[New-bugs-announce] [issue39602] importlib: lazy loading can result in reimporting a submodule

Pox TheGreat report at bugs.python.org
Mon Feb 10 13:19:50 EST 2020

New submission from Pox TheGreat <poxthegreat at gmail.com>:

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]
	path = parent_module.__path__

Maybe we could check if name in sys.modules after the __path__ attribute access?

components: Library (Lib)
files: LazyImport.zip
messages: 361705
nosy: Pox TheGreat
priority: normal
severity: normal
status: open
title: importlib: lazy loading can result in reimporting a submodule
type: behavior
versions: Python 3.7, Python 3.8
Added file: https://bugs.python.org/file48889/LazyImport.zip

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