The tricky bit here is that a Python 3 string is a sequence of Unicode characters, which is not the same as a sequence of ASCII characters.

• In Python2, the str type and the bytes type are synonyms, and there is a separate type, unicode, that represents a sequence of Unicode characters. This makes it something of a mystery, if you have a string: is it a sequence of bytes, or is it a sequence of characters in some character-set?

• In Python3, str now means unicode and we use bytes for what used to be str. Given a string—a sequence of Unicode characters—we use encode to convert it to some byte-sequence that can represent it, if there is such a sequence:

  >>> 'hello'.encode('ascii')
  b'hello'
  >>> 'sch\N{latin small letter o with diaeresis}n'
  'schön'
  >>> 'sch\N{latin small letter o with diaeresis}n'.encode('utf-8')
  b'sch\xc3\xb6n'
  

  but:

  >>> 'sch\N{latin small letter o with diaeresis}n'.encode('ascii')
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  UnicodeEncodeError: 'ascii' codec can't encode character '\xf6' in position 3: ordinal not in range(128)
  

Once you have the bytes object, you already know what to do. In Python2, if you have a str, you have a bytes object; in Python3, use .encode with your chosen encoding.

Based on Jon Clements's answer, try the codes on python3.7. I have the error like this:

>>> s = '1234'    
>>> hexlify(s)    
Traceback (most recent call last):    
  File "<pyshell#13>", line 1, in <module>    
    hexlify(s)    
TypeError: a bytes-like object is required, not 'str'

Solved by the following codes:

>>> str = '1234'.encode()    
>>> hexlify(str).decode()   
'31323334'

    #!/usr/bin/python3
    """
    Program name:  txt_to_ASC.py

    The program transfers
    a string of letters -> the corresponding
      string of hexadecimal ASCII-codes,
    eg.  the -> 746865

    Only letters in [abc...xyzABC...XYZ] should be input.

    """
    print("Transfer letters to hex ASCII-codes")
    print("Input range is [abc...xyzABC...XYZ].")
    print()
    string = input("Input set of letters, eg. the: ")
    print("hex ASCII-code: " + " "*15, end = "")

    def str_to_hasc(x):
        global glo

        byt = bytes(x, 'utf-8')
        bythex = byt.hex()

        for b1 in bythex:
            y = print(b1, end = "")
            glo = str(y)
        return glo

    str_to_hasc(string)

>>> import binascii
>>> s = b'SHASHI'
>>> myWord = binascii.b2a_hex(s)
>>> myWord
b'534841534849'
>>> binascii.a2b_hex(myWord)
b'SHASHI'


>>> bytearray.fromhex("534841534849").decode()
'SHASHI'