gh-151907: Avoid creating temporary objects in some comprehensions

[ZeroIntensity]

The following code:

[i for i in range(42)]

is essentially turned into:

for i in range(42):
    pass

• Issue: Avoid construction of objects in comprehensions without targets #151907

[iritkatriel]

Can we instead just wrap the LIST_APPEND in if !avoid_creation?

[ZeroIntensity]

I think we'd also need to wrap the LIST_EXTEND with that too; I opted for this approach to avoid duplicating that logic.

But do we need the separate VISIT path for the Starred case at all? I might be able to refactor it to look like this:

ISIT(c, expr, elt);
if (elt->kind == Starred_kind) {
    op = LIST_EXTEND;
}
else {
    op = LIST_APPEND;
}
if (!avoid_creation) {
    ADDOP_I(c, elt_loc, op, depth + 1);
}

[iritkatriel]

I think this might change semantics for the dict case - if a key is not hashable.

[ZeroIntensity]

Yeah, good catch. This breaks set too:

{x for x in [[]]}  # No error!

I'll just remove set and dict for now, but it might still be possible to avoid object creation by calling PyObject_Hash. Not worth doing that here though.

[iritkatriel]

Yeah, good catch. This breaks set too:

{x for x in [[]]}  # No error!

I'll just remove set and dict for now, but it might still be possible to avoid object creation by calling PyObject_Hash. Not worth doing that here though.

Maybe also add tests for these cases.

[sergey-miryanov]

I believe you need to add a test where the loop body has a side effect. IIUC, the body with thread.join() should be turned into a method call, rather than just pass.