Why does adding a trailing comma after a variable name make it a tuple?

It is the commas, not the parentheses, which are significant. The Python tutorial says:

A tuple consists of a number of values separated by commas

Parentheses are used for disambiguation in other places where commas are used, for example, enabling you to nest or enter a tuple as part of an argument list.

See the Python Tutorial section on Tuples and Sequences

Because this is the only way to write a tuple literal with one element. For list literals, the necessary brackets make the syntax unique, but because parantheses can also denote grouping, enclosing an expression in parentheses doesn't turn it into a tuple: you need a different syntactic element, in this case the comma.

Update

See above for a much better answer.

Original Answer

In python a tuple is indicated by parenthesis.

Tuples are not indicated by the parentheses. Any expression can be enclosed in parentheses, this is nothing special to tuples. It just happens that it is almost always necessary to use parentheses because it would otherwise be ambiguous, which is why the __str__ and __repr__ methods on a tuple will show them.

I stand corrected (all I've been doing today. Sigh).

For instance:

abc = ('my', 'string')

What about single element tuples? The parenthesis notation still holds.

abc = ('mystring',)

For all tuples, the parenthesis can be left out but the comma needs to be left in.

abc = 'mystring', # ('mystring',)

Or

abc = 'my', 'string', # ('my', 'string',)

So in effect what you were doing was to create a single element tuple as opposed to a string.

The documentation clearly says:

An expression list containing at least one comma yields a tuple. The length of the tuple is the number of expressions in the list. The expressions are evaluated from left to right.