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    <p>The basic problem you're running into is that <code>__xxx__</code> methods are only looked up on the class, which means all instances of the same class will use the same <code>__xxx__</code> methods. This suggests using a method similar to what Cat Plus Plus suggested; however, you also don't want your users to have to worry about even more special names (such as <code>_call_impl</code> and <code>_negate</code>).</p> <p>If you don't mind the possibly mind-melting power of metaclasses, that is the route to take. A metaclass can add in the <code>_negate</code> attribute automatically (and name mangle it to avoid clashes), as well as take the <code>__call__</code> that your user wrote and rename it to <code>_call</code>, then create a new <code>__call__</code> that calls the old <code>__call__</code> (now called <code>_call</code> ;) and then negates the result, if necessary, before returning it.</p> <p>Here's the code:</p> <pre><code>import copy import inspect class MetaFunction(type): def __new__(metacls, cls_name, cls_bases, cls_dict): result_class = type.__new__(metacls, cls_name, cls_bases, cls_dict) if '__call__' in cls_dict: original_call = cls_dict['__call__'] args, varargs, kwargs, defaults = inspect.getargspec(original_call) args = args[1:] if defaults is None: defaults = [''] * len(args) else: defaults = [''] * (len(args) - len(defaults)) + list(defaults) signature = [] for arg, default in zip(args, defaults): if default: signature.append('%s=%s' % (arg, default)) else: signature.append(arg) if varargs is not None: signature.append(varargs) if kwargs is not None: signature.append(kwargs) signature = ', '.join(signature) passed_args = ', '.join(args) new_call = ( """def __call__(self, %(signature)s): result = self._call(%(passed_args)s) if self._%(cls_name)s__negate: result = -result return result""" % { 'cls_name':cls_name, 'signature':signature, 'passed_args':passed_args, }) eval_dict = {} exec new_call in eval_dict new_call = eval_dict['__call__'] new_call.__doc__ = original_call.__doc__ new_call.__module__ = original_call.__module__ new_call.__dict__ = original_call.__dict__ setattr(result_class, '__call__', new_call) setattr(result_class, '_call', original_call) setattr(result_class, '_%s__negate' % cls_name, False) negate = """def __neg__(self): "returns an instance of the same class that returns the negation of __call__" negated = copy.copy(self) negated._%(cls_name)s__negate = not self._%(cls_name)s__negate return negated""" % {'cls_name':cls_name} eval_dict = {'copy':copy} exec negate in eval_dict negate = eval_dict['__neg__'] negate.__module__ = new_call.__module__ setattr(result_class, '__neg__', eval_dict['__neg__']) return result_class class Base(object): __metaclass__ = MetaFunction class Power(Base): def __init__(self, power): "power = the power to raise to" self.power = power def __call__(self, number): "raises number to power" return number ** self.power </code></pre> <p>and an example:</p> <pre><code>--&gt; square = Power(2) --&gt; neg_square = -square --&gt; square(9) 81 --&gt; neg_square(9) -81 </code></pre> <p>While the metaclass code itself can be complex, the resulting objects can be very easy to use. To be fair, most of the code, and the complexity, in <code>MetaFunction</code> is due to re-writing <code>__call__</code> in order to preserve the call signature and make introspection useful... so instead of seeing <code>__call__(*args, *kwargs)</code> in help, you see this:</p> <pre><code>Help on Power in module test object: class Power(Base) | Method resolution order: | Power | Base | __builtin__.object | | Methods defined here: | | __call__(self, number) | raises number to power | | __init__(self, power) | power = the power to raise to | | __neg__(self) | returns an instance of the same class that returns the negation of __call__ </code></pre>
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    1. POImplementing __neg__ in a generic way for all subclasses in Python
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    1. USEthan Furman
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    1. COWhile brain melting, this is the kind of abstraction I was ultimately looking for. Thanks for sharing this! I have to spend some time and study your code.
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