Alex Rivera | Logout

Why does adding a trailing comma after an expression create a tuple?

Asked 2010-09-20T10:43:50.947
44

Why does adding a trailing comma after an expression create a tuple with the expression's value? E.g. in this code:

>>> abc = 'mystring',
>>> print(abc)
('mystring',)

Why is the printed output ('mystring',), and not just mystring?

python syntax tuples
Edit
Report

1 Answer

6

Make sure to read this great answer by Ben James.

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.

For instance:

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

What about single element tuples?

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

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.

answered 2010-09-20T10:51:21.430

Your Answer