Introduce importlib.abc. The module contains various ABCs related to imports

(mostly stuff specified by PEP 302). There are two ABCs, PyLoader and
PyPycLoader, which help with implementing source and source/bytecode loaders by
implementing load_module in terms of other methods. This removes a lot of
gritty details loaders typically have to worry about.

Lib/importlib/abc.py

@@ -0,0 +1,110 @@
+"""Abstract base classes related to import."""
+from . import _bootstrap
+from . import machinery
+import abc
+import types
+
+
+class Loader(metaclass=abc.ABCMeta):
+
+    """Abstract base class for import loaders.
+
+    See PEP 302 for details.
+
+    """
+
+    def load_module(self, fullname:str) -> types.ModuleType:
+        raise NotImplementedError
+
+Loader.register(machinery.BuiltinImporter)
+Loader.register(machinery.FrozenImporter)
+
+
+class Finder(metaclass=abc.ABCMeta):
+
+    """Abstract base class for import finders.
+
+    See PEP 302 for details.
+
+    """
+
+    @abc.abstractmethod
+    def find_module(self, fullname:str, path:[str]=None) -> Loader:
+        raise NotImplementedError
+
+Finder.register(machinery.BuiltinImporter)
+Finder.register(machinery.FrozenImporter)
+Finder.register(machinery.PathFinder)
+
+
+class Importer(Finder, Loader):
+
+    """Abstract base class for importers."""
+
+
+
+class ResourceLoader(Loader):
+
+    """Abstract base class for loaders which can return data from the back-end
+    storage.
+
+    This ABC represents one of the optional protocols specified by PEP 302.
+
+    """
+
+    @abc.abstractmethod
+    def get_data(self, path:str) -> bytes:
+        raise NotImplementedError
+
+
+class InspectLoader(Loader):
+
+    """Abstract base class for loaders which supports introspection.
+
+    This ABC represents one of the optional protocols specified by PEP 302.
+
+    """
+
+    @abc.abstractmethod
+    def is_package(self, fullname:str) -> bool:
+        return NotImplementedError
+
+    @abc.abstractmethod
+    def get_code(self, fullname:str) -> types.CodeType:
+        return NotImplementedError
+
+    @abc.abstractmethod
+    def get_source(self, fullname:str) -> str:
+        return NotImplementedError
+
+
+class PyLoader(_bootstrap.PyLoader, InspectLoader):
+
+    """Abstract base class that implements the core parts needed to load Python
+    source code."""
+
+    # load_module and get_code are implemented.
+
+    @abc.abstractmethod
+    def source_path(self, fullname:str) -> object:
+        raise NotImplementedError
+
+
+class PyPycLoader(_bootstrap.PyPycLoader, PyLoader):
+
+    """Abstract base class that implements the core parts needed to load Python
+    source and bytecode."""
+
+    # Implements load_module and get_code.
+
+    @abc.abstractmethod
+    def source_mtime(self, fullname:str) -> int:
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    def bytecode_path(self, fullname:str) -> object:
+        raise NotImplementedError
+
+    @abc.abstractmethod
+    def write_bytecode(self, fullname:str, bytecode:bytes):
+        raise NotImplementedError