How to create a tuple with only one element

('a') is not a tuple, but just a string.

You need to add an extra comma at the end to make python take them as tuple: -

>>> a = [('a',), ('b',), ('c', 'd')]
>>> a
[('a',), ('b',), ('c', 'd')]
>>> 

Your first two examples are not tuples, they are strings. Single-item tuples require a trailing comma, as in:

>>> a = [('a',), ('b',), ('c', 'd')]
>>> a
[('a',), ('b',), ('c', 'd')]

why is a tuple converted to a string when it only contains a single string?

a = [('a'), ('b'), ('c', 'd')]

Because those first two elements aren't tuples; they're just strings. The parenthesis don't automatically make them tuples. You have to add a comma after the string to indicate to python that it should be a tuple.

>>> type( ('a') )
<type 'str'>

>>> type( ('a',) )
<type 'tuple'>

To fix your example code, add commas here:

>>> a = [('a',), ('b',), ('c', 'd')]

             ^       ^

From the Python Docs:

A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective.

If you truly hate the trailing comma syntax, a workaround is to pass a list to the tuple() function:

x = tuple(['a'])