def fallback_findfile(filename):
    """
    :param str filename:
    :return: try to find the full filename, e.g. in modules, etc
    :rtype: str|None
    """
    mods = [m for m in sys.modules.values() if m and hasattr(m, "__file__") and filename in m.__file__]
    if len(mods) == 0:
        return None
    alt_fn = mods[0].__file__
    if alt_fn[-4:-1] == ".py":
        alt_fn = alt_fn[:-1]  # *.pyc or whatever
    if not os.path.exists(alt_fn) and alt_fn.startswith("./"):
        # Maybe current dir changed.
        alt_fn2 = _cur_pwd + alt_fn[1:]
        if os.path.exists(alt_fn2):
            return alt_fn2
        # Try dirs of some other mods.
        for m in ["__main__", "better_exchook"]:
            if hasattr(sys.modules.get(m), "__file__"):
                alt_fn2 = os.path.dirname(sys.modules[m].__file__) + alt_fn[1:]
                if os.path.exists(alt_fn2):
                    return alt_fn2
    return alt_fn