach every other index by
 repeated index accesses. The list [4, 0, 3, 1, 2] is a permutation: for example index 0
 stores a 4; index 4 stores a 2; index 2 stores a 3; index 3 stores a 1; index 1 stores a 0,
 bringing us back to that starting position after visiting every index.'''
 def __init__(self, p, start):
 self.p = p
 self.start = start
 def __iter__(self):
 class P_iter:
 def __init__(self, p, start):
 self.p = p
 self.start = start
 self.curr = start
 self.stahp = False
 def __next__(self):
 if not self.stahp:
 current = self.curr
 self.curr = self.p[self.curr]
 self.stahp = self.curr == self.start
 return current
 else:
 raise StopIteration
 return P_iter(self.p, self.start)
class Permutation2:
 '''similar to the description above, but its __iter__ method should be implemented by a
 generator (not a P_iter nested class)'''
 def __init__(self,p,start):
 self.p = p
 self.start = start
 def __iter__(self):
 curr = self.start
 stahp = False
 while not stahp:
 yield curr
 curr = self.p[curr]
 stahp = curr == self.start
 else:
 raise StopIteration

I'm looking to clean the code a bit and maybe make it simpler. The code is fully working currently and generates the desired output.

• Jeff Mercado: Your indentation looks off a bit in a number of places. You'll want to check that.
• LucyBen: @JeffMercado - I updated the code formatting.
• LucyBen: @200_success - The current code is not broken, it works fine. Just need a review. Thanks
• 200_success: Whitespace is significant in Python, therefore it was broken code. Thanks for fixing it. I've reopened the question.
• Jeff Mercado: @200_success: Still not quite bug-free. The doc strings, the whole __iter__() method of the first implementation... in this particular case, it's clear to see the intent... but it's still broken code.

Answer 1

Well Permutation has basically equivalent code to Permutation2, but is doing things the hard way. Therefore, I'll just look at Permutation2, particularly __iter__().

 def __iter__(self):
 curr = self.start
 stahp = False
 while not stahp:
 yield curr
 curr = self.p[curr]
 stahp = curr == self.start
 else:
 raise StopIteration

You shouldn't need to explicitly raise StopIteration. Iteration automatically stops when the flow of control reaches the end of the function.

I find the LoLcat spelling of "stop" amusing. However, flag variables are generally a bad idea. You would be better off writing if curr == self.start: break, for example. However, I'd just choose to write it like this:

def __iter__(self):
 yield self.start
 curr = self.p[curr]
 while curr != self.start:
 yield curr
 curr = self.p[curr]

(Once again, the lack of a do-while loop in Python makes life difficult.)

Take care to indent your code very carefully in Python, as whitespace is significant. Four spaces is a very strong norm, established in PEP 8. You've used five spaces at the first level.

Answer 2

• P_iter should have an __iter__ method to fully comply with the iterator protocol:

  def __iter__(self):
   return self
  

• Defining the P_iter class inside a method hurts readability and makes the class unavailable for anyone who wants to check the type of an iterator object.

• In Permutation2.__iter__ I would use this approach to avoid the stahp variable :

  def __iter__(self):
   curr = self.start
   while True:
   yield curr
   curr = self.p[curr]
   if curr == self.start:
   return