How to convert between bytes and strings in Python 3?

This is a Python 101 type question, but it had me baffled for a while when I tried to use a package that seemed to convert my string input into bytes.

As you will see below I found the answer for myself, but I felt it was worth recording here because of the time it took me to unearth what was going on. It seems to be generic to Python 3, so I have not referred to the original package I was playing with; it does not seem to be an error (just that the particular package had a .tostring() method that was clearly not producing what I understood as a string...)

My test program goes like this:

import mangler                                 # spoof package

stringThing = """
<Doc>
    <Greeting>Hello World</Greeting>
    <Greeting>你好</Greeting>
</Doc>
"""

# print out the input
print('This is the string input:')
print(stringThing)

# now make the string into bytes
bytesThing = mangler.tostring(stringThing)    # pseudo-code again

# now print it out
print('\nThis is the bytes output:')
print(bytesThing)

The output from this code gives this:

This is the string input:

<Doc>
    <Greeting>Hello World</Greeting>
    <Greeting>你好</Greeting>
</Doc>


This is the bytes output:
b'\n<Doc>\n    <Greeting>Hello World</Greeting>\n    <Greeting>\xe4\xbd\xa0\xe5\xa5\xbd</Greeting>\n</Doc>\n'

So, there is a need to be able to convert between bytes and strings, to avoid ending up with non-ascii characters being turned into gobbledegook.