Weird discarded comparison lurking in untested code about zodb

[tseaver]

• It won't be an UnboundLocalError, because maxoid is a (long-unused, BBB) parameter to read_index.
• Even better, I don't think that index.maxKey() without args can raise a ValueError: that only happens when an explicit comparison key is passed, but no key is greater.

[kilink]

Calling maxKey() on an empty BTree will throw a ValueError:

>>> x = BTrees.OOBTree.OOBTree()
>>> x.maxKey()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: empty tree

[tseaver]

Ah, OK. I don't think it is ever possible to have load an empty fsTree from a file, though: it is always at least going to have the root object in it, or it wouldn't have been created at all.

[jmuchemb]

The index is empty when a FileStorage DB is being created. It's also possible to create one without adding a root object. The following code actually causes a ValueError twice:

FileStorage("foo").close()
FileStorage("foo")

So the except clause is important and there's no need to add a unit test. 9618301 clarifies the code.