Subtle unexpected behaviour with a function used as class-attribute

Question

I have a piece of code that was broken when I changed an attribute and made it class-attribute, I would love to understand why.

So, I started with

def linInterp(x1, x2, y1, y2, x):
    return y1 + (x - x1) * (y2 - y1) / (x2 - x1)

class Inter1DLineal(object):
    def __init__(self):
        self.interpSegment = linInterp

if __name__ == "__main__":
    inter = Inter1DLineal()
    print(inter.interpSegment(0.2, 0.6, 0, 1, 0.4))

which works alright. I know I could implement interpSegment as a proper method in Inter1DLineal, for instance as

@classmethod
def interpSegment(cls, *args, **kwargs):
    return linInterp(*args, **kwargs)

but I thought I was fine with an attribute of type function.

What surprised me was what happened next. I decided that, because interpSegment is common to all instances, I should make it a class attribute.

def linInterp(x1, x2, y1, y2, x):
    return y1 + (x - x1) * (y2 - y1) / (x2 - x1)

class Inter1DLineal(object):
    interpSegment = linInterp
    def __init__(self):
        pass

if __name__ == "__main__":
    inter = Inter1DLineal()
    print(inter.interpSegment(0.2, 0.6, 0, 1, 0.4))

But this broke the code. It seems the call to interpSegment is passing now passing 6 attributes, not 5. Can someone explain why this happens? What would the most Pythonistic way of doing this?

Why does it need to be on the class at all? It accesses/affects neither the class nor an instance. — Ignacio Vazquez-Abrams, Mar 20 '14 at 08:23
See [How does assignment of a function as a class attribute become a method in Python?](http://stackoverflow.com/q/2307653/222914) — Janne Karila, Mar 20 '14 at 08:25
@Ignacio of course you are right, this is just a toy example to show the behaviour I do not understand. — zeycus, Mar 20 '14 at 08:45
@JanneKarila thanks, I see that is exactly what underlies my question. — zeycus, Mar 20 '14 at 08:47

sshashank124 · Accepted Answer · 2014-03-20T08:48:45.347

1

well you are passing the inter as the first parameter which is the self parameter. So essentially, in the "__main__" code, you are passing something as follows:

interpSegment(inter, 0.2, 0.6, 0, 1, 0.4)

You should instead call it as:

Inter1DLineal.interpSegment(0.2, 0.6, 0, 1, 0.4)

However, let me also state that what you were doing first where interpSegment was in __init__ or as a method on its own is a better way of doing it.

edited Mar 20 '14 at 08:48

answered Mar 20 '14 at 08:37

sshashank124

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thanks for your answer. Of course you are right. But what is still a bit mystifying is why the same call works in one case, and not in the other. Hopefully I will understand better when I read the post suggested by Janne. Anyway, I will go for implementing interpSegment as a method. – zeycus Mar 20 '14 at 09:16
No problem. Glad I could be of help – sshashank124 Mar 20 '14 at 09:19

Subtle unexpected behaviour with a function used as class-attribute

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