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POProgrammatically create arithmetic special methods in Python, (aka factory function HOWTO)
 

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  1. POProgrammatically create arithmetic special methods in Python, (aka factory function HOWTO)
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    <p>My idea is to create particular function objects that can be summed/subtracted/... together, returning a new function object that has the same properties. This example code, hopefully, demonstrates the idea:</p> <pre><code>from FuncObj import Func # create some functions quad = Func(lambda x: x**2) cube = Func(lambda x: x**3) # now combine functions as you like plus = quad + cube minus = quad - cube other = quad * quad / cube # and these can be called plus(1) + minus(32) * other(5) </code></pre> <p>I have written the following code, which is hopefully commented and documented enough to explain what i want to achieve.</p> <pre><code>import operator class GenericFunction(object): """ Base class providing arithmetic special methods. Use derived class which must implement the __call__ method. """ # this way of defining special methods works well def __add__(self, operand): """ This is an example of a special method i want to implement. """ obj = GenericFunction() # this is a trick from Alex Martelli at # http://stackoverflow.com/questions/1705928/problem-with-making-object-callable-in-python # to allow per-instance __call__ methods obj.__class__ = type(obj.__class__.__name__, (obj.__class__,), {}) obj.__class__.__call__ = lambda s, ti: self(ti) + operand(ti) return obj # on the other hand this factory function seems buggy def _method_factory(operation, name): """ Method factory. Parameters ---------- op : callable an arithmetic operator from the operator module name : str the name of the special method that will be created Returns ------- method : callable the __***__ special method """ def method(s, operand): obj = GenericFunction() obj.__class__ = type(obj.__class__.__name__, (obj.__class__,), {}) obj.__class__.__call__ = lambda s, ti: operation(s(ti), operand(ti)) return obj return method __sub__ = _method_factory(operator.__sub__, '__sub__') __mul__ = _method_factory(operator.__mul__, '__mul__') __truediv__ = _method_factory(operator.__truediv__, '__div__') class Func(GenericFunction): """ A customizable callable object. Parameters ---------- func : callable """ def __init__(self, func): self.func = func def __call__(self, *args): return self.func(*args) if __name__ == '__main__': # create some functions quad = Func(lambda x: x**2) cube = Func(lambda x: x**3) # now combine functions poly_plus = quad + cube poly_minus = quad - cube # this is the expected behaviour, and it works well # since the __add__ method is defined correctly. assert quad(1) + cube(1) == poly_plus(1) # this, and the others with * and / result in a "maximum recursion depth exceeded" assert quad(1) - cube(1) == poly_minus(1) </code></pre> <p>I think I'm missing something important but I can't see it.</p> <p><strong>EDIT</strong> </p> <p>After Dietrich answer i forgot to mention a corner case. Suppose i want to subclass GenericInput and i need to customize the <strong>call</strong> method__, without passing a callable to the constructor. I have to examples, (actually this is the code for which i originally posted this question).</p> <pre><code>class NoiseInput(GenericInput): def __init__(self, sigma, a, b, t): """ A band-pass noisy input. """ self._noise = lfilter(b, a, np.random.normal(0, 1, len(t))) self._noise *= sigma/self._noise.std() self._spline = InterpolatedUnivariateSpline(t, self._noise, k=2) def __call__(self, ti): """ Compute value of the input at a given time. """ return self._spline(ti) class SineInput(GenericInput): def __init__(self, A, fc): self.A = A self.fc = fc def __call__(self, ti): return self.A*np.sin(2*np.pi*ti*self.fc) </code></pre> <p>In this case there is some more work to do.</p>
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    1. COin your example, does `plus(1)` mean `quad(1) + cube(1)`?
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    2. CO@inspectorG4dget Yes, that is the intended behaviour.
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