Re: [Fab-user] external role definitions

[Michael Gliwinski]

On Friday 19 Nov 2010 02:41:28 Jeff Forcier wrote:
> Hey Michael,
> 
> As Morgan said, we'd love to see what you have, for sure! A couple
> additional points below.

OK, I don't have it anywhere public, it's really just few lines of code, so 
pasted them below.

[...]
> I'm not sure how much of the Fab source you peeked at, but roledefs
> are already lazy, so hopefully your load_from() method is just
> populating the dict values with callables -- that's all that should be
> necessary for this aspect of your setup to work, off the top of my
> head.

Yes, I knew about that, but in this case I'm actually loading the whole dict 
instead of each role individually.  I think it would actually be more 
complicated to try to split it, but see below.

[...]
> That's certainly valid. Part of me says the feature is a bit on the
> esoteric side, but on the other hand I've always been a fan of adding
> "power user" features provided the common use case isn't compromised
> and said feature doesn't add significant complexity. Based on your
> description, your change probably passes both those tests :)

Well, my use case probably is, but I'm thinking that this can be used for as 
simple thing as just separating role definitions into another file.

> Let us know where we can check it out and I'm sure we can offer
> further advice/questions/etc, and if it looks good I'll definitely add
> a ticket for figuring out how best to include it.

Well, here goes (stripped most docstrings for brevity):


from os import path

import contextlib
import functools
import urllib2
import urlparse

import yaml

def urlopen(url, *args, **kwargs):
    """Open URL and return a file-like object for its resource.

    This is just a wrapper for urllib2.urlopen to handle local paths (URLs
    without scheme) and make the returned object behave like a context 
manager.
    """
    if not urlparse.urlsplit(url).scheme:
        url = 'file://' + path.abspath(path.normpath(path.expanduser(url)))
    return contextlib.closing(urllib2.urlopen(url, *args, **kwargs))

class RoleDefs(dict):
    _loaded = False
    _sources = None

    def load_from(self, sources):
        if isinstance(sources, basestring) \
           or not hasattr(sources, '__iter__'):
            sources = [sources]
        self._sources = sources

    @staticmethod
    def source_loader(url):
        with urlopen(url, 'r') as fp:
            return yaml.load(fp)

    def __missing__(self, key):
        if self._loaded or not self._sources: raise KeyError(key)
        self._loaded = True
        sources = list(self._sources)
        while sources:
            src = sources.pop(0)
            loader = src
            if not callable(src):
                loader = functools.partial(self.source_loader, src)
            try:
                self.update(loader())
                break
            except Exception:
                if not sources: raise KeyError(key)
        return self[key]


-- 
Michael Gliwinski
Henderson Group Information Services
9-11 Hightown Avenue, Newtownabby, BT36 4RT
Phone: 028 9034 3319

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