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Python: Cool Tricks with the unary * Operator (2024)

by lucasoshiro on 3/31/25, 8:14 PM with 1 comments

by zahlman on 3/31/25, 8:47 PM
```
    (*my_list,) # = (1, 2, 3), note the comma to ensure it's a tuple!
```
This is off the original topic a bit, but it deserves a bit of elaboration.
There's a saying in Python circles that "the comma makes the tuple". Grammatically, () are just grouping parentheses. With simple values, (foo) is just foo while (foo,) is a tuple containing foo (a common gotcha; see e.g. https://stackoverflow.com/questions/12876177). But (*my_list) would actually be a syntax error. Whereas the parentheses aren't necessary for this expression by itself:
```
    >>> *my_list,
    (1, 2, 3)
```
The problem is that it's basically impossible to use this sort of tuple expression - even if a star isn't involved - as part of anything more complex without parentheses:
```
    >>> 1, + ()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: bad operand type for unary +: 'tuple'
    >>> () + 1,
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: can only concatenate tuple (not "int") to tuple
    >>> 1,[0] # doesn't index into the singleton tuple
    (1, [0])
```
The comma has very low precedence, which is fixed by adding parentheses:
```
    >>> 1,*3
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: Value after * must be an iterable, not int
    >>> (1,)*3
    (1, 1, 1)*
```