What does "del sys.modules[module]" actually do?

As everyone knows, you can do del sys.modules[module] to delete an imported module. So I was thinking: how is this different from rewriting sys.modules? An interesting fact is, rewriting sys.modules can't truely delete a module.

# a_module.pyprint("a module imported")

Then

import sysdef func1():    import a_module    # del sys.modules['a_module']    sys.modules = {        k: v for k, v in sys.modules.items() if 'a_module' not in k}    print('a_module' not in sys.modules)  # Truedef func2():    import a_modulefunc1()  # a module importedfunc2()  # no output here

If I use del sys.modules['a_module'], calling func2() also prints a module imported, which implies that a_module is successfully deleted.

My question is: What does del sys.modules[module] actually do, besides changing the dictionary?