Eugene Alterman <eug...@gmail.com> writes: > a = 1, 2, 3 > > b = *a, # assignment - OK > b += *a, # augmented assignment - syntax error > > Need to enclose in parenthesis: > > b += (*a,) > > Why isn't it allowed with an augmented assignment, while it is OK with a > regular assignment? >
Syntactically (i.e. according to the grammar): The right-hand side of an assignment has as one of the alternative a starred_expression, which includes starred and unstarred expressions. The right-hand side of an augmented assignment has an expression_list there. (The other option is both cases is a yield_expression.) And if it is a list (i.e. there is a comma in it), it is a tuple. Semantically: x += y is more or less equivalent to x = x + y, except that x is only evaluated once, and y is treated as a unity (think implicit parentheses around it). So the y is basically part of an expression. But starred expressions are not allowed in expressions, except within explicit parentheses. -- Piet van Oostrum <pie...@vanoostrum.org> WWW: http://piet.vanoostrum.org/ PGP key: [8DAE142BE17999C4] -- https://mail.python.org/mailman/listinfo/python-list
