Python tuple ... is not a tuple? What does the comma do?

Question

I was looking at code in my course material and had to write a function which adds the value 99 to either a list or tuple. The final code looks like this:

def f(l):
    print(l)
    l += 99,
    print(l)

f([1,2,3])
f((1,2,3))

This was used to show something different but I'm getting somewhat hung up on the line l += 99,. What this does, is create an iterable that contains the 99 and list as well as tuple support the simple "addition" of such an object to create a new instance/add a new element.

What I don't really get is what exactly is created using the syntax element,? If I do an assignment like x = 99, the type(x) will be tuple but if I try run x = tuple(99) it will fail as the 99 is not iterable. So is there:

• Some kind of intermediate iterable object created using the syntax element,?
• Is there a special function defined that would allow the calling of tuple without an iterable and somehow , is mapped to that?

Edit: In case anyone wonders why the accepted answer is the one it is: The explanation for my second question made it. I should've been more clear with my question but that += is what actuallly got me confused and this answer includes information on this.

Answer 1

If the left-hand argument of = is a simple name, the type of argument currently bound to that name is irrelevant. tuple(99) fails because tuple's argument is not iterable; it has nothing to do with whether or not x already refers to an instance of tuple.

99, creates a tuple with a single argument; parentheses are only necessary to separate it from other uses of commas. For example, foo((99,100)) calls foo with a single tuple argument, while foo(99,100) calls foo with two distinct int arguments.