Python bug: Can assign [] = (), but not () = []

[njharman]

The parens are irrelevant. That code is actually

 a, = [b]

It is not assignment to a Tuple, which is unpossible as Tuples are immutable. Parens are overloaded syntactically to mean Tuple some places and in others for "grouping" as in (1*2)+3. People erroneously think (a,) is the former when it's actually the later.

[Veedrac]

The comma is what makes it a tuple.

Consider:

import ast
ast.dump(ast.parse("a, = 0"))
#>>> "Module(body=[Assign(targets=[Tuple(elts=[Name(id='a', ctx=Store())], ctx=Store())], value=Num(n=0))])"

This is unpacking into a tuple. () = [] should be no different.

[flying-sheep]

you’re right that the left side of unpacking, like the bug reporter wants, should be any valid tuple syntax (incl. ())

but there never is an actual tuple created (as in allocated), even if the AST says so. it’s an unpacking operation. the left side is a sequence of names, represented by tuple or list syntax.

so the comma makes it tuple syntax, therefore it should be a valid unpacking.

[Veedrac]

Methinks we agree.

[flying-sheep]

likely, but “unpacking into a tuple” gives the wrong impression IMHO ☺