Issue #17621: Introduce importlib.util.LazyLoader.
diff --git a/Lib/importlib/util.py b/Lib/importlib/util.py
index 6d73b1d..e50ef6d 100644
--- a/Lib/importlib/util.py
+++ b/Lib/importlib/util.py
@@ -1,5 +1,5 @@
"""Utility code for constructing importers, etc."""
-
+from . import abc
from ._bootstrap import MAGIC_NUMBER
from ._bootstrap import cache_from_source
from ._bootstrap import decode_source
@@ -12,6 +12,7 @@
from contextlib import contextmanager
import functools
import sys
+import types
import warnings
@@ -200,3 +201,94 @@
return fxn(self, module, *args, **kwargs)
return module_for_loader_wrapper
+
+
+class _Module(types.ModuleType):
+
+ """A subclass of the module type to allow __class__ manipulation."""
+
+
+class _LazyModule(types.ModuleType):
+
+ """A subclass of the module type which triggers loading upon attribute access."""
+
+ def __getattribute__(self, attr):
+ """Trigger the load of the module and return the attribute."""
+ # All module metadata must be garnered from __spec__ in order to avoid
+ # using mutated values.
+ # Stop triggering this method.
+ self.__class__ = _Module
+ # Get the original name to make sure no object substitution occurred
+ # in sys.modules.
+ original_name = self.__spec__.name
+ # Figure out exactly what attributes were mutated between the creation
+ # of the module and now.
+ attrs_then = self.__spec__.loader_state
+ attrs_now = self.__dict__
+ attrs_updated = {}
+ for key, value in attrs_now.items():
+ # Code that set the attribute may have kept a reference to the
+ # assigned object, making identity more important than equality.
+ if key not in attrs_then:
+ attrs_updated[key] = value
+ elif id(attrs_now[key]) != id(attrs_then[key]):
+ attrs_updated[key] = value
+ self.__spec__.loader.exec_module(self)
+ # If exec_module() was used directly there is no guarantee the module
+ # object was put into sys.modules.
+ if original_name in sys.modules:
+ if id(self) != id(sys.modules[original_name]):
+ msg = ('module object for {!r} substituted in sys.modules '
+ 'during a lazy load')
+ raise ValueError(msg.format(original_name))
+ # Update after loading since that's what would happen in an eager
+ # loading situation.
+ self.__dict__.update(attrs_updated)
+ return getattr(self, attr)
+
+ def __delattr__(self, attr):
+ """Trigger the load and then perform the deletion."""
+ # To trigger the load and raise an exception if the attribute
+ # doesn't exist.
+ self.__getattribute__(attr)
+ delattr(self, attr)
+
+
+class LazyLoader(abc.Loader):
+
+ """A loader that creates a module which defers loading until attribute access."""
+
+ @staticmethod
+ def __check_eager_loader(loader):
+ if not hasattr(loader, 'exec_module'):
+ raise TypeError('loader must define exec_module()')
+ elif hasattr(loader.__class__, 'create_module'):
+ if abc.Loader.create_module != loader.__class__.create_module:
+ # Only care if create_module() is overridden in a subclass of
+ # importlib.abc.Loader.
+ raise TypeError('loader cannot define create_module()')
+
+ @classmethod
+ def factory(cls, loader):
+ """Construct a callable which returns the eager loader made lazy."""
+ cls.__check_eager_loader(loader)
+ return lambda *args, **kwargs: cls(loader(*args, **kwargs))
+
+ def __init__(self, loader):
+ self.__check_eager_loader(loader)
+ self.loader = loader
+
+ def create_module(self, spec):
+ """Create a module which can have its __class__ manipulated."""
+ return _Module(spec.name)
+
+ def exec_module(self, module):
+ """Make the module load lazily."""
+ module.__spec__.loader = self.loader
+ module.__loader__ = self.loader
+ # Don't need to worry about deep-copying as trying to set an attribute
+ # on an object would have triggered the load,
+ # e.g. ``module.__spec__.loader = None`` would trigger a load from
+ # trying to access module.__spec__.
+ module.__spec__.loader_state = module.__dict__.copy()
+ module.__class__ = _LazyModule