Assertion rewriting reorders an operand when a later call rebinds its name

[RonnyPfannschmidt]

The rewriter hoists sub-expressions into statements, but leaves a plain name as a bare load evaluated when the enclosing expression is assembled — that is, after the statements belonging to the operands that follow it. If one of those rebinds the name, the earlier operand sees a value Python would never have given it.

count = 0

def bump():
    global count
    count = 99
    return 0

def test_left_operand_is_read_too_late():
    assert count == bump()

Python evaluates the left operand first, so this compares 0 == 0 and passes. Rewritten, it compares 99 == 0 and fails. Same with nonlocal, and in argument position:

value = "a"

def bump():
    global value
    value = "b"
    return "x"

def test_earlier_argument_is_read_too_late():
    assert (value, bump()) == ("a", "x")

Both pass under --assert=plain.

#14447 fixes this class for the walrus operator, via a visit_operand() helper that freezes an operand into a temporary when a later operand rebinds its name. The detection is _walrus_targets(), which scans for ast.NamedExpr — so a function that rebinds through global/nonlocal is invisible to it.

The conservative fix is to freeze a bare name whenever anything later can execute arbitrary code (Call, Await, NamedExpr) rather than only when a walrus targets it. It is cheaper than it sounds: an operand that is already hoisted needs no freeze, so assert len(items) == expected is unchanged; only the bare-name-followed-by-a-call shape gains one assignment.

Tested on main (f306da747), Python 3.12. Related: #14445, #14819.