Chainable mutating methods are not idiomatic Python; returning None is idiomatic for mutating methods. ISTR that the reasons here regard readability and clearly distinguishing mutations from queries, but in any case if you don't follow the idiom in libraries you create, they won't behave like Python's builtins and standard library, which, whatever you think about chaining on its own, will cause some context switching for most Python users when trying to work with it, increasing the cognitive load.
The builtins are broken, that is why there are new builtins liked `sorted()`, list.sort() returning None is not an improvement. The standard libraries are a _mess_, there is absolutely know cohesion in their api design, or even naming conventions (go peruse the standard lib). Much of what is Pythonic is someones opinion on how a nanny-language should behave.
At this point I think calling something idiomatic or Pythonic is a thought terminating statement. I need to see refutation.
sorted returns a new object, not mutates the original object, so it follows the convention GP is talking about. sort() also follows that rule, since it mutates, it returns None.
In terms of refutation, some things are just conventions, and breaking them has to be done with the understanding that the benefits outweigh the costs. If python were a clean slate, totally new language, you might have a discussion about the merits of mutating chaining, but as it is, the convention is not to do that, and the benefits don't buy enough to violate everyone's expectations.
Counterpoint, object.next() was removed from python3 and replaced with next(object). It both modifies state and returns data. I really prefered next() as a method since it does change the state of a generator.
