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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 

 

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. 

 

@synchronized(_post_import_hooks_lock) 

def register_post_import_hook(hook, name): 

    # 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. 

 

        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 discover_post_import_hooks(group): 

    try: 

        import pkg_resources 

    except ImportError: 

        return 

 

    for entrypoint in pkg_resources.iter_entry_points(group=group): 

        def proxy_post_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) 

 

        register_post_import_hook(proxy_post_import_hook, 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_loader() from 

                # the importlib 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. 

 

                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