294 lines
10 KiB
Python
294 lines
10 KiB
Python
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"""This module implements a post import hook mechanism styled after what is
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described in PEP-369. Note that it doesn't cope with modules being reloaded.
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"""
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import sys
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import threading
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PY2 = sys.version_info[0] == 2
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if PY2:
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string_types = basestring,
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find_spec = None
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else:
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string_types = str,
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from importlib.util import find_spec
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# The dictionary registering any post import hooks to be triggered once
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# the target module has been imported. Once a module has been imported
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# and the hooks fired, the list of hooks recorded against the target
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# module will be truncated but the list left in the dictionary. This
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# acts as a flag to indicate that the module had already been imported.
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_post_import_hooks = {}
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_post_import_hooks_init = False
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_post_import_hooks_lock = threading.RLock()
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# Register a new post import hook for the target module name. This
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# differs from the PEP-369 implementation in that it also allows the
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# hook function to be specified as a string consisting of the name of
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# the callback in the form 'module:function'. This will result in a
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# proxy callback being registered which will defer loading of the
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# specified module containing the callback function until required.
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def _create_import_hook_from_string(name):
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def import_hook(module):
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module_name, function = name.split(':')
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attrs = function.split('.')
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__import__(module_name)
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callback = sys.modules[module_name]
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for attr in attrs:
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callback = getattr(callback, attr)
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return callback(module)
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return import_hook
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def register_post_import_hook(hook, name):
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# Create a deferred import hook if hook is a string name rather than
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# a callable function.
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if isinstance(hook, string_types):
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hook = _create_import_hook_from_string(hook)
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with _post_import_hooks_lock:
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# Automatically install the import hook finder if it has not already
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# been installed.
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global _post_import_hooks_init
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if not _post_import_hooks_init:
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_post_import_hooks_init = True
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sys.meta_path.insert(0, ImportHookFinder())
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# Check if the module is already imported. If not, register the hook
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# to be called after import.
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module = sys.modules.get(name, None)
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if module is None:
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_post_import_hooks.setdefault(name, []).append(hook)
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# If the module is already imported, we fire the hook right away. Note that
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# the hook is called outside of the lock to avoid deadlocks if code run as a
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# consequence of calling the module import hook in turn triggers a separate
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# thread which tries to register an import hook.
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if module is not None:
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hook(module)
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# Register post import hooks defined as package entry points.
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def _create_import_hook_from_entrypoint(entrypoint):
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def import_hook(module):
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__import__(entrypoint.module_name)
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callback = sys.modules[entrypoint.module_name]
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for attr in entrypoint.attrs:
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callback = getattr(callback, attr)
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return callback(module)
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return import_hook
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def discover_post_import_hooks(group):
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try:
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import pkg_resources
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except ImportError:
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return
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for entrypoint in pkg_resources.iter_entry_points(group=group):
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callback = _create_import_hook_from_entrypoint(entrypoint)
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register_post_import_hook(callback, entrypoint.name)
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# Indicate that a module has been loaded. Any post import hooks which
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# were registered against the target module will be invoked. If an
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# exception is raised in any of the post import hooks, that will cause
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# the import of the target module to fail.
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def notify_module_loaded(module):
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name = getattr(module, '__name__', None)
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with _post_import_hooks_lock:
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hooks = _post_import_hooks.pop(name, ())
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# Note that the hook is called outside of the lock to avoid deadlocks if
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# code run as a consequence of calling the module import hook in turn
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# triggers a separate thread which tries to register an import hook.
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for hook in hooks:
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hook(module)
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# A custom module import finder. This intercepts attempts to import
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# modules and watches out for attempts to import target modules of
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# interest. When a module of interest is imported, then any post import
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# hooks which are registered will be invoked.
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class _ImportHookLoader:
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def load_module(self, fullname):
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module = sys.modules[fullname]
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notify_module_loaded(module)
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return module
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class _ImportHookChainedLoader:
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def __init__(self, loader):
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self.loader = loader
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if hasattr(loader, "load_module"):
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self.load_module = self._load_module
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if hasattr(loader, "create_module"):
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self.create_module = self._create_module
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if hasattr(loader, "exec_module"):
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self.exec_module = self._exec_module
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def _set_loader(self, module):
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# Set module's loader to self.loader unless it's already set to
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# something else. Import machinery will set it to spec.loader if it is
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# None, so handle None as well. The module may not support attribute
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# assignment, in which case we simply skip it. Note that we also deal
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# with __loader__ not existing at all. This is to future proof things
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# due to proposal to remove the attribue as described in the GitHub
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# issue at https://github.com/python/cpython/issues/77458. Also prior
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# to Python 3.3, the __loader__ attribute was only set if a custom
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# module loader was used. It isn't clear whether the attribute still
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# existed in that case or was set to None.
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class UNDEFINED: pass
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if getattr(module, "__loader__", UNDEFINED) in (None, self):
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try:
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module.__loader__ = self.loader
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except AttributeError:
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pass
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if (getattr(module, "__spec__", None) is not None
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and getattr(module.__spec__, "loader", None) is self):
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module.__spec__.loader = self.loader
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def _load_module(self, fullname):
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module = self.loader.load_module(fullname)
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self._set_loader(module)
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notify_module_loaded(module)
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return module
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# Python 3.4 introduced create_module() and exec_module() instead of
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# load_module() alone. Splitting the two steps.
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def _create_module(self, spec):
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return self.loader.create_module(spec)
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def _exec_module(self, module):
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self._set_loader(module)
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self.loader.exec_module(module)
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notify_module_loaded(module)
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class ImportHookFinder:
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def __init__(self):
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self.in_progress = {}
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def find_module(self, fullname, path=None):
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# If the module being imported is not one we have registered
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# post import hooks for, we can return immediately. We will
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# take no further part in the importing of this module.
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with _post_import_hooks_lock:
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if fullname not in _post_import_hooks:
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return None
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# When we are interested in a specific module, we will call back
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# into the import system a second time to defer to the import
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# finder that is supposed to handle the importing of the module.
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# We set an in progress flag for the target module so that on
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# the second time through we don't trigger another call back
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# into the import system and cause a infinite loop.
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if fullname in self.in_progress:
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return None
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self.in_progress[fullname] = True
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# Now call back into the import system again.
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try:
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if not find_spec:
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# For Python 2 we don't have much choice but to
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# call back in to __import__(). This will
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# actually cause the module to be imported. If no
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# module could be found then ImportError will be
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# raised. Otherwise we return a loader which
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# returns the already loaded module and invokes
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# the post import hooks.
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__import__(fullname)
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return _ImportHookLoader()
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else:
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# For Python 3 we need to use find_spec().loader
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# from the importlib.util module. It doesn't actually
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# import the target module and only finds the
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# loader. If a loader is found, we need to return
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# our own loader which will then in turn call the
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# real loader to import the module and invoke the
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# post import hooks.
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loader = getattr(find_spec(fullname), "loader", None)
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if loader and not isinstance(loader, _ImportHookChainedLoader):
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return _ImportHookChainedLoader(loader)
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finally:
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del self.in_progress[fullname]
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def find_spec(self, fullname, path=None, target=None):
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# Since Python 3.4, you are meant to implement find_spec() method
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# instead of find_module() and since Python 3.10 you get deprecation
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# warnings if you don't define find_spec().
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# If the module being imported is not one we have registered
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# post import hooks for, we can return immediately. We will
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# take no further part in the importing of this module.
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with _post_import_hooks_lock:
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if fullname not in _post_import_hooks:
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return None
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# When we are interested in a specific module, we will call back
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# into the import system a second time to defer to the import
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# finder that is supposed to handle the importing of the module.
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# We set an in progress flag for the target module so that on
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# the second time through we don't trigger another call back
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# into the import system and cause a infinite loop.
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if fullname in self.in_progress:
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return None
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self.in_progress[fullname] = True
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# Now call back into the import system again.
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try:
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# This should only be Python 3 so find_spec() should always
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# exist so don't need to check.
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spec = find_spec(fullname)
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loader = getattr(spec, "loader", None)
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if loader and not isinstance(loader, _ImportHookChainedLoader):
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spec.loader = _ImportHookChainedLoader(loader)
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return spec
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finally:
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del self.in_progress[fullname]
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# Decorator for marking that a function should be called as a post
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# import hook when the target module is imported.
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def when_imported(name):
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def register(hook):
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register_post_import_hook(hook, name)
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return hook
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return register
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