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    <p>If it's just <code>pow()</code> giving you headaches, you can easily redefine it to return <code>NaN</code> under whatever circumstances you like.</p> <pre><code>def pow(x, y): return x ** y if x == x else float("NaN") </code></pre> <p>If <code>NaN</code> can be used as an exponent you'd also want to check for that; this raises a <code>ValueError</code> exception except when the base is 1 (apparently on the theory that 1 to any power, even one that's not a number, is 1).</p> <p>(And of course <code>pow()</code> actually takes three operands, the third optional, which omission I'll leave as an exercise...)</p> <p>Unfortunately the <code>**</code> operator has the same behavior, and there's no way to redefine that for built-in numeric types. A possibility to catch this is to write a subclass of <code>float</code> that implements <code>__pow__()</code> and <code>__rpow__()</code> and use that class for your <code>NaN</code> values. </p> <p>Python doesn't seem to provide access to any flags set by calculations; even if it did, it's something you'd have to check after each individual operation.</p> <p><strong>In fact, on further consideration,</strong> I think the best solution might be to simply use an instance of a dummy class for missing values. Python will choke on any operation you try to do with these values, raising an exception, and you can catch the exception and return a default value or whatever. There's no reason to proceed with the rest of the calculation if a needed value is missing, so an exception should be fine.</p>
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    1. POPropagation of NaN through calculations
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    1. COI don't see how that works. `NaN != NaN` so your `if` is always going to be true.
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    2. COJust replace `x != NaN` with `x == x`.
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    3. COAnd I'm not sure; maybe `pow` is the only one, maybe it's not... I guess using `NaN` for missing data, neat as it sounds, is not really practical... :(
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