It's possible for a ReadOnlyDirectory to be created without all fields.

[jpivarski]

ReadOnlyDirectory has a few attributes that usually get filled with meaningful values when fSeekKeys != 0. But in that special case:

uproot5/src/uproot/reading.py

Lines 1405 to 1407 in 914dd25

	if self._fSeekKeys == 0:
	self._header_key = None
	self._keys = []

the self._keys_lookup and self._len attributes don't get set (any others? scroll down to check). They should be set in any case, possibly before that if-branch so that conditional initialization doesn't bite us again.

[jpivarski]

By the way, this should be back-ported to main-v4 and make a new v4 release, not just main/v5. It's important enough to warrant a new v4 release.