What is the syntax rule for having trailing commas in tuple definitions?

Question

In the case of a single element tuple, the trailing comma is required.

a = ('foo',)

What about a tuple with multiple elements? It seems that whether the trailing comma exists or not, they are both valid. Is this correct? Having a trailing comma is easier for editing in my opinion. Is that a bad coding style?

a = ('foo1', 'foo2')
b = ('foo1', 'foo2',)

Answer 1

It is only required for single-item tuples to disambiguate defining a tuple or an expression surrounded by parentheses.

(1)  # the number 1 (the parentheses are wrapping the expression `1`)
(1,) # a 1-tuple holding a number 1

For more than one item, it is no longer necessary since it is perfectly clear it is a tuple. However, the trailing comma is allowed to make defining them using multiple lines easier. You could add to the end or rearrange items without breaking the syntax because you left out a comma on accident.

e.g.,

someBigTuple = (
                   0,
                   1,
                   2,
                   3,
                   4,
                   5,
                   6,
                   7,
                   8,
                   9,
                   10,
                   #...
                   10000000000,
               )

Note that this applies to other collections (e.g., lists and dictionaries) too and not just tuples.