PYTHON-3084 MongoClient/Database/Collection should not implement Iterable

[ShaneHarvey]

As noted in PYTHON-3084, this was the only way to get the typing.Iterable instance check to fail. We could also remove __next__+next (from client/Database only) but I kept them around so that users get better error messages TypeError: 'MongoClient' object is not iterable vs TypeError: name must be an instance of str.

[ShaneHarvey]

Notice the old code was accidentally testing Database, not collection.

[ShaneHarvey]

I opened https://jira.mongodb.org/browse/PYTHON-3184 for this.

[ShaneHarvey]

Ah it turns out this approach does not work quite the same on PyPy. I'm not surprised since it feels like an undefined behavior hack:

FAIL: test_iteration (test_client.ClientUnitTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/mongo-python-driver/mongo-python-driver/test/test_client.py", line 216, in test_iteration
    for _ in client:  # type: ignore[misc] # error: "None" not callable  [misc]
TypeError: 'NoneType' object is not callable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/mongo-python-driver/mongo-python-driver/test/test_client.py", line 217, in test_iteration
    break
AssertionError: "'MongoClient' object is not iterable" does not match "'NoneType' object is not callable"

[ShaneHarvey]

With further research it appears that setting __iter__ = None is actually well defined behavior:

Setting a special method to None indicates that the corresponding operation is not available. For example, if a class sets iter() to None, the class is not iterable, so calling iter() on its instances will raise a TypeError (without falling back to getitem()). 2

https://docs.python.org/3/reference/datamodel.html#special-method-names

This is exactly the behavior we want so I'll adjust the assertions to pass on PyPy.

[ShaneHarvey]

I opened https://foss.heptapod.net/pypy/pypy/-/issues/3716 for PyPy to consider improving this error message.

[blink1073]

LGTM!