root/releases/pkgcore/0.1.4/dev-notes/config.rst @ marienz%2540gentoo.org-20060913163754-b5e2ca73c1b17bac

=====================================
 Config use and implementation notes
=====================================

Using the manager
=================

Normal use
----------

To get at the user's configuration::

 from pkgcore.config import load_config
 config = load_config()
 main_repo = config.get_default('repo')
 spork_repo = config.repo['spork']

Usually this is everything you need to know about the manager. Some
things to be aware of:

- Some of the managed sources of configuration data may be slow, so
  accessing a source is delayed for as long as possible. Some things
  require accessing all sources though and should therefore be
  avoided. The easiest one to trigger is config.repo.keys() or the
  equivalent list(config.sections('repo')). This has to get the
  "class" setting for every available config section, which might be
  slow.
- For the same reason the manager does not know what type names exist
  (there is no hardcoded list of them, so the only way to get that
  information would be loading all config sections). This is why you
  can get this::

   >>> load_config().section('repo') # typo, should be "sections"
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   TypeError: '_ConfigMapping' object is not callable

  This constructed a dictlike object for accessing all config sections
  of the type "section", then tried to call it.

Testcase use
------------

For testing of high-level scripts it can be useful to construct a
config manager containing hardcoded values::

 from pkgcore.config import basics, central

 config = central.ConfigManager([{
     'repo' = basics.HardCodedConfigSection({'class': my_repo,
                                             'data': ['1', '2']}),
     'cont' = basics.ConfigSectionFromStringDict({'class': 'pkgcore.my.cont',
                                                  'ref': 'repo'}),
     }])

What this does should be fairly obvious. Be careful you do not use the
same ConfigSection object in more than one place: caching will not
behave the way you want. See `Adding a config source`_ for details.

Adding a configurable
=====================

You often do not really *have* to do anything to make something a
valid "class" value, but it is clearer and it is necessary in certain
cases.

Adding a class
--------------

To make a class available, do this::

 from pkgcore.config import ConfigHint, errors

 class MyRepo(object):

     pkgcore_config_type = ConfigHint({'cache': 'section_ref'},
                                      typename='repo')

     def __init__(self, repo):
         try:
             self.initialize(repo)
         except SomeRandomException:
             raise errors.InstantiationError('eep!')

The first ConfigHint arg tells the config system what kind of
arguments you take. Without it it assumes arguments with no default
are strings and guesses for the other args based on the type of the
default value. So if you have no default values or they are just None
you should tell the system about your args.

The second one tells it you fulfill the repo "protocol", meaning your
instances will show up in load_config().repo.

ConfigHint takes some more arguments, see the api docs for details.

Adding a callable
-----------------

To make a callable available you can do this::

 from pkgcore.config import configurable, errors

 @configurable({'cache': 'section_ref'}, typename=repo)
 def my_repo(repo):
     # do stuff

configurable is just a convenience function that applies a ConfigHint.

Exception handling
------------------

If you raise an exception when the config system calls you it will
catch the exception and wrap it in an InstantiationError. This is good
for calling code since catching and printing those provides the user
with a readable description of what happened. It is less good for
developers since the raising of a new exception kills the traceback
printed in debug mode. You will have a traceback that "ends" in the
config code handling instantiation.

You can improve this by raising an InstantiationError yourself. If you
do this the config system will be able to add the extra information
needed for a user-friendly error message to it without raising a new
exception, meaning debug mode will give a traceback leading right back
to your code raising the InstantiationError.

Adding a config source
======================

Config sources are pretty straightforward: they are mappings from a
section name to a ConfigSection subclass. The only tricky thing is the
combination of section references and caching. The general rule is "do
not expose the same ConfigSection in more than one way". If you do it
will be collapsed and instantiated once for every way it is exposed,
which is usually not what you want. An example::

 from pkgcore.config import basics, configurable

 def example():
     return object()

 @configurable({'ref': 'section_ref'})
 def nested(ref):
     return ref

 multi = basics.HardCodedConfigSection({'class': example})

 myconf = {
     'multi': multi,
     'bad': basics.HardCodedConfigSection({'class': nested, 'ref': multi})
     'good': basics.ConfigSectionFromStringDict({'class': 'nested',
                                                 'ref': 'multi'})

If you feed this to the ConfigManager and instantiate everything
"multi" and "good" will be identical but "bad" will be a different
object. For an explanation of why this happens see the implementation
notes in the next section.

You trigger a similar problem if you create a custom ConfigSection
subclass that bypasses central's collapse_named_section for named
section refs. If you somehow get at the referenced ConfigSection and
hand it to collapse_section you will most likely circumvent caching.
Only use collapse_section for unnamed sections.

ConfigManager tries not to extract more things from this mapping than
it has to. Specifically, it will not call __getitem__ before it needs
to instantiate the section or needs to know its type. However it
*will* iterate over the keys (section names) immediately to find
autoloads. If this is a problem (getting those names is slow) then
make sure the manager knows your config is "remote".

Implementation notes
====================

This code has evolved quite a bit over time. The current code/design
tries among other things to:

- Allow sections to contain both named and nameless/inline references
  to other sections.
- Allow serialization of the loaded config.
- Not do unnecessary work (if possibly not recollapse configs,
  definitely not trigger unnecessary imports, access configs
  unnecessarily, reinstantiate configs)
- Provide both end-user error messages that are complete enough to
  track down a problem in a complex nested config and tracebacks that
  reach back to actual buggy code for developers.

The support for nameless sections means neither ConfigSection nor
CollapsedConfig have a name attribute. This makes the error handling
code a bit tricky as it has to tag in the name at various points, but
this works better than enforcing names where it does not make sense
(means lots of unnecessary duplication of names when dealing with
dicts of HardCoded/StringBasedConfigSections).

The suppport for serialization of the loaded config means section_refs
cannot be instantiated straight away. The object used for
serialization is the CollapsedConfig which gives you both the actual
values and the type they have. If the CollapsedConfig contained
arbitrary instantiated objects serializing them would be impossible.
So it contains nested CollapsedConfigs instead.

Not doing unnecessary work is done by caching in two places. The
simple one is CollapsedConfig caching its instantiated value. This is
pretty straightforward. The more subtle one is ConfigManager caching
CollapsedConfigs by name. It is obviously a good idea to cache these
(if we didn't we would have to cache the instantiated value in the
ConfigManager). An alternative would be caching them by their
ConfigSection. This has the minor disadvantage of keeping the
ConfigSection in memory, and the larger one that it may break caching
for weird config sources that generate ConfigSections on demand. The
downside of caching by name is we have to make sure nothing generates
a CollapsedConfig for a named section in a way other than
collapse_named_section (handing the ConfigSection to collapse_section
bypasses caching).

This means a ConfigSection has to return a CollapsedConfig for a
section_ref get_value call, not a ConfigSection. If it was a
ConfigSection that central then collapsed and the reference was
actually to a named section caching is bypassed.

The need for a section name starting with "autoload" is also there to
avoid unnecessary work. Without this we would have to figure out the
typename of every section. While we can do that without entirely
collapsing the config we cannot avoid importing the "class", which
means load_config() would import most of pkgcore. That should
definitely be avoided.