Why return None?

[Erik Max Francis]

Martin DeMello wrote:

> Yes, but why? I mean, is there either an advantage to returning None
> or
> some inherent danger in returning self?

The "inherent danger" is that the user might think that it returns a new
object rather than mutating the original.  Returning a new object vs.
mutating the argument and returning None is merely a convention, but
it's one used consistently in the Python standard library.

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