Imports a module. A single point of truth for importing modules to
    be documented by `pydoc`. In particular, it makes sure that the top
    module in `module_name` can be imported by using only the paths in
    `pydoc.import_path`.

    If a module has already been imported, then its corresponding entry
    in `sys.modules` is returned. This means that modules that have
    changed on disk cannot be re-imported in the same process and have
    its documentation updated.

def import_module(module_name):
    """
    Imports a module. A single point of truth for importing modules to
    be documented by `pydoc`. In particular, it makes sure that the top
    module in `module_name` can be imported by using only the paths in
    `pydoc.import_path`.

    If a module has already been imported, then its corresponding entry
    in `sys.modules` is returned. This means that modules that have
    changed on disk cannot be re-imported in the same process and have
    its documentation updated.
    """
    if import_path != sys.path:
        # Such a kludge. Only restrict imports if the `import_path` has
        # been changed. We don't want to always restrict imports, since
        # providing a path to `imp.find_module` stops it from searching
        # in special locations for built ins or frozen modules.
        #
        # The problem here is that this relies on the `sys.path` not being
        # independently changed since the initialization of this module.
        # If it is changed, then some packages may fail.
        #
        # Any other options available?

        # Raises an exception if the parent module cannot be imported.
        # This hopefully ensures that we only explicitly import modules
        # contained in `pydoc.import_path`.
        imp.find_module(module_name.split('.')[0], import_path)

    if module_name in sys.modules:
        return sys.modules[module_name]
    else:
        __import__(module_name)
        return sys.modules[module_name]