byte types

[Gribouillis]

i don't understand this suggestion.

Here is an example with a generic function ddouble() defined for three types in python 2 and 3

from itertools import chain
import sys

if sys.version_info < (3,):
    from singledispatch import singledispatch
else:
    from functools import singledispatch
    unicode = str

@singledispatch
def ddouble(item):
    raise NotImplementedError

@ddouble.register(unicode)
def _(s):
    return ''.join(x+x for x in s)

@ddouble.register(bytearray)
def _(s):
    return bytearray(chain.from_iterable(zip(*(s, s))))

@ddouble.register(bytes)
def _(s):
    return bytes(ddouble(bytearray(s)))

if __name__ == "__main__":
    data = b'Hello world!'
    udata = data.decode()
    print(repr(ddouble(data)))
    print(repr(ddouble(udata)))

Output:
b'HHeelllloo  wwoorrlldd'
'HHeelllloo  wwoorrlldd'

i prefer to name functions based on API design before i code

The types of a function's parameters are an important part of the API. You actually want function overloading. Generic functions are definitely a way.