The weirdest thing is that it already works pretty well in output of re.compile.

    >>> re.compile('', re.I | re.M)
    re.compile('', re.IGNORECASE|re.MULTILINE)
    >>> _.flags  # re.UNICODE == 32 added automatically
    42

So the only thing we should enhance is the output of .flags, and it seems that all the necessary code is already in the source code of __repr__ of SRE_Pattern objects.

Also, I suppose that means you've given up on the autocreation (since the values _are_ semantical here), and I suppose you'll require all the declared values to be powers of 2. With those conditions, I think this is a good enhancement of Python.