use lazy config loader in confit

This lets you, for example, import beets in a library or the REPL and use it
immediately without calling "beets.config.read()" first.

beets/__init__.py

@@ -20,4 +20,4 @@ from beets.util import confit
 
 Library = beets.library.Library
 
-config = confit.Configuration('beets', __name__, False)
+config = confit.LazyConfig('beets', __name__)

beets/ui/__init__.py

@@ -689,18 +689,13 @@ class SubcommandsOptionParser(optparse.OptionParser):
 
 # The root parser and its main function.
 
-def _raw_main(args, load_config=True):
+def _raw_main(args):
     """A helper function for `main` without top-level exception
     handling.
     """
-    # Load global configuration files.
-    if load_config:
-        config.read()
-
     # Temporary: Migrate from 1.0-style configuration.
     from beets.ui import migrate
-    if load_config:
-        migrate.automigrate()
+    migrate.automigrate()
 
     # Get the default subcommands.
     from beets.ui.commands import default_commands

beets/util/confit.py

@@ -677,3 +677,36 @@ class Configuration(RootView):
         if not os.path.isdir(appdir):
             os.makedirs(appdir)
         return appdir
+
+class LazyConfig(Configuration):
+    """A Configuration at reads files on demand when it is first
+    accessed. This is appropriate for using as a global config object at
+    the module level.
+    """
+    def __init__(self, appname, modname=None):
+        super(LazyConfig, self).__init__(appname, modname, False)
+        self._materialized = False  # Have we read the files yet?
+        self._lazy_prefix = []  # Pre-materialization calls to set().
+        self._lazy_suffix = []  # Calls to add().
+
+    def resolve(self):
+        if not self._materialized:
+            self._materialized = True
+            self.read()
+            self.sources += self._lazy_suffix
+            self.sources[:0] = self._lazy_prefix
+        return super(LazyConfig, self).resolve()
+
+    def add(self, value):
+        super(LazyConfig, self).add(value)
+        if not self._materialized:
+            # Buffer additions to end.
+            self._lazy_suffix += self.sources
+            del self.sources[:]
+
+    def set(self, value):
+        super(LazyConfig, self).set(value)
+        if not self._materialized:
+            # Buffer additions to beginning.
+            self._lazy_prefix[:0] = self.sources
+            del self.sources[:]

test/_common.py

@@ -92,6 +92,7 @@ class TestCase(unittest.TestCase):
     def setUp(self):
         # A "clean" source list including only the defaults.
         beets.config.sources = []
+        beets.config._materialized = True
         beets.config.read(user=False, defaults=True)
 
         # Direct paths to a temporary directory. Tests can also use this

test/test_ui.py

@@ -478,7 +478,7 @@ class ConfigTest(_common.TestCase):
         if config_yaml:
             config_data = yaml.load(config_yaml, Loader=confit.Loader)
             config.set(config_data)
-        ui._raw_main(args + ['test'], False)
+        ui._raw_main(args + ['test'])
 
     def test_paths_section_respected(self):
         def func(lib, opts, args):