Make Dispatcher a singleton after first instantiation

This allows plugins to re-instantiate the Dispatcher
(or, now, use the instance cached in `current_dispatcher`)
without knowing what plugins have been loaded by dotbot.

This fixes plugins that want to dispatch to other plugins.

Fixes #339

dotbot/dispatcher.py

@@ -5,8 +5,23 @@ from .context import Context
 from .messenger import Messenger
 from .plugin import Plugin
 
+current_dispatcher = None
+
 
 class Dispatcher:
+    def __new__(cls, *args, **kwargs):
+        # After dotbot instantiates this class, the instance will be cached.
+        # Subsequent instantiations (say, by plugins) will return the same instance.
+        # This is needed because plugins don't have access to the entire configuration
+        # (for example, they won't know which plugins have been loaded).
+        # This ensures a consistent configuration is used.
+        global current_dispatcher
+        if current_dispatcher is None:
+            instance = object.__new__(cls)
+            instance.is_initialized = False
+            current_dispatcher = instance
+        return current_dispatcher
+
     def __init__(
         self,
         base_directory,
@@ -16,6 +31,9 @@ class Dispatcher:
         options=Namespace(),
         plugins=None,
     ):
+        if self.is_initialized:
+            return
+        self.is_initialized = True
         self._log = Messenger()
         self._setup_context(base_directory, options)
         plugins = plugins or []

tests/conftest.py

@@ -12,6 +12,7 @@ import pytest
 import yaml
 
 import dotbot.cli
+import dotbot.dispatcher
 
 
 def get_long_path(path):
@@ -312,3 +313,9 @@ def run_dotbot(dotfiles):
             dotbot.cli.main()
 
     yield runner
+
+
+@pytest.fixture(autouse=True)
+def reset_current_dispatcher():
+    yield
+    dotbot.dispatcher.current_dispatcher = None

tests/test_dispatcher.py

@@ -0,0 +1,22 @@
+import dotbot.dispatcher
+
+
+def test_dispatcher_instantiation(home, dotfiles, run_dotbot):
+    """Verify that the dispatcher caches itself as a singleton."""
+
+    assert dotbot.dispatcher.current_dispatcher is None
+
+    dotfiles.write_config([])
+    run_dotbot()
+
+    # Verify the dispatcher has been cached.
+    assert dotbot.dispatcher.current_dispatcher is not None
+
+    existing_id = id(dotbot.dispatcher.current_dispatcher)
+    new_instance = dotbot.dispatcher.Dispatcher("bogus")
+
+    # Verify the new and existing instances are the same.
+    assert existing_id == id(new_instance)
+
+    # Verify the singleton was not overridden.
+    assert existing_id == id(dotbot.dispatcher.current_dispatcher)