I want to get the source code of the currently running program, do some transformations (at the source-level and also at the bytecode-level), and output the new source code.

@typechecked
def module_to_bytecode(module:ModuleType)->Bytecode: # ?
    #with open(module.__file__, "rb") as file:
    #    file.seek(16)
    #    mbc:bytes = file.read( )
    try:
        return marshal.load(module.__file__)
    except ValueError as e:
        raise IntrospectionError('failed to convert moduletype to bytecode') from e

@typechecked
def decompile_module(module:ModuleType)->str:
    """ handles .pyc ? """

    assembly:Bytecode = module_to_bytecode(module)
    try:
        return decompile(assembly) # TODO test
    except Exception as e:
        raise IntrospectionError(f"failed to decompile moduletype") from e
#if extra_tests:
#    print(decompile_module(getmodule(currentframe())))
    
@typechecked
def introspect()->str:
    """ returns the source code for the currently running module """

    cf:FrameType  = currentframe()
    if cf is None:
        raise IntrospectionError("Running on an implementation without Python stack frame support?")
    cm:ModuleType = getmodule(cf)
    try:
        return getsource(cm)
    except OSError as e:
        return decompile_module(cm)