Python: How to sum up all elements from a list in the CLASS

Question

class Neuron:
    def __init__(self, inbound_neurons=[], label=''):
        self.label = label
        self.inbound_neurons = inbound_neurons
        self.outbound_neurons = []
        self.value = None

        for n in self.inbound_neurons:
            n.outbound_neurons.append(self)

    def forward(self):
        raise NotImplemented

class Input(Neuron):
    def __init__(self):
        Neuron.__init__(self)

    def forward(self, value=None):
        if value is not None:
            self.value = value

class Add(Neuron):
    def __init__(self, *inputs):
        Neuron.__init__(self, inputs)

    def forward(self):
        for n in self.inputs:
            self.value = self.value + n

Add() is the subclass of class Neuron, I have met some difficulties to use loop to add all the elements of the inputs array.

If the problems you see are that you keep having more outbound neurons than expected, read the first section of [this](http://amir.rachum.com/blog/2013/07/06/python-common-newbie-mistakes-part-1/) blog post. — Shachar Ohana, Jan 14 '17 at 07:52

score 2 · Accepted Answer · answered Jan 16 '17 at 03:45

class Add(Neuron):

    def __init__(self, *inputs):
        Neuron.__init__(self, inputs)

    def forward(self):
        self.value = 0
        for n in self.inbound_neurons:
            self.value = self.value + n.value

        return(self.value)

The function 'forward' in Class Add has a loop to sum all elements of inbound_neurons.

score 1 · Answer 2 · answered Jan 14 '17 at 07:57

Firt off this line of code should be:

for n in self.inbound_neurons:
        self.outbound_neurons.append(self)

self.inputs was never defined in your Class. In order to loop through the inputs, you'd have to have:

    def __init__(self, *inputs):
        self.inputs = inputs
        Neuron.__init__(self, inputs)

However, it looks like inputs would be a list with two items, another list and maybe a string in it. These will not concate together. It looks like instead you want to sum the total of self.inbound_neurons.

Thanks, this is a good answer. Because of I am not familiar with using the class and subclass in Python, I want to sum all elements in the "inputs" in the subclass Add. — Jeremy_Tamu, Jan 14 '17 at 19:06

score 1 · Answer 3 · edited May 23 '17 at 10:29

It's not related to your question, but VERY important: you should NOT use mutable data types (as list) for function/method defaults.

Your code should be updated like this:

class Neuron:
    def __init__(self, inbound_neurons=None, label=''):
        self.inbound_neurons = inbound_neurons or []
        # ...

Why you should do this way is explained here: "Least Astonishment" and the Mutable Default Argument

Python: How to sum up all elements from a list in the CLASS

3 Answers3