I was researching about python codegolf and saw someone use the unpacking operator in a strange way:
*s,='abcde'
I know that the unpacking operator basically iterates over a sequence. So I know that
s=[*'abcde']
will “unpack” the abcde string and save ['a', 'b', 'c', 'd', 'e'] in variable s.
Can someone explain as thoroughly as possible how does the
*s,='abcde'
statement work? I know it does the same thing as s=[*'abcde'] but it accomplishes it in a different way. Why is the unpacking iterator on the variable, instead of the string? Why is there a comma right after the variable name?
Answers:
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Method 1
This is Iterable Unpacking. You may have seen it in other places to assign values to multiple variables from a single expression
a, b, c = [1, 2, 3]
This syntax includes a * to indicate that this variable should be a list containing the elements from the iterable that weren’t explicitly assigned to another variable.
a, *b, c = [1, 2, 3, 4, 5] print(b) # [2, 3, 4]
So, what’s going on in your example? There’s only a single variable name being assigned to, so it’s going to take all the items not assigned to another variable, which in this case is all of them. If you try just
*s='abcde'
you’ll get
SyntaxError: starred assignment target must be in a list or tuple
Which is why that comma is there, as a trailing comma is how you indicate a single-value tuple.
Method 2
The trailing comma is required only to create a single tuple (a.k.a. a singleton); it is optional in all other cases. A single expression without a trailing comma doesn’t create a tuple, but rather yields the value of that expression.
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