classification
Title: importlib: lazy loading can result in reimporting a submodule
Type: behavior Stage:
Components: Library (Lib) Versions: Python 3.8, Python 3.7
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: Pox TheGreat
Priority: normal Keywords:

Created on 2020-02-10 18:19 by Pox TheGreat, last changed 2020-02-10 18:19 by Pox TheGreat.

Files
File name Uploaded Description Edit
LazyImport.zip Pox TheGreat, 2020-02-10 18:19 example
Messages (1)
msg361705 - (view) Author: Pox TheGreat (Pox TheGreat) * Date: 2020-02-10 18:19
Using the LazyLoader class one can modify the sys.meta_path finders so that every import mechanism becomes lazy. This method has been used in Mercurial and by Facebook.

My problem is that if I have a package (pa) which imports a submodule (a) in the __init__.py and accesses its attributes (or uses a from list) then that submodule is imported (executed) twice without any warning.

I traced back the problem to importlib._bootstrap.py / _find_and_load_unlocked. There is a check there if the submodule has already been imported by the parent package, but the submodule will be imported just after that check because of the _LazyModule and the __path__ attribute access of the submodule.

# Crazy side-effects!
if name in sys.modules:
	return sys.modules[name]
parent_module = sys.modules[parent]
try:
	path = parent_module.__path__


Maybe we could check if name in sys.modules after the __path__ attribute access?
History
Date User Action Args
2020-02-10 18:19:50Pox TheGreatcreate