The urllib package serves as a namespace only. There are other modules under urllib like request and parse.
For optimization importing urllib doesn’t import other modules under it. Because doing so would consume processor cycles and memory, but people may not need those other modules.
Individual modules under urllib must be imported separately depending on the needs.
Try these, the first one fails but the second succeeds because when flask is imported flask itself imports urllib.parse.
python3 -c 'import urllib, sys;print(sys.modules["urllib.parse"])'
python3 -c 'import flask, sys;print(sys.modules["urllib.parse"])'