无法将赋值表达式与下标一起使用

Question

if session['dp'] := current_user.avatar :
    ^ SyntaxError: cannot use assignment expressions with subscript

为什么Python禁止使用walrus操作符

Answer 1

在python-dev邮件列表中给出的赋值表达式中不允许更复杂的赋值，这是有道理的

特别是Chris Angelico：

Assignment to arbitrary targets would also mean permitting iterable unpacking, which is not desired ("x, y := 3, 4"??), and there weren't enough use-cases for attribute/item assignment to justify creating a rule of "you can assign to any single target, but can't unpack". In the future, if such use-cases are found, the grammar can be expanded.

https://mail.python.org/pipermail/python-dev/2018-July/154628.html

来自圭多本人：

Also nobody had a use case.

https://mail.python.org/pipermail/python-dev/2018-July/154631.html

这可能是一个尽可能接近的解释。据推测，如果有需求，该功能可能会在未来的某个版本中扩展

Answer 2

因为，正如替代名称（命名表达式）所示，the left hand side of the walrus operator is to be a ^{}。因此，根据定义，您的问题中提到的表达式以及（例如）函数调用不允许在此表单中分配

文件还规定：

Single assignment targets other than a single NAME are not supported

为了进一步说明这个论点，可以注意到，如果表达式为^{，则cPython显式地checks：

if (target->kind != Name_kind) {
    const char *expr_name = get_expr_name(target);
    if (expr_name != NULL) {
        ast_error(c, n, "cannot use assignment expressions with %s", expr_name);
    }
    return NULL;
}