if function error in Python 3

I am encountering issues in python with the if function in an exercise for my computer programming class in college, and am running into a problem with the 2nd if statement in the following program. It says that there is a syntax error:

my_text = input("enter plaintext: ")
distance = int(input("enter distance: "))
def caesar(my_text , distance)

for ch in my_text:
    if ch.isalpha():
        alphabet = ord(ch) + distance
        if alphabet < 26:
            alphabet -=26
        else:
            alphabet = (alphabet)%26
        final_letter = chr(alphabet)
    ciphertext = ""
    ciphertext += final_letter
    print (("cipher text is: ") , ciphertext
return (ciphertext)

caesar(my_text , distance)

The assignment is to write a code for using the caesar cipher to encrypt messages. Thanks, and please let me know if there are an other mistakes, as I am new to python and am prone to making mistakes