# How does the forward method get called in this pyTorch conv net?

In this example network from pyTorch tutorial

import torch
import torch.nn as nn
import torch.nn.functional as F

class Net(nn.Module):

def __init__(self):
super(Net, self).__init__()
# 1 input image channel, 6 output channels, 3x3 square convolution
# kernel
self.conv1 = nn.Conv2d(1, 6, 3)
self.conv2 = nn.Conv2d(6, 16, 3)
# an affine operation: y = Wx + b
self.fc1 = nn.Linear(16 * 6 * 6, 120)  # 6*6 from image dimension
self.fc2 = nn.Linear(120, 84)
self.fc3 = nn.Linear(84, 10)

def forward(self, x):
# Max pooling over a (2, 2) window
x = F.max_pool2d(F.relu(self.conv1(x)), (2, 2))
# If the size is a square you can only specify a single number
x = F.max_pool2d(F.relu(self.conv2(x)), 2)
x = x.view(-1, self.num_flat_features(x))
x = F.relu(self.fc1(x))
x = F.relu(self.fc2(x))
x = self.fc3(x)
return x

def num_flat_features(self, x):
size = x.size()[1:]  # all dimensions except the batch dimension
num_features = 1
for s in size:
num_features *= s
return num_features

net = Net()
print(net)

net = Net()
input = torch.randn(1, 1, 32, 32)
out = net(input)
print(out)


Why is the method forward() not explicitely called? I mean how does just calling net(output) calls forward() ? (which is what happens as far as I understand) By the way I dont understand what this line means:

super(Net, self).__init__()


I can imagine super() is calling the constructor of a parent class but …?

If you look at the Module implementation of pyTorch, you'll see that forward is a method called in the special method __call__ :

class Module(object):
...
def __call__(self, *input, **kwargs):
...
result = self.forward(*input, **kwargs)


As you construct a Net class by inheriting from the Module class and you override the default behavior of the __init__ constructor, you also need to explicitly call the parent's one with super(Net, self).__init__().

• Thanks, great answer. This is something they could have put in the comments. – Machupicchu Aug 30 '19 at 12:02
• But so the __call__ merhod is called by the constructor of the Module class if i understood right? – Machupicchu Aug 30 '19 at 12:05
• Nope, it is called when you call an instance of the class, so in the example : out = net(input). See : stackoverflow.com/questions/9663562/… – Elliot Aug 30 '19 at 12:08
• I dont see any explicit call to _\call_ if i can say so lol – Machupicchu Aug 30 '19 at 12:09
• great thanks for your fantastic answers! – Machupicchu Aug 30 '19 at 13:21