[BUG][MERGE] 'bzr upgrade' broken by lazy imports

[Aaron Bentley]

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Wouter van Heyst wrote:
> On Tue, Nov 07, 2006 at 12:18:45PM -0600, John Arbash Meinel wrote:
>>My initial feeling is to just change most code paths that use
>>'bzrilb.foo' to being 'from bzrlib import foo' at the beginning, and use
>>'foo.bar' later on.
> 
> 
> The fully explicit form can't be used with lazy importing?

I think it can be done, but the problem is that missing dependencies
will not be caught.  This is a failing of Python that is exacerbated by
lazy importing.

When you import a package, it doesn't import its sub-modules.  So
'import bzrlib' does not import 'bzrlib.workingtree'.  So if you try to
use 'bzrlib.workingtree' later, you get an AttributeError.

But if you first import another package, and that package imports
workingtree, then the bzrlib package will get a 'workingtree' attribute.
(It can do 'from bzrlib import workingtree' or 'import bzrlib.workingtree'.)

So in general, using packagename.module leaves you blind to possible
missing imports.  And with lazy importing, the number of imports is
reduced, which exposes this kind of issue.

Aaron
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