Python is used by very different groups of people and how we decide which features are Common Python and which are Situational Python must take that into account. If something is common to one group only e.g. data scientists, or enterpise web application developers, it should be considered Situational Python - even if the use case is very common for that group.
The When of Python is a fledgling community initiative. The goal is to effectively shrink Python so it fits our brains by providing guidance on when we should use particular language features (and when we should not).