[Python-Dev] PEP 420 - dynamic path computation is missing rationale (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Wed May 23 15:02:15 CEST 2012

On Wed, May 23, 2012 at 10:31 PM, Eric V. Smith <eric at trueblade.com> wrote:

On 05/22/2012 09:49 PM, PJ Eby wrote:

It shouldn't - all you should need is to use getattr(sys.modules[self.modname], self.attr) instead of referencing a parent path object directly. The problem isn't the lookup, it's coming up with self.modname and self.attr. As it currently stands, PathFinder.findmodule is given the parent path, not the module name and attribute name used to look up the parent path using sys.modules and getattr.

Right, that's what PJE and I were discussing. Instead of passing in the path object directly, you can instead pass an object that lazily retrieves the path object in its iter method:

class LazyIterable:
    """On iteration, retrieves a reference to a named iterable and

returns an iterator over that iterable""" def init(self, modname, attribute): self.modname = modname self.attribute = attribute def iter(self): mod = import_module(self.modname) # Will almost always get a hit directly in sys.modules return iter(getattr(mod, self.attribute)

Where importlib currently passes None or sys.path as the path argument to find_module(), instead pass "LazyIterable('sys', 'path')" and where it currently passes package.path, instead pass "LazyIterable(package.name, 'path')".

The existing for loop iteration and tuple() calls should then take care of the lazy lookup automatically.

That way, the only code that needs to know the values of modname and attribute is the code that already has access to those values.

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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