Let lookup_global accept unicode as input

[SimonKing]

In #28444, bytes_to_str was made more tolerant: It now accepts as input not only bytes but also str. A str input is returned unaltered.

bytes_to_str is also used in sage.structure.factory.lookup_global. It turns out that a pickle created in py3 may not be unpicklable in py2 because the stored name of the global is a unicode, but a bytes or str is expected.

Obvious fix: Try to convert unicode to str in lookup_global.

[nbruin]

That needs to go into a differently-named routine. You're clearly not converting bytes to str here. You're taking *some* input and trying to ensure it's "str".

There is of course a way to convert a unicode object to py2: s.encode("utf8"). Which in Py3 will actually result in a bytes object. That's a pretty clear clue that this behaviour does NOT belong in a routine named bytes_to_str.

[SimonKing]

Replying to nbruin:

That needs to go into a differently-named routine. You're clearly not converting bytes to str here. You're taking *some* input and trying to ensure it's "str".

There is of course a way to convert a unicode object to py2: s.encode("utf8"). Which in Py3 will actually result in a bytes object. That's a pretty clear clue that this behaviour does NOT belong in a routine named bytes_to_str.

OK, that somehow makes sense to me. So, I will not touch it.

Question: Is the change that I have introduced in #28444 OK, then? At least it made it possible to read a py2 pickle in py3.

[SimonKing]

Rather than stretching byte_to_str beyond its specifications, it may be better to fix the function that actually needs a conversion from unicode to str, namely lookup_glocal. See the new ticket description.

[SimonKing]

See #28414 for a situation where the fix from this ticket actually solves a problem.

---

New commits:

​2e0d11e	Pass unpickling options to pickle.load, default encoding 'latin1'. Accept both str and bytes in mtx_unpickle
​8c105cd	Make str_to_bytes/bytes_to_str accept both str and bytes input.
​c8a0748	Add tests for #28444
​89ac77a	Fix keyword for py3-only test
​3693024	Fix doc strings in sage.misc.persist
​f0828ee	Add a comment regarding the expected data type for an unpickle helper
​ba41ebe	Fix two typos in a comment
​4a33e77	Allow names of type bytes, str and unicode in lookup_global

[chapoton]

red branch

[embray]

Ticket retargeted after milestone closed

[mkoeppe]

Batch modifying tickets that will likely not be ready for 9.1, based on a review of the ticket title, branch/review status, and last modification date.

[gh-mwageringel]

As we do not support Python 2 anymore, should we close this as outdated? I imagine there is rarely a need to load a py3 pickle in a Python-2-based Sage now.

[mkoeppe]

That was my thought too.

[gh-mwageringel]

Ok.

[slelievre]

If this needs to be reopened, use the milestone sage-9.1.1