Mechanism for de-registering registered SQL functions

[simonw]

I used a custom SQL function in a migration script and then realized that it should be de-registered before the end of the script to avoid leaking into the calling code.

[simonw]

For that particular case I realized I'd quite like to have a mechanism for applying functions for a block of code and then de-registering them at the end - a context manager.

I played with this idea a bit:

with db.register_functions(md5, md5_random):
    db.query(...)

[simonw]

Normally in Python/sqlite3 you de-register a function by passing None to it.

You can't do that with db.register_function() at the moment because a fn of None does something else:

sqlite-utils/sqlite_utils/db.py

Lines 461 to 464 in 1260bdc

	if fn is None:
	return register
	else:
	register(fn)

[simonw]

Here's a prototype: 62f6738

Still needs tests and documentation (and some more thought to make sure it's doing the right thing).