def Presentation(pptx=None):
    """
    Return a |Presentation| object loaded from *pptx*, where *pptx* can be
    either a path to a ``.pptx`` file (a string) or a file-like object. If
    *pptx* is missing or ``None``, the built-in default presentation
    "template" is loaded.
    """
    if pptx is None:
        pptx = _default_pptx_path()

    presentation_part = Package.open(pptx).main_document_part

    if not _is_pptx_package(presentation_part):
        tmpl = "file '%s' is not a PowerPoint file, content type is '%s'"
        raise ValueError(tmpl % (pptx, presentation_part.content_type))

    return presentation_part.presentation