LookupError

LookupError is a base class for IndexError and KeyError:

LookupError.__subclasses__()
# [IndexError, KeyError, encodings.CodecRegistryError]

KeyError.mro()
# [KeyError, LookupError, Exception, BaseException, object]

IndexError.mro()
# [IndexError, LookupError, Exception, BaseException, object]

The main purpose of this intermediate exception is to simplify a bit lookup for deeply nested structures when any of these two exceptions may occur:

try:
    username = resp['posts'][-1]['authors'][0]['name']
except LookupError:
    username = None