# sys_path_modify.py
    import imp
    import os
    import sys

    base_dir = os.path.dirname(__file__) or '.'
    print('Base directory:', base_dir)

    # Insert the package_dir_a directory at the front of the path.
    package_dir_a = os.path.join(base_dir, 'package_dir_a')
    sys.path.insert(0, package_dir_a)

    # Import the example module
    import example
    print('Imported example from:', example.__file__)
    print('\t', example.DATA)

    # Make package_dir_b the first directory in the search path
    package_dir_b = os.path.join(base_dir, 'package_dir_b')
    sys.path.insert(0, package_dir_b)

    # Reload the module to get the other version
    imp.reload(example)
    print('Reloaded example from:', example.__file__)
    print('\t', example.DATA)

    $ python3 sys_path_modify.py

    Base directory: .
    Imported example from: ./package_dir_a/example.py
             This is example A
    Reloaded example from: ./package_dir_b/example.py
             This is example B        

    # sys_path_hooks_noisy.py
    import sys

    
    class NoisyImportFinder:

        PATH_TRIGGER = 'NoisyImportFinder_PATH_TRIGGER'

        def __init__(self, path_entry):
            print('Checking {}:'.format(path_entry), end=' ')
            if path_entry != self.PATH_TRIGGER:
                print('wrong finder')
                raise ImportError()
            else:
                print('works')
            return

        def find_module(self, fullname, path=None):
            print('Looking for {!r}'.format(fullname))
            return None


    sys.path_hooks.append(NoisyImportFinder)

    for hook in sys.path_hooks:
        print('Path hook: {}'.format(hook))

    sys.path.insert(0, NoisyImportFinder.PATH_TRIGGER)

    try:
        print('importing target_module')
        import target_module
    except Exception as e:
        print('Import failed:', e)

    $ python3 sys_path_hooks_noisy.py

    Path hook: <class 'zipimport.zipimporter'>
    Path hook: <function FileFinder.path_hook.<locals>.path_hook_for_FileFinder at 0x100734950>
    Path hook: <class '__main__.NoisyImportFinder'>
    importing target_module
    Checking NoisyImportFinder_PATH_TRIGGER: works
    Looking for 'target_module'
    Import failed: No module named 'target_module'