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"""Import modules on demand.
See https://bugs.python.org/issue17621 for background.
"""
import contextlib
import importlib.machinery
import os
import sys
from importlib.util import LazyLoader
# global flag controlling lazy import support
_disabled = False
# modules that have issues when lazily imported
_skip = frozenset(
[
"__builtin__",
"__future__",
"builtins",
"grp",
"pwd",
"OpenSSL.SSL", # pyopenssl
]
)
class _LazyLoader(LazyLoader):
"""LazyLoader with extended support for disabling and skipping modules."""
def exec_module(self, module):
"""Make the module load lazily."""
if _disabled or module.__name__ in _skip:
self.loader.exec_module(module)
else:
super().exec_module(module)
# custom loaders using our extended LazyLoader
_extensions_loader = _LazyLoader.factory(importlib.machinery.ExtensionFileLoader)
_bytecode_loader = _LazyLoader.factory(importlib.machinery.SourcelessFileLoader)
_source_loader = _LazyLoader.factory(importlib.machinery.SourceFileLoader)
def _filefinder(path):
"""Return a custom FileFinder using our lazy loaders."""
return importlib.machinery.FileFinder(
path,
(_extensions_loader, importlib.machinery.EXTENSION_SUFFIXES),
(_source_loader, importlib.machinery.SOURCE_SUFFIXES),
(_bytecode_loader, importlib.machinery.BYTECODE_SUFFIXES),
)
def enable():
"""Enable lazy loading for all future module imports."""
if os.environ.get("SNAKEOIL_DEMANDIMPORT", "y").lower() not in (
"n",
"no",
"0",
"false",
):
sys.path_hooks.insert(0, _filefinder)
def is_enabled():
"""Determine if lazy loading is currently enabled."""
return _filefinder in sys.path_hooks and not _disabled
def disable():
"""Disable lazy loading for all future module imports."""
try:
while True:
sys.path_hooks.remove(_filefinder)
except ValueError:
pass
@contextlib.contextmanager
def disabled():
"""Context manager for temporarily disabling lazy imports.
Example usage:
>>> from importlib.util import _LazyModule
>>> from snakeoil import demandimport
>>> demandimport.enable()
>>>
>>> with demandimport.disabled():
>>> from module import submodule
>>> assert not isinstance(submodule, _LazyModule)
"""
global _disabled
enabled = is_enabled()
if enabled:
_disabled = True
try:
yield
finally:
if enabled:
_disabled = False
@contextlib.contextmanager
def enabled():
"""Context manager for temporarily enabling lazy imports.
Useful as a workaround for avoiding circular import issues.
Example usage:
>>> from importlib.util import _LazyModule
>>> from snakeoil import demandimport
>>>
>>> with demandimport.enabled():
>>> from module import submodule
>>> assert isinstance(submodule, _LazyModule)
>>> from module2 import submodule2
>>> assert not isinstance(submodule2, _LazyModule)
"""
global _disabled
enabled = is_enabled()
if not enabled:
_disabled = False
enable()
try:
yield
finally:
if not enabled:
_disabled = True
|
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