def _install(modules):
    """
    Take a mapping defining a package and turn it into real C{ModuleType}
    instances in C{sys.modules}.

    Consider these example::

        a = {"foo": "bar"}
        b = {"twisted": {"__version__": "42.6"}}
        c = {"twisted": {"plugin": {"getPlugins": stub}}}

    C{_install(a)} will place an item into C{sys.modules} with C{"foo"} as the
    key and C{"bar" as the value.

    C{_install(b)} will place an item into C{sys.modules} with C{"twisted"} as
    the key.  The value will be a new module object.  The module will have a
    C{"__version__"} attribute with C{"42.6"} as the value.

    C{_install(c)} will place an item into C{sys.modules} with C{"twisted"} as
    the key.  The value will be a new module object with a C{"plugin"}
    attribute.  An item will also be placed into C{sys.modules} with the key
    C{"twisted.plugin"} which refers to that module object.  That module will
    have an attribute C{"getPlugins"} with a value of C{stub}.

    @param modules: A mapping from names to definitions of modules.  The names
        are native strings like C{"twisted"} or C{"unittest"}.  Values may be
        arbitrary objects.  Any value which is not a dictionary will be added to
        C{sys.modules} unmodified.  Any dictionary value indicates the value is
        a new module and its items define the attributes of that module.  The
        definition of this structure is recursive, so a value in the dictionary
        may be a dictionary to trigger another level of processing.

    @return: L{None}
    """
    result = {}
    _makePackages(None, modules, result)
    sys.modules.update(result)

    Take a mapping defining a package and turn it into real C{ModuleType}
    instances in C{sys.modules}.

    Consider these example::

        a = {"foo": "bar"}
        b = {"twisted": {"__version__": "42.6"}}
        c = {"twisted": {"plugin": {"getPlugins": stub}}}

    C{_install(a)} will place an item into C{sys.modules} with C{"foo"} as the
    key and C{"bar" as the value.

    C{_install(b)} will place an item into C{sys.modules} with C{"twisted"} as
    the key.  The value will be a new module object.  The module will have a
    C{"__version__"} attribute with C{"42.6"} as the value.

    C{_install(c)} will place an item into C{sys.modules} with C{"twisted"} as
    the key.  The value will be a new module object with a C{"plugin"}
    attribute.  An item will also be placed into C{sys.modules} with the key
    C{"twisted.plugin"} which refers to that module object.  That module will
    have an attribute C{"getPlugins"} with a value of C{stub}.

    @param modules: A mapping from names to definitions of modules.  The names
        are native strings like C{"twisted"} or C{"unittest"}.  Values may be
        arbitrary objects.  Any value which is not a dictionary will be added to
        C{sys.modules} unmodified.  Any dictionary value indicates the value is
        a new module and its items define the attributes of that module.  The
        definition of this structure is recursive, so a value in the dictionary
        may be a dictionary to trigger another level of processing.

    @return: L{None}
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_twisted.py