Implement importlib.abc.SourceLoader and deprecate PyLoader and PyPycLoader.

SourceLoader is a simplification of both PyLoader and PyPycLoader. If one only
wants to use source, then they need to only implement get_data and
get_filename. To also use bytecode -- sourceless loading is not supported --
then two abstract methods -- path_mtime and set_data -- need to be implemented.
Compared to PyLoader and PyPycLoader, there are less abstract methods
introduced and bytecode files become an optimization controlled by the ABC and
hidden from the user (this need came about as PEP 3147 showed that not treating
bytecode as an optimization can cause problems for compatibility).

PyLoader is deprecated in favor of SourceLoader. To be compatible from Python
3.1 onwards, a subclass need only use simple methods for source_path and
is_package. Otherwise conditional subclassing based on whether Python 3.1 or
Python 3.2 is being is the only change. The documentation and docstring for
PyLoader explain what is exactly needed.

PyPycLoader is deprecated also in favor of SourceLoader. Because PEP 3147
shifted bytecode path details so much, there is no foolproof way to provide
backwards-compatibility with SourceLoader. Because of this the class is simply
deprecated and users should move to SourceLoader (and optionally PyLoader for
Python 3.1). This does lead to a loss of support for sourceless loading
unfortunately.

At some point before Python 3.2 is released, SourceLoader will be moved over to
importlib._bootstrap so that the core code of importlib relies on the new code
instead of the old PyPycLoader code. This commit is being done now so that
there is no issue in having the API in Python 3.1a1.

1 files changed, 260 insertions, 10 deletions

diff --git a/Lib/importlib/abc.py b/Lib/importlib/abc.py
index d6f4520..6a688d1 100644
--- a/Lib/importlib/abc.py
+++ b/Lib/importlib/abc.py
@@ -1,8 +1,16 @@
 """Abstract base classes related to import."""
 from . import _bootstrap
 from . import machinery
+from . import util
 import abc
+import imp
+import io
+import marshal
+import os.path
+import sys
+import tokenize
 import types
+import warnings
 
 
 class Loader(metaclass=abc.ABCMeta):
@@ -58,19 +66,19 @@ class InspectLoader(Loader):
     def is_package(self, fullname:str) -> bool:
         """Abstract method which when implemented should return whether the
         module is a package."""
-        return NotImplementedError
+        raise NotImplementedError
 
     @abc.abstractmethod
     def get_code(self, fullname:str) -> types.CodeType:
         """Abstract method which when implemented should return the code object
         for the module"""
-        return NotImplementedError
+        raise NotImplementedError
 
     @abc.abstractmethod
     def get_source(self, fullname:str) -> str:
         """Abstract method which should return the source code for the
         module."""
-        return NotImplementedError
+        raise NotImplementedError
 
 InspectLoader.register(machinery.BuiltinImporter)
 InspectLoader.register(machinery.FrozenImporter)
@@ -92,33 +100,273 @@ class ExecutionLoader(InspectLoader):
         raise NotImplementedError
 
 
-class PyLoader(_bootstrap.PyLoader, ResourceLoader, ExecutionLoader):
+class SourceLoader(ResourceLoader, ExecutionLoader):
 
-    """Abstract base class to assist in loading source code by requiring only
-    back-end storage methods to be implemented.
+    """Abstract base class for loading source code (and optionally any
+    corresponding bytecode).
 
-    The methods get_code, get_source, and load_module are implemented for the
-    user.
+    To support loading from source code, the abstractmethods inherited from
+    ResourceLoader and ExecutionLoader need to be implemented. To also support
+    loading from bytecode, the optional methods specified directly by this ABC
+    is required.
+
+    Inherited abstractmethods not implemented in this ABC:
+
+        * ResourceLoader.get_data
+        * ExecutionLoader.get_filename
+
+    """
+
+    def path_mtime(self, path:str) -> int:
+        """Optional method that returns the modification time for the specified
+        path.
+
+        Implementing this method allows the loader to read bytecode files.
+
+        """
+        raise NotImplementedError
+
+    def set_data(self, path:str, data:bytes) -> None:
+        """Optional method which writes data to a file path.
+
+        Implementing this method allows for the writing of bytecode files.
+
+        """
+        raise NotImplementedError
+
+    def is_package(self, fullname):
+        """Concrete implementation of InspectLoader.is_package by checking if
+        the path returned by get_filename has a filename of '__init__.py'."""
+        filename = os.path.basename(self.get_filename(fullname))
+        return os.path.splitext(filename)[0] == '__init__'
+
+    def get_source(self, fullname):
+        """Concrete implementation of InspectLoader.get_source."""
+        path = self.get_filename(fullname)
+        try:
+            source_bytes = self.get_data(path)
+        except IOError:
+            raise ImportError("source not available through get_data()")
+        encoding = tokenize.detect_encoding(io.BytesIO(source_bytes).readline)
+        return source_bytes.decode(encoding[0])
+
+    def get_code(self, fullname):
+        """Concrete implementation of InspectLoader.get_code.
+
+        Reading of bytecode requires path_mtime to be implemented. To write
+        bytecode, set_data must also be implemented.
+
+        """
+        source_path = self.get_filename(fullname)
+        bytecode_path = imp.cache_from_source(source_path)
+        source_mtime = None
+        if bytecode_path is not None:
+            try:
+                source_mtime = self.path_mtime(source_path)
+            except NotImplementedError:
+                pass
+            else:
+                try:
+                    data = self.get_data(bytecode_path)
+                except IOError:
+                    pass
+                else:
+                    magic = data[:4]
+                    raw_timestamp = data[4:8]
+                    if (len(magic) == 4 and len(raw_timestamp) == 4 and
+                            magic == imp.get_magic() and
+                            marshal._r_long(raw_timestamp) == source_mtime):
+                        return marshal.loads(data[8:])
+        source_bytes = self.get_data(source_path)
+        code_object = compile(source_bytes, source_path, 'exec',
+                                dont_inherit=True)
+        if (not sys.dont_write_bytecode and bytecode_path is not None and
+                source_mtime is not None):
+            # If e.g. Jython ever implements imp.cache_from_source to have
+            # their own cached file format, this block of code will most likely
+            # throw an exception.
+            data = bytearray(imp.get_magic())
+            data.extend(marshal._w_long(source_mtime))
+            data.extend(marshal.dumps(code_object))
+            try:
+                self.set_data(bytecode_path, data)
+            except (NotImplementedError, IOError):
+                pass
+        return code_object
+
+    @util.module_for_loader
+    def load_module(self, module):
+        """Concrete implementation of Loader.load_module.
+
+        Requires ExecutionLoader.get_filename and ResourceLoader.get_data to be
+        implemented to load source code. Use of bytecode is dictated by whether
+        get_code uses/writes bytecode.
+
+        """
+        name = module.__name__
+        code_object = self.get_code(name)
+        module.__file__ = self.get_filename(name)
+        module.__cached__ = imp.cache_from_source(module.__file__)
+        module.__package__ = name
+        is_package = self.is_package(name)
+        if is_package:
+            module.__path__  = [os.path.dirname(module.__file__)]
+        else:
+            module.__package__ = module.__package__.rpartition('.')[0]
+        module.__loader__ = self
+        exec(code_object, module.__dict__)
+        return module
+
+
+class PyLoader(SourceLoader):
+
+    """Implement the deprecated PyLoader ABC in terms of SourceLoader.
+
+    This class has been deprecated! It is slated for removal in Python 3.4.
+    If compatibility with Python 3.1 is not needed then implement the
+    SourceLoader ABC instead of this class. If Python 3.1 compatibility is
+    needed, then use the following idiom to have a single class that is
+    compatible with Python 3.1 onwards::
+
+        try:
+            from importlib.abc import SourceLoader
+        except ImportError:
+            from importlib.abc import PyLoader as SourceLoader
+
+
+        class CustomLoader(SourceLoader):
+            def get_filename(self, fullname):
+                # Implement ...
+
+            def source_path(self, fullname):
+                '''Implement source_path in terms of get_filename.'''
+                try:
+                    return self.get_filename(fullname)
+                except ImportError:
+                    return None
+
+            def is_package(self, fullname):
+                filename = os.path.basename(self.get_filename(fullname))
+                return os.path.splitext(filename)[0] == '__init__'
 
     """
 
     @abc.abstractmethod
+    def is_package(self, fullname):
+        raise NotImplementedError
+
+    @abc.abstractmethod
     def source_path(self, fullname:str) -> object:
         """Abstract method which when implemented should return the path to the
-        sourced code for the module."""
+        source code for the module."""
         raise NotImplementedError
 
+    def get_filename(self, fullname):
+        """Implement get_filename in terms of source_path.
+
+        As get_filename should only return a source file path there is no
+        chance of the path not existing but loading still being possible, so
+        ImportError should propagate instead of being turned into returning
+        None.
 
-class PyPycLoader(_bootstrap.PyPycLoader, PyLoader):
+        """
+        warnings.warn("importlib.abc.PyLoader is deprecated and is "
+                            "slated for removal in Python 3.4; "
+                            "use SourceLoader instead. "
+                            "See the importlib documentation on how to be "
+                            "compatible with Python 3.1 onwards.",
+                        PendingDeprecationWarning)
+        path = self.source_path(fullname)
+        if path is None:
+            raise ImportError
+        else:
+            return path
+
+PyLoader.register(_bootstrap.PyLoader)
+
+
+class PyPycLoader(PyLoader):
 
     """Abstract base class to assist in loading source and bytecode by
     requiring only back-end storage methods to be implemented.
 
+    This class has been deprecated! Removal is slated for Python 3.4. Implement
+    the SourceLoader ABC instead. If Python 3.1 compatibility is needed, see
+    PyLoader.
+
     The methods get_code, get_source, and load_module are implemented for the
     user.
 
     """
 
+    def get_filename(self, fullname):
+        """Return the source or bytecode file path."""
+        path = self.source_path(fullname)
+        if path is not None:
+            return path
+        path = self.bytecode_path(fullname)
+        if path is not None:
+            return path
+        raise ImportError("no source or bytecode path available for "
+                            "{0!r}".format(fullname))
+
+    def get_code(self, fullname):
+        """Get a code object from source or bytecode."""
+        warnings.warn("importlib.abc.PyPycLoader is deprecated and slated for "
+                            "removal in Python 3.4; use SourceLoader instead. "
+                            "If Python 3.1 compatibility is required, see the "
+                            "latest documentation for PyLoader.",
+                        PendingDeprecationWarning)
+        source_timestamp = self.source_mtime(fullname)
+        # Try to use bytecode if it is available.
+        bytecode_path = self.bytecode_path(fullname)
+        if bytecode_path:
+            data = self.get_data(bytecode_path)
+            try:
+                magic = data[:4]
+                if len(magic) < 4:
+                    raise ImportError("bad magic number in {}".format(fullname))
+                raw_timestamp = data[4:8]
+                if len(raw_timestamp) < 4:
+                    raise EOFError("bad timestamp in {}".format(fullname))
+                pyc_timestamp = marshal._r_long(raw_timestamp)
+                bytecode = data[8:]
+                # Verify that the magic number is valid.
+                if imp.get_magic() != magic:
+                    raise ImportError("bad magic number in {}".format(fullname))
+                # Verify that the bytecode is not stale (only matters when
+                # there is source to fall back on.
+                if source_timestamp:
+                    if pyc_timestamp < source_timestamp:
+                        raise ImportError("bytecode is stale")
+            except (ImportError, EOFError):
+                # If source is available give it a shot.
+                if source_timestamp is not None:
+                    pass
+                else:
+                    raise
+            else:
+                # Bytecode seems fine, so try to use it.
+                return marshal.loads(bytecode)
+        elif source_timestamp is None:
+            raise ImportError("no source or bytecode available to create code "
+                                "object for {0!r}".format(fullname))
+        # Use the source.
+        source_path = self.source_path(fullname)
+        if source_path is None:
+            message = "a source path must exist to load {0}".format(fullname)
+            raise ImportError(message)
+        source = self.get_data(source_path)
+        code_object = compile(source, source_path, 'exec', dont_inherit=True)
+        # Generate bytecode and write it out.
+        if not sys.dont_write_bytecode:
+            data = bytearray(imp.get_magic())
+            data.extend(marshal._w_long(source_timestamp))
+            data.extend(marshal.dumps(code_object))
+            self.write_bytecode(fullname, data)
+        return code_object
+
+
     @abc.abstractmethod
     def source_mtime(self, fullname:str) -> int:
         """Abstract method which when implemented should return the
@@ -137,3 +385,5 @@ class PyPycLoader(_bootstrap.PyPycLoader, PyLoader):
         bytecode for the module, returning a boolean representing whether the
         bytecode was written or not."""
         raise NotImplementedError
+
+PyPycLoader.register(_bootstrap.PyPycLoader)