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    <p>I realise that a month has passed since this was asked, but I've come across a similar problem (i.e. <code>pow(float('nan'), 1)</code> throws an exception in some Python implementations, e.g. Jython 2.52b2), and I found the above answers weren't quite what I was looking for.</p> <p>Using a MissingData type as suggested by 6502 seems like the way to go, but I needed a concrete example. I tried Ethan Furman's NullType class but found that that this didn't work with any arithmetic operations as it doesn't coerce data types (see below), and I also didn't like that it explicitly named each arithmetic function that was overriden.</p> <p>Starting with Ethan's example and tweaking code I found <a href="http://code.activestate.com/recipes/307618-class-with-default-method-handler/" rel="nofollow">here</a>, I arrived at the class below. Although the class is heavily commented you can see that it actually only has a handful of lines of functional code in it.</p> <p>The key points are: <strong><em>1.</em></strong> Use coerce() to return two NoData objects for mixed type (e.g. NoData + float) arithmetic operations, and two strings for string based (e.g. concat) operations. <strong><em>2.</em></strong> Use getattr() to return a callable NoData() object for all other attribute/method access <strong><em>3.</em></strong> Use call() to implement all other methods of the NoData() object: by returning a NoData() object</p> <p>Here's some examples of its use. </p> <pre><code>&gt;&gt;&gt; nd = NoData() &gt;&gt;&gt; nd + 5 NoData() &gt;&gt;&gt; pow(nd, 1) NoData() &gt;&gt;&gt; math.pow(NoData(), 1) Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; TypeError: nb_float should return float object &gt;&gt;&gt; nd &gt; 5 NoData() &gt;&gt;&gt; if nd &gt; 5: ... print "Yes" ... else: ... print "No" ... No &gt;&gt;&gt; "The answer is " + nd 'The answer is NoData()' &gt;&gt;&gt; "The answer is %f" % (nd) Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; TypeError: float argument required, not instance &gt;&gt;&gt; "The answer is %s" % (nd) 'The answer is ' &gt;&gt;&gt; nd.f = 5 &gt;&gt;&gt; nd.f NoData() &gt;&gt;&gt; nd.f() NoData() </code></pre> <p>I noticed that using pow with NoData() calls the ** operator and hence works with NoData, but using math.pow does not as it first tries to convert the NoData() object to a float. I'm happy using the non math pow - hopefully 6502 etc were using math.pow when they had problems with pow in their comments above.</p> <p>The other issue I can't think of a way of solving is the use with the format (%f) operator... No methods of NoData are called in this case, the operator just fails if you don't provide a float. Anyway here's the class itself.</p> <pre><code>class NoData(): """NoData object - any interaction returns NoData()""" def __str__(self): #I want '' returned as it represents no data in my output (e.g. csv) files return '' def __unicode__(self): return '' def __repr__(self): return 'NoData()' def __coerce__(self, other_object): if isinstance(other_object, str) or isinstance(other_object, unicode): #Return string objects when coerced with another string object. #This ensures that e.g. concatenation operations produce strings. return repr(self), other_object else: #Otherwise return two NoData objects - these will then be passed to the appropriate #operator method for NoData, which should then return a NoData object return self, self def __nonzero__(self): #__nonzero__ is the operation that is called whenever, e.g. "if NoData:" occurs #i.e. as all operations involving NoData return NoData, whenever a #NoData object propagates to a test in branch statement. return False def __hash__(self): #prevent NoData() from being used as a key for a dict or used in a set raise TypeError("Unhashable type: " + self.repr()) def __setattr__(self, name, value): #This is overridden to prevent any attributes from being created on NoData when e.g. "NoData().f = x" is called return None def __call__(self, *args, **kwargs): #if a NoData object is called (i.e. used as a method), return a NoData object return self def __getattr__(self,name): #For all other attribute accesses or method accesses, return a NoData object. #Remember that the NoData object can be called (__call__), so if a method is called, #a NoData object is first returned and then called. This works for operators, #so e.g. NoData() + 5 will: # - call NoData().__coerce__, which returns a (NoData, NoData) tuple # - call __getattr__, which returns a NoData object # - call the returned NoData object with args (self, NoData) # - this call (i.e. __call__) returns a NoData object #For attribute accesses NoData will be returned, and that's it. #print name #(uncomment this line for debugging purposes i.e. to see that attribute was accessed/method was called) return self </code></pre>
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    1. COI meant Jython 2.5.2b2, not 2.52b2
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