Python: AttributeError: 'int' object has no attribute 'isalpha'

Question

I'm looking to create a Caesar's Cipher function in python using client/server. The client sends a message with a key and the server encrypts it. Server Code:

import socket

def getCaesar(message, key):
    result=""
    for l in message:
        if l.isalpha():
            num = ord(l)
            num += key

            if l.isupper():
                if num > ord('Z'):
                    num -= 26
                elif num < ord('A'):
                    num += 26

            elif l.islower():
                if num > ord('z'):
                    num -= 26
                elif num < ord('a'):
                    num += 26

            result += chr(num)
        else:
            result += l
return result    

serverSock=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
host=socket.gethostname()
port=4000

serverSock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

serverSock.bind((host,port))
serverSock.listen(5)
print("Listenting for requests")
while True:
    s,addr=serverSock.accept()
    print("Got connection from ",addr)
    print("Receiving...")

    message=s.recv(1024)
    key=s.recv(1024)

    resp=getCaesar(message, key)
    print('Ciphertext: ')
    print(resp)

serverSock.close()

The line that keeps getting called out is line 6:' if l isalpha(): ' and gives me the error: AttributeError: 'int' object has no attribute 'isalpha'. What does this error mean?

Client Program:

import socket

def getMessage():
    print('Enter your message:')
    return input()

def getKey():
    key = 0
    while True:
        print('Enter the key number (1-%s)' % (26))
        key = int(input())
        if (key >= 1 and key <= 26):
            return key

s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
host=socket.gethostname()
port=4000
s.connect((host,port))


message = getMessage()
key = getKey()

message=message.encode()


s.send(message)
s.send(bytes(key))
cipher= s.recv(1024)

print('Ciphertext: ')
print(cipher)
s.close()

Answer 1

In python 3, socket recv returns an immutable sequence of bytes. Each element has type int and is in the range [0, 255]. isalpha is a method of str and is not a method of int.

If you'd like to treat the server response as a string, you can decode the bytes.

string = response.decode('utf-8')

for c in string:
    if c.isalpha():
        [...]

Answer 2

as mentioned in above answer, isalpha() is the method of string so apply explicit string casting for object l, Instead of l.isalpha() use this str(l).isalpha()