What does a star (asterisk) do in f-string?

There are two alternatives for f_expression: a comma separated list of or_exprs, optionally preceded by asterisks, or a single yield_expression. Note the yield_expression does not allow an asterisk.

I assume the intention was that the comma-separated list alternative is only chosen when there's at least one comma, but the grammar doesn't actually say that. I feel like the repetition operator at the end should have been a + instead of a *.

So f"{*1}" would be a syntax error because there's an asterisk, but no comma. f"{*1,*2}" is syntactically valid, but a type error because 1 and 2 aren't iterable. f"{*[1], *[2]}" is valid and acts the same as f"{1,2}". So the asterisk is allowed because it acts as the splat operator in tuples, which can be written without parentheses in f-expressions.

Note that using or_expr as the operand to * does not mean that a bitwise or-operator has to be used there - it just means that the bitwise or-operator is the first operator in the precedence-hierachy that would be allowed as an operand to *. So it's just about setting the precedence of prefix * vs. other expressions. I believe or_expr is consistently used as the operand to prefix * everywhere in the grammar (that is, everywhere where prefix * is followed by an expression as opposed to a parameter name).