If we drop the requirement for pickle round-tripping then I would add a requirement that sentinel is unpicklable, to prevent accidents.

Irit  

On Fri, May 14, 2021 at 8:38 PM Tal Einat <taleinat@gmail.com> wrote:

I'll try to organize my thoughts a bit here. This is a bit long,
welcome to skip to the final sentence for the "tl;dr".

Features one may want for a sentinel:
1. Unique from other objects
2. Globally unique, i.e. unique from other such sentinels (no consensus here)
3. Clear repr (the original subject of this thread) - significant
since these often appear in function signatures
4. Survives pickling round-trip (I brought this up since I was bitten
by this once, but others have mentioned that this is usually
irrelevant)
5. Can be given a clear type signature (this was brought up on
twitter[1]) - significant since without this nobody can add full type
signatures even if they want to

The common `SENTINEL = object()` idiom fails #3, #4 and #5. This is
what I've been using for years, and I now think that it isn't good
enough. This not having a nice repr is what started this thread.

I'd also personally prefer something simple, ideally without a new
class or module.

There are several simple idioms using existing features that seem like
good options, so let's review those:

1. Ellipsis, a.k.a. `...`. This has all of the features outlined above
except #2. My main issue with using Ellipsis for this is that it could
be surprising and confusing for devs first encountering such use, and
could be relatively awkward to figure out.

2. An instance of a one-off class. dataclasses.MISSING is an example
of this. It is defined thus:

class _MISSING:
    pass
MISSING = _MISSING()

Besides failing #4 (surviving pickle round-trips), its repr isn't
great: <dataclasses._MISSING object at 0x7fe14b1e2e80>. That is easily
overcome by implementing __repr__.

3. A one-off class:

class MISSING: pass

This has all of the above features except #5: having a clear type
signature (since its type is type). Using a class as a value this way
could be surprising, though. It's repr also isn't great: <class
'dataclasses._MISSING'>.

4. A value of an single-valued enum, for example (from [1]):

class _UNSET(enum.Enum):
    token = enum.auto()

This has all of the features above and is simple, just requiring a
comment to explain what it is. It's repr is a bit awkward though:

>>> repr(_UNSET.token)
'<_UNSET.token: 1>'


All of these are in use by some developers, though not necessarily in
the stdlib. None is perfect, though all are probably good enough.
Since pickling is likely not relevant in most cases, I'm currently in
favor of #2 making sure to implement a nice __repr__.

- Tal

[1] https://twitter.com/nolar/status/1392962447166877697
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