def char_shift(char, shift):
	start = ord('a') if char.isupper() else ord('A')
	char = ord(char) - start
	shift = (char + shift) % 26
	return chr(shift+start)

def caesar(message, shift):
	out = ''
	for letter in message:
		if letter.isalpha():
			letter = char_shift(letter, shift)
		out += letter

		# or in one line
		# out += char_shift(letter, shift) if letter.isalpha() else letter

	return out

def decrypt_caesar(message, shift):
	return caesar(message, shift - (shift*2))

# Add your own assert statements to test your code.

sentence = 'But the Caesar Cipher is [still used](http://en.wikipedia.org/wiki/ROT13)'
shift = 300

assert(decrypt_caesar(caesar(sentence, shift), 300) == sentence )