enter string into list according to a known rule

[buran]

I don't really understand what you want to do. We don't know what Data is in your example (str, bytearay, list. something else?)

spam = bytearray(b'0P\x14}\x04\xf8\xff3')
data = [f"{byte:02x}" for byte in spam]
print(f'Full Data: {data}')
start=0
length = 4
data = [f"{byte:02x}" for byte in spam[start:start + length]]
print(f'Partial data: {data}')

Output:
Full Data: ['30', '50', '14', '7d', '04', 'f8', 'ff', '33']
Partial data: ['30', '50', '14', '7d']