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

• zahlman 2y ago
  (*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 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)*