Combining Flag members with |, & and ^ walks both operands on every call to check that neither carries a None value, raising a clear TypeError when one does. That check only matters in the rare case where a value really is None; for every normal combination of two valid flags it adds work to each operation. Flag arithmetic is hot wherever flags model option sets — the re module's own flags are an IntFlag, so each re.compile(pattern, re.I | re.M) combines them, and permission systems, feature toggles and styling layers do the same in tight loops.

The two values the guard inspects are already in hand: one is the member's own value, the other is the operand value computed just above. This runs the scan over the operands only when one of those is actually None, so a valid combination skips it entirely. The offending-flag walk, the TypeError and its message are unchanged for the invalid cases.

Running flag combinations mined from the top-1000 corpus, including the re flag sets, improves from 72.6 µs to 49.5 µs, 47% faster.

import re, functools, operator, pyperf

combos_str = ["re.DOTALL|re.I", "re.DOTALL|re.M", "re.DOTALL|re.VERBOSE",
    "re.DOTALL|re.VERBOSE|re.M", "re.IGNORECASE|re.VERBOSE", "re.I|re.M",
    "re.I|re.S", "re.I|re.S|re.X", "re.MULTILINE|re.DOTALL",
    "re.MULTILINE|re.DOTALL|re.VERBOSE", "re.M|re.S", "re.S|re.M",
    "re.VERBOSE|re.I", "re.VERBOSE|re.M"]
M = {"I": re.I, "IGNORECASE": re.I, "M": re.M, "MULTILINE": re.M, "S": re.S,
     "DOTALL": re.S, "X": re.X, "VERBOSE": re.X, "A": re.A, "ASCII": re.A}
combos = [[M[p.split(".")[1]] for p in c.split("|")] for c in combos_str]

def run():
    for flags in combos:
        combined = functools.reduce(operator.or_, flags)
        flags[0] in combined

runner = pyperf.Runner()
runner.bench_func("re.RegexFlag combine %d real corpus combos" % len(combos), run)

Resolves #150817.