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230 lines
7.7 KiB
Python
230 lines
7.7 KiB
Python
"""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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else:
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import importlib
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string_types = str,
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from .decorators import synchronized
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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 truncacted 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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@synchronized(_post_import_hooks_lock)
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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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# 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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# Determine if any prior registration of a post import hook for
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# the target modules has occurred and act appropriately.
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hooks = _post_import_hooks.get(name, None)
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if hooks is None:
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# No prior registration of post import hooks for the target
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# module. We need to check whether the module has already been
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# imported. If it has we fire the hook immediately and add an
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# empty list to the registry to indicate that the module has
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# already been imported and hooks have fired. Otherwise add
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# the post import hook to the registry.
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module = sys.modules.get(name, None)
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if module is not None:
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_post_import_hooks[name] = []
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hook(module)
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else:
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_post_import_hooks[name] = [hook]
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elif hooks == []:
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# A prior registration of port import hooks for the target
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# module was done and the hooks already fired. Fire the hook
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# immediately.
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module = sys.modules[name]
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hook(module)
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else:
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# A prior registration of port import hooks for the target
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# module was done but the module has not yet been imported.
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_post_import_hooks[name].append(hook)
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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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@synchronized(_post_import_hooks_lock)
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def notify_module_loaded(module):
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name = getattr(module, '__name__', None)
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hooks = _post_import_hooks.get(name, None)
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if hooks:
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_post_import_hooks[name] = []
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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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def load_module(self, fullname):
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module = self.loader.load_module(fullname)
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notify_module_loaded(module)
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return 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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@synchronized(_post_import_hooks_lock)
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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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if not fullname 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 PY2:
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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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try:
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import importlib.util
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loader = importlib.util.find_spec(fullname).loader
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except (ImportError, AttributeError):
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loader = importlib.find_loader(fullname, path)
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if loader:
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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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# 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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