Python list comprehension - access last created element?

Question

Is it possible to access the previous element generated in a list comprehension.

I am working on some toy encryption stuff. Given the key as an arbitrarily large in

Answer 1

You could use a helper object to store all the internal state while iterating over the sequence:

class Encryption:
  def __init__(self, key, init_value):
    self.key = key
    self.previous = init_value
  def next(self, element):
    self.previous = element ^ self.previous ^ self.key
    return self.previous

enc = Encryption(...)
cipher = [enc.next(e) for e in message]

That being said, adding the previously encrypted element into the xor doesn't make your algorithm any harder to break than just xor'ing every element with the key. An attacker can just xor any character in the cipher text with the previous encrypted character and so cancel out the xor that was done during encryption.