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POImplementing __neg__ in a generic way for all subclasses in Python
 

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  1. POImplementing __neg__ in a generic way for all subclasses in Python
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    <p>I apologize in advance for the rather long question.</p> <p>I'm implementing callable objects and would like them to behave somewhat like (mathematical) functions. I have a base class whose <code>__call__</code> method raises <code>NotImplementedError</code> so users must subclass to define <code>__call__</code>. My question is: how can I define the special method <code>__neg__</code> in the <em>base class</em> so subclasses immediately have the expected behavior without having the implement <code>__neg__</code> in each subclass? My sense of the expected behavior is that if <code>f</code> is an instance of (a subclass of) the base class with a properly defined <code>__call__</code>, then <code>-f</code> should be an instance of the same class as <code>f</code>, possessing all the same attributes as <code>f</code>, except for <code>__call__</code>, which should return the negative of <code>f</code>'s <code>__call__</code>.</p> <p>Here's an example of what I mean:</p> <pre><code>class Base(object): def __call__(self, *args, **kwargs): raise NotImplementedError, 'Please subclass' def __neg__(self): def call(*args, **kwargs): return -self(*args, **kwargs) mBase = type('mBase', (Base,), {'__call__': call}) return mBase() class One(Base): def __init__(self data): self.data = data def __call__(self, *args, **kwargs): return 1 </code></pre> <p>This has the expected behavior:</p> <pre><code>one = One() print one() # Prints 1 minus_one = -one print minus_one() # Prints -1 </code></pre> <p>though it's not exactly what I'd like since <code>minus_one</code> is not an instance of the same class as <code>one</code> (but I could live with that).</p> <p>Now I'd like the new instance <code>minus_one</code> to inherit all attributes and methods of <code>one</code>; only the <code>__call__</code> method should change. So I could change <code>__neg__</code> to</p> <pre><code> def __neg__(self): def call(*args, **kwargs): return -self(*args, **kwargs) mBase = type('mBase', (Base,), {'__call__': call}) new = mBase() for n, v in inspect.getmembers(self): if n != '__call__': setattr(new, n, v) return new </code></pre> <p>This seems to work. My question is: are there cons to this strategy? Implementing a generic <code>__neg__</code> must be a standard exercise but I couldn't find anything on it on the web. Are there recommended alternatives?</p> <p>Thanks in advance for any comments.</p>
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    1. COThis can be solved quite nicely with metaclasses. ;)
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