ord('a')

97

ord('b')

98

ord('A')

65

chr(97)

'a'

def encode(n,s):
    result = ""
    for c in s:
        if c >= 'a' and c <= 'z':
            result = result + chr(((ord(c) - ord('a')) + n)%26 + ord('a'))
        else:
            result = result + c
    return result

encode(13,'my name is Raj')

'zl anzr vf Rnw'

chr((ord('a')-ord('a')+13)%26 + ord('a'))

'n'

def encode(n,s):
    result = ""
    for c in s:
        if c >= 'a' and c <= 'z':
            result = result + chr(((ord(c) - ord('a')) + n)%26 + ord('a'))
        else:
            result = result + c
    return result