I know the comment says that this will change post 1.16.

Does this improve the functionality? Is this likely to break anyone in the wild?

My theory is that anyone who has used this previously would have started explicitly listing their arguments, rather than continue to receive the warning. The only improvement I intended was to remove the warning, and allow many users to skip straight to the recommended defaults.

If you would like to close this however, in line with the policy of minimal changes, then you should do so.

I don't know that I want to close it, as much as I'd like to see it fully explored and either ruled out or in. We do want to make minimal changes in some cases; in other cases, we want fix long-standing annoyances not to mention add new features.

Just to throw out another idea - we could change the default behaviour and update the warning to report that change.

The warning has been there since PIL 1.1.6 (December 3, 2006). The frombuffer function was added in 1.1.4 (May 10, 2003).

Looking up the original reason for the warning:

• The default arguments for "frombuffer" doesn't match "fromstring"
  and the documentation; this is a bug, and will most likely be fixed
  in a future version. In this release, PIL prints a warning message
  instead. To silence the warning, change any calls of the form
  "frombuffer(mode, size, data)" to

  frombuffer(mode, size, data, "raw", mode, 0, 1)
  

https://bitbucket.org/effbot/pil-2009-raclette/src/cd403356263f039a4a48a1111c7f5cc38686e481/CHANGES#lines-306

fromstring has since been replaced with frombytes:

    def fromstring(self, *args, **kw):
        raise NotImplementedError("fromstring() has been removed. "
                                  "Please call frombytes() instead.")

Is it still the case that the default args for frombuffer don't match frombytes and the docs?

The docs for writing your own file decoder state that

If omitted, the orientation defaults to 1

How about we update the warning to concretely say:

-the frombuffer defaults may change in a future release
+the frombuffer defaults will change in Pillow X

And then do the change in version X? Version 7.0.0 isn't a lot of notice, but the warning's been there for ages.

PR to change the wording to "the frombuffer defaults will change in Pillow 7.0.0": #4086.