What's the best practice for handling single-value tuples in Python?

You need to somehow test for the type, if it's a string or a tuple. I'd do it like this:

keywords = library.get_keywords()
if not isinstance(keywords, tuple):
    keywords = (keywords,) # Note the comma
for keyword in keywords:
    do_your_thang(keyword)

For your first problem, I'm not really sure if this is the best answer, but I think you need to check yourself whether the returned value is a string or tuple and act accordingly.

As for your second problem, any variable can be turned into a single valued tuple by placing a , next to it:

>>> x='abc'
>>> x
'abc'
>>> tpl=x,
>>> tpl
('abc',)

Putting these two ideas together:

>>> def make_tuple(k):
...     if isinstance(k,tuple):
...             return k
...     else:
...             return k,
... 
>>> make_tuple('xyz')
('xyz',)
>>> make_tuple(('abc','xyz'))
('abc', 'xyz')

Note: IMHO it is generally a bad idea to use isinstance, or any other form of logic that needs to check the type of an object at runtime. But for this problem I don't see any way around it.

Your tuple_maker doesn't do what you think it does. An equivalent definition of tuple maker to yours is

def tuple_maker(input):
    return input

What you're seeing is that tuple_maker("a string") returns a string, while tuple_maker(["str1","str2","str3"]) returns a list of strings; neither return a tuple!

Tuples in Python are defined by the presence of commas, not brackets. Thus (1,2) is a tuple containing the values 1 and 2, while (1,) is a tuple containing the single value 1.

To convert a value to a tuple, as others have pointed out, use tuple.

>>> tuple([1])
(1,)
>>> tuple([1,2])
(1,2)