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 integer, an initialization value, and a list of elements as the message to encrypt. I need to xor each element with the previous ciphered element and the key. The following loop would do.

previous = initialization_value
cipher = []
for element in message:
    previous = element ^ previous ^ key
    cipher.append(previous)

I feel like it should be possible to turn this into a list comprehension, but I am not exactly sure how to handle both the initial value or accessing the previous value generated.
Is it possible and if so what would the comprehension be?

Answer 1

There isn’t a good, Pythonic way to do this with a list comprehension. The best way to think about list comprehensions is as a replacement for map and filter. In other words, you’d use a list comprehension whenever you need to take a list and

• Use its elements as input for some expression (e.g. squaring the elements)

• Remove some of its elements based on some condition

What these things have in common is that they each only look at a single list element at a time. This is a good rule of thumb; even if you could theoretically write the code you showed as a list comprehension, it would be awkward and unpythonic.