Python constructing a class that uses a function from within the same class

Question

I'm trying to get a construct a class that calls a function from within the same class but I'm having trouble doing so.

I've had a look at:

Python function pointers within the same Class

But I've had no luck with the approach presented. I've created this simple example:

#### Libraries ####
# Third Party Libraries  
import numpy as np

#### Defines the activation functions ####
class sigmoid_class(object):
    def __init__(self, z):
        self.z = z

    def activation_fn(self):
        """The sigmoid function"""
        return 1.0/(1.0+np.exp(-self.z))

    def prime(self):
        """The derivative of the sigmoid function"""
        return activation_fn(self.z)*(1-activation_fn(self.z))

a = sigmoid_class(0)
print(a.prime())

when I test with print(a.activation_fn(0)) I get the desired output, but when I try `print(a.prime())

I get `NameError: name 'activation_fn' is not defined

I'm not sure how to fix this so that it functions properly

use self.activation_fn() activation_fn is a member function of the class, so you need to give it the class instance (which is self) in order to call it. — Keozon
– Keozon, Commented Jul 18, 2016 at 22:13
I'm wondering whether the argument shouldn't be self -- not self.z as activation_fn signature is just self — joel goldstick
– joel goldstick, Commented Jul 18, 2016 at 22:14
@Keozon: Actually that should be self.activation_fn(). activation_fn doesn't take the z value as argument - it looks it up from self — Martin Bonner supports Monica
– Martin Bonner supports Monica, Commented Jul 18, 2016 at 22:15
@MartinBonner RIght, that's what I get for not looking close enough. Edited. — Keozon
– Keozon, Commented Jul 18, 2016 at 22:16

Martin Bonner supports Monica · Accepted Answer · 2016-07-18 22:17:05Z

4

It should actually be:

def prime(self):
    """The derivative of the sigmoid function"""
    return self.activation_fn()*(1-self.activation_fn())

Note that activation_fn does not need to be passed the z value as well - it looks that up from self.

answered Jul 18, 2016 at 22:17

Martin Bonner supports Monica

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Comments

joel goldstick · Accepted Answer · 2016-07-18 22:17:35Z

3

So combining both points:

def prime(self):
    """The derivative of the sigmoid function"""
    return self.activation_fn()*(1-self.activation_fn())

activation_fn finds self.z for itself

answered Jul 18, 2016 at 22:17

joel goldstick

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djhaskin987 · Accepted Answer · 2016-07-18 22:14:07Z

2

def prime(self):
    """The derivative of the sigmoid function"""
    return self.activation_fn(self.z)*(1-self.activation_fn(self.z))

Or so. note the self.activation_fn instead of just activation_fn.

answered Jul 18, 2016 at 22:14

djhaskin987

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jhwang · Accepted Answer · 2016-07-18 22:16:05Z

1

The comments are correct, you need reference self. because activation_fn is not defined in the scope of the method itself, and your method also doesn't take in any arguments.

answered Jul 18, 2016 at 22:16

jhwang

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