Things on this page are fragmentary and immature notes/thoughts of the author. Please read with your own judgement!

Generally speaking, prefer to return a new object rather than mutating the existing object. This is true in most programming languages besides C who seeks for best peformance.

The pandas package in Python keeps a good balance between good design and performance. By default, pandas always return a copy. When a DataFrame is large (and performance matteers), in-place changes on the DataFrame (rather than copy) can be made by specifying the option inplace=True.

References

Idiomatic Python - return new object or mutate the existing one?