Print a string as hexadecimal bytes

You can transform your string to an integer generator. Apply hexadecimal formatting for each element and intercalate with a separator:

>>> s = "Hello, World!"
>>> ":".join("{:02x}".format(ord(c)) for c in s)
'48:65:6c:6c:6f:2c:20:57:6f:72:6c:64:21

':'.join(x.encode('hex') for x in 'Hello, World!')

For Python 2.x:

':'.join(x.encode('hex') for x in 'Hello, World!')

The code above will not work with Python 3.x. For 3.x, the code below will work:

':'.join(hex(ord(x))[2:] for x in 'Hello, World!')

Some complements to Fedor Gogolev's answer:

First, if the string contains characters whose ASCII code is below 10, they will not be displayed as required. In that case, the correct format should be {:02x}:

>>> s = "Hello Unicode \u0005!!"
>>> ":".join("{0:x}".format(ord(c)) for c in s)
'48:65:6c:6c:6f:20:75:6e:69:63:6f:64:65:20:5:21:21'
                                           ^

>>> ":".join("{:02x}".format(ord(c)) for c in s)
'48:65:6c:6c:6f:20:75:6e:69:63:6f:64:65:20:05:21:21'
                                           ^^

Second, if your "string" is in reality a "byte string" -- and since the difference matters in Python 3 -- you might prefer the following:

>>> s = b"Hello bytes \x05!!"
>>> ":".join("{:02x}".format(c) for c in s)
'48:65:6c:6c:6f:20:62:79:74:65:73:20:05:21:21'

Please note there is no need for conversion in the above code as a bytes object is defined as "an immutable sequence of integers in the range 0 <= x < 256".