Yes, it is a bad idea. The reason is that if in-place and non-in-place operations have apparently identical output, then programmers will frequently mix up in-place operations and non-in-place operations (List.sort() vs. sorted()) and that results in hard-to-detect errors.
In-place operations returning themselves can allow you to perform "method chaining", however, this is bad practice because you may bury functions with side-effects in the middle of a chain by accident.
To prevent errors like this, method chains should only have one method with side-effects, and that function should be at the end of the chain. Functions before that in the chain should transform the input without side-effects (for instance, navigating a tree, slicing a string, etc.). If in-place operations return themselves then a programmer is bound to accidentally use it in place of an alternative function that returns a copy and therefore has no side effects (again, List.sort() vs. sorted()) which may result in an error that is difficult to debug.
This is the reason Python standard library functions always either return a copy or return None and modify objects in-place, but never modify objects in-place and also return themselves. Other Python libraries like Django also follow this practice (see this very similar question about Django).
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