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"""This module implements a post import hook mechanism styled after what is
described in PEP-369. Note that it doesn't cope with modules being reloaded.
"""
import sys
import threading
PY2 = sys.version_info[0] == 2
PY3 = sys.version_info[0] == 3
if PY3:
import importlib
string_types = str,
else:
string_types = basestring,
from .decorators import synchronized
# The dictionary registering any post import hooks to be triggered once
# the target module has been imported. Once a module has been imported
# and the hooks fired, the list of hooks recorded against the target
# module will be truncacted but the list left in the dictionary. This
# acts as a flag to indicate that the module had already been imported.
_post_import_hooks = {}
_post_import_hooks_init = False
_post_import_hooks_lock = threading.RLock()
# Register a new post import hook for the target module name. This
# differs from the PEP-369 implementation in that it also allows the
# hook function to be specified as a string consisting of the name of
# the callback in the form 'module:function'. This will result in a
# proxy callback being registered which will defer loading of the
# specified module containing the callback function until required.
def _create_import_hook_from_string(name):
def import_hook(module):
module_name, function = name.split(':')
attrs = function.split('.')
__import__(module_name)
callback = sys.modules[module_name]
for attr in attrs:
callback = getattr(callback, attr)
return callback(module)
return import_hook
@synchronized(_post_import_hooks_lock)
def register_post_import_hook(hook, name):
# Create a deferred import hook if hook is a string name rather than
# a callable function.
if isinstance(hook, string_types):
hook = _create_import_hook_from_string(hook)
# Automatically install the import hook finder if it has not already
# been installed.
global _post_import_hooks_init
if not _post_import_hooks_init:
_post_import_hooks_init = True
sys.meta_path.insert(0, ImportHookFinder())
# Determine if any prior registration of a post import hook for
# the target modules has occurred and act appropriately.
hooks = _post_import_hooks.get(name, None)
if hooks is None:
# No prior registration of post import hooks for the target
# module. We need to check whether the module has already been
# imported. If it has we fire the hook immediately and add an
# empty list to the registry to indicate that the module has
# already been imported and hooks have fired. Otherwise add
# the post import hook to the registry.
module = sys.modules.get(name, None)
if module is not None:
_post_import_hooks[name] = []
hook(module)
else:
_post_import_hooks[name] = [hook]
elif hooks == []:
# A prior registration of port import hooks for the target
# module was done and the hooks already fired. Fire the hook
# immediately.
module = sys.modules[name]
hook(module)
else:
# A prior registration of port import hooks for the target
# module was done but the module has not yet been imported.
_post_import_hooks[name].append(hook)
# Register post import hooks defined as package entry points.
def _create_import_hook_from_entrypoint(entrypoint):
def import_hook(module):
__import__(entrypoint.module_name)
callback = sys.modules[entrypoint.module_name]
for attr in entrypoint.attrs:
callback = getattr(callback, attr)
return callback(module)
return import_hook
def discover_post_import_hooks(group):
try:
import pkg_resources
except ImportError:
return
for entrypoint in pkg_resources.iter_entry_points(group=group):
callback = _create_import_hook_from_entrypoint(entrypoint)
register_post_import_hook(callback, entrypoint.name)
# Indicate that a module has been loaded. Any post import hooks which
# were registered against the target module will be invoked. If an
# exception is raised in any of the post import hooks, that will cause
# the import of the target module to fail.
@synchronized(_post_import_hooks_lock)
def notify_module_loaded(module):
name = getattr(module, '__name__', None)
hooks = _post_import_hooks.get(name, None)
if hooks:
_post_import_hooks[name] = []
for hook in hooks:
hook(module)
# A custom module import finder. This intercepts attempts to import
# modules and watches out for attempts to import target modules of
# interest. When a module of interest is imported, then any post import
# hooks which are registered will be invoked.
class _ImportHookLoader:
def load_module(self, fullname):
module = sys.modules[fullname]
notify_module_loaded(module)
return module
class _ImportHookChainedLoader:
def __init__(self, loader):
self.loader = loader
def load_module(self, fullname):
module = self.loader.load_module(fullname)
notify_module_loaded(module)
return module
class ImportHookFinder:
def __init__(self):
self.in_progress = {}
@synchronized(_post_import_hooks_lock)
def find_module(self, fullname, path=None):
# If the module being imported is not one we have registered
# post import hooks for, we can return immediately. We will
# take no further part in the importing of this module.
if not fullname in _post_import_hooks:
return None
# When we are interested in a specific module, we will call back
# into the import system a second time to defer to the import
# finder that is supposed to handle the importing of the module.
# We set an in progress flag for the target module so that on
# the second time through we don't trigger another call back
# into the import system and cause a infinite loop.
if fullname in self.in_progress:
return None
self.in_progress[fullname] = True
# Now call back into the import system again.
try:
if PY3:
# For Python 3 we need to use find_spec().loader
# from the importlib.util module. It doesn't actually
# import the target module and only finds the
# loader. If a loader is found, we need to return
# our own loader which will then in turn call the
# real loader to import the module and invoke the
# post import hooks.
try:
import importlib.util
loader = importlib.util.find_spec(fullname).loader
except (ImportError, AttributeError):
loader = importlib.find_loader(fullname, path)
if loader:
return _ImportHookChainedLoader(loader)
else:
# For Python 2 we don't have much choice but to
# call back in to __import__(). This will
# actually cause the module to be imported. If no
# module could be found then ImportError will be
# raised. Otherwise we return a loader which
# returns the already loaded module and invokes
# the post import hooks.
__import__(fullname)
return _ImportHookLoader()
finally:
del self.in_progress[fullname]
# Decorator for marking that a function should be called as a post
# import hook when the target module is imported.
def when_imported(name):
def register(hook):
register_post_import_hook(hook, name)
return hook
return register
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