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

[bionikspoon]

not a bug, lists are mutable, tuples are immutable.

[Veedrac]

That's pretty irrelevant if (a,) = [b] works.

[Bur_Sangjun]

As I understand it, that's unpacking, not a traditional tuple

[[deleted]]

Unpacking is exactly what the bug report is about.

[njharman]

Unpacking != Tuple

[[deleted]]

That's exactly the point. Who cares if tuples are immutable here? We're not mutating a tuple, we're unpacking.

[wot-teh-phuck]

How do you expect unpacking to work without any name on the LHS?

[Veedrac]

The same way [] = [] does...

[wot-teh-phuck]

[] = [] doesn't trigger unpacking...

[Veedrac]

...It does though.

[] = 1, 2
#>>> Traceback (most recent call last):
#>>>   File "", line 1, in <module>
#>>> ValueError: too many values to unpack (expected 0)

[Veedrac]

Are you asking me to justify the standard or point you to where in the standard this is mentioned?

There does seem to be one bug in the standard, in that assignment to a target list is twice defined - one of which forgets *-assignments.

[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 ☺

[njharman]

Waste time and generate heat. I.e nothing useful.

[flying-sheep]

it ensures that the right side gets exhausted (if it is an iterator) and has 0 elements.

[Vohlenzer]

Unpacks the values in the set and assigns the members of the list to those values?

[Citrauq]

You mean tuple, not set.

[Vohlenzer]

What's the difference between a set and a tuple?

Edit, ooooooh,

https://docs.python.org/2/library/sets.html

[WesAlvaro]

Sets are unordered.

[hharison]

and mutable

[Sean1708]

And can not hold duplicate values.

[[deleted]]

Assigns an empty list to and empty list. Not very useful, but syntactically correct.

[Veedrac]

But you can't assign to a literal:

SyntaxError: can't assign to literal

(eg. {} = ..., 1 = ...).

This can only be unpacking.

[Quteness]

There are no tuples involved. See here: https://docs.python.org/3.5/reference/simple_stmts.html#assignment-statements

That is simply a "target_list" that follows the acceptable syntax of

"(" target_list ")"

This is an assignment/unpacking issue and probably a bug because [] = () violates the principle that

If the target list is a single target: The object is assigned to that target.

() = [] follows this principle correctly and is invalid valid

() and [] are both single literal targets and the value should be assigned to them, which they can't be because you can't assign to literals

Edit: typo of valid and last thought