Add a `default` argument to `operator.attrgetter` and `operator.itemgetter`

Feature or enhancement

Proposal:

The proposal if for these functions to receive a default argument.

The main motivation is operator.attrgetter, which implements nested attribute access, but is missing getattr's default value support. operator.itemgetter also gets the default argument for consistency.

For operator.attrgetter, only AttributeErrors should be suppressed, and for operator.itemgetter, KeyError.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/enhancing-getattr-to-support-nested-attribute-access-with-dotted-strings/74305/12

首先检查针对 operator.attrgetter 和 operator.itemgetter 提议的行为，然后阅读链接的 Discourse 讨论，了解尚未解决的设计问题。跟踪这两个函数现有的实现和测试；完成的标准是支持默认参数，同时仅对 attrgetter 抑制 AttributeError，仅对 itemgetter 抑制 KeyError。