[Python-Dev] Re: Christmas Wishlist (original) (raw)

Edward Loper edloper at gradient.cis.upenn.edu
Mon Dec 15 13:44:56 EST 2003

Barry Warsaw <barry at python.org> wrote:

Actually, if truth be told, I'd love to just ban local imports. I understand that might be controversial though .

+1 (asuming that you're talking about package-local imports).

I've seen a couple people get bitten by the fact that a module gets loaded twice if there are separate local & global imports for it:

 % mkdir pkg

 % touch pkg/__init__.py

 % cat >pkg/a.py
 print 'A is being imported (not re-used)'
 class A: pass

 % cat >pkg/b.py
 from a import A as LocalA
 from pkg.a import A as GlobalA
 print isinstance(LocalA(), GlobalA)
 print isinstance(GlobalA(), LocalA)

 % PYTHONPATH=.; python pkg/b.py
 A is being imported (not re-used)
 A is being imported (not re-used)
 0
 0

Since pkg/a gets loaded twice, we end up with two versions of class A, which are not compatible. In practice this usually comes up if a package uses local imports between submodules, and then an outside user uses a global import to get a submodule.

-Edward

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