On 05/21/2012 07:25 PM, Nick Coghlan wrote:
> As a simple example to back up PJE's explanation, consider:
> 1. encodings becomes a namespace package
> 2. It sometimes gets imported during interpreter startup to initialise
> the standard io streams
> 3. An application modifies sys.path after startup and wants to
> contribute additional encodings
>
> Searching the entire parent path for new portions on every import would
> be needlessly slow.
>
> Not recognising new portions would be needlessly confusing for users. In
> our simple case above, the application would fail if the io
> initialisation accessed the encodings package, but work if it did not
> (e.g. when all streams are utf-8).
>
> PEP 420 splits the difference via an automatically invalidated cache:
> when you iterate over a namespace package __path__ object, it rescans
> the parent path for new portions *if and only if* the contents of the
> parent path have changed since the previous scan.
That seems like a pretty convincing example to me.
Personally I'm +1 on putting dynamic computation into the PEP, at least
for top-level namespace packages, and probably for all namespace packages.
The code is not very large or complicated, and with the proposed removal
of the restriction that sys.path cannot be replaced, I think it behaves
well.
But Guido can decide against it without hurting my feelings.
Eric.
P.S.: Here's the current code in the pep-420 branch. This code still has
the restriction that sys.path (or parent_path in general) can't be
replaced. I'll fix that if we decide to keep the feature.
class _NamespacePath:
def __init__(self, name, path, parent_path, path_finder):
self._name = name
self._path = path
self._parent_path = parent_path
self._last_parent_path = tuple(parent_path)
self._path_finder = path_finder
def _recalculate(self):
# If _parent_path has changed, recalculate _path
parent_path = tuple(self._parent_path) # Make a copy
if parent_path != self._last_parent_path:
loader, new_path = self._path_finder(self._name, parent_path)
# Note that no changes are made if a loader is returned, but we
# do remember the new parent path
if loader is None:
self._path = new_path
self._last_parent_path = parent_path # Save the copy
return self._path
def __iter__(self):
return iter(self._recalculate())
def __len__(self):
return len(self._recalculate())
def __repr__(self):
return "_NamespacePath" + repr((self._path, self._parent_path))
def __contains__(self, item):
return item in self._recalculate()
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