python - 不能使用带下标的赋值表达式

Question

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

为什么 Python 禁止使用海象运算符？

Answer 1

决定禁止在 python-dev 邮件列表上给出的赋值表达式中进行更复杂的赋值是有一定理由的。

特别是来自 Chris Angelico 的:

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

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

来自 Guido 本人:

Also nobody had a use case.

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

这可能是尽可能接近的解释。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。