You should split your problem into smaller problems. First you should have a function which shifts one char:
import string
def shift_char(char, n=13):
if len(char) != 1:
raise ValueError('String must be size of 1')
if not isinstance(char, str):
raise ValueError('Input must be a str')
if char.lower() in string.ascii_lowercase:
idx = string.ascii_lowercase.index(char.lower())
idx += n
idx %= len(string.ascii_lowercase) # look for modulo operation, what this does
# it's often very handy for operations like this one
else:
return char
if char.islower():
return string.ascii_lowercase[idx]
else:
return string.ascii_uppercase[idx]Output:
>>> shift_char('H')
'U'
>>> shift_char('H', 2)
'J'
>>> shift_char('Z', 1) # begins shifts one to right and begins at the beginning of the list
'A'
>>> shift_char('H', 1337)
'S'
A str is also iterable. You can do the classic for loop to iterate over every single character of the str object and append it to a list or you just use the built-in map function for this task:
map_object = map(shift_char, 'Hello World!')
# first argument is the function, the second is an iterable.
# Each element of the iterable is called with the function shift_char
# shift_char('H'), shift_char('e'), shift_char('l'), ...Since Python 3 the map function returns an lazy evaluated generator object.
Output:
>>> map_object
<map at 0x7f6a1430f710>
So you need a function to iterate over it. You can use a for loop to iterate over the map object or list or tuple or put it as an argument into the join method of str:
for c in map_object:
print(c)
# now the map object is exhausted. If you want to use it more than once, you've to create a new one
map_object = map(shift_char, 'Hello World!')
# now demonstration of the join method of the class str
''.join(map_object) # iterates over map_object and join between them '' == str of len 0
# you can also write this as str.join('', map_object)
# now you want to set the keyword argument n. You can do this with the partial function from functools
from functools import partial
shift10 = partial(shift_char, n=10)
# first argument is the function itself, and n is the keyword argument of shift_char
# we create the map_object:
map_object = map(shift_char, 'Hello World!')
# and then using the join method
encoded_text = ''.join(map_object)
# or compact writing:
print(''.join(map(shift10, 'Hello World!')))
# make a function for this task
def encode(text, n):
shift_func = partial(shift_char, n=n)
return ''.join(map(shift_func, text))Now make the test:
text = 'Hello World 123 !!!'
encoded = encode(text, 13)
decoded = encode(encoded, 13)
print(text)
print(encoded)
print(decoded)
print(text == decoded)
You should see:
Output:
Hello World 123 !!!
Uryyb Jbeyq 123 !!!
Hello World 123 !!!
True
Now think about it how to extend the shift_char function, to modify numbers and maybe punctuation as well.
You can find it in string.digits and string.punctuation. Use the length of them for the modulo operation (%).
Hopefully this was not too much.
