[Python-ideas] Dunder method to make object str-like

[Chris Angelico]

This is a spin-off from the __fspath__ discussion on python-dev, in
which a few people said that a more general approach would be better.

Proposal: Objects should be allowed to declare that they are
"string-like" by creating a dunder method (analogously to __index__
for integers) which implies a loss-less conversion to str.

This could supersede the __fspath__ "give me the string for this path"
protocol, or could stand in parallel with it.

Obviously str will have this dunder method, returning self. Most other
core types (notably 'object') will not define it. Absence of this
method implies that the object cannot be treated as a string.

String methods will be defined as accepting string-like objects. For
instance, "hello"+foo will succeed if foo is string-like.

Downside: Two string-like objects may behave unexpectedly - foo+bar
will concatenate strings if either is an actual string, but if both
are other string-like objects, depends on the implementation of those
objects.

Bikeshedding:

1) What should the dunder method be named? __str_coerce__? __stringlike__?

2) Which standard types are sufficiently string-like to be used thus?

3) Should there be a bytes equivalent?

4) Should there be a format string "coerce to str"? "{}".format(x) is
equivalent to str(x), but it might be nice to be able to assert that
something's stringish already.

Open season!