[docs] [issue9583] Document startup option/environment interaction

[Buck Golemon]

Buck Golemon <buck.golemon at amd.com> added the comment:

If I understand this code, it means that PYTHONOPTIMIZE set to 1 or 2 works as expected, but set to 0, gives a flag value of 1.

static int
add_flag(int flag, const char *envs)
{
        int env = atoi(envs);
        if (flag < env)
                flag = env;
        if (flag < 1)
                flag = 1;
        return flag;
}


Read literally, the man page indicates that any integer value will give a flag value of 2. 

I agree my shell script is probably unusual, but I believe setting this environment value to zero and expecting the feature to be off (given no contradicting options) is reasonable.

I petition to remove the second if statement above (environment value of "0" yields no flag).

I'd also love to provide a numeric argument to -O, to dynamically set this value more readily, but that is lower importance.

I can implement these and run the unit tests if required.

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<http://bugs.python.org/issue9583>
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