当你在 python 中打印某个类的对象时会打印什么？

Question

I want to ask about this specific example, taken from the official 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)

net = Net()
print(net)

And the output is

Net(
  (conv1): Conv2d(1, 6, kernel_size=(3, 3), stride=(1, 1))
  (conv2): Conv2d(6, 16, kernel_size=(3, 3), stride=(1, 1))
  (fc1): Linear(in_features=576, out_features=120, bias=True)
  (fc2): Linear(in_features=120, out_features=84, bias=True)
  (fc3): Linear(in_features=84, out_features=10, bias=True)
)

As I understand, this code defines a child class Net of nn.Module , and defines in its initializer the members conv1 , conv2 , etc. These members are printed when print(net) is called. Based on this obsevation, I thought that if I add the line self.x = 0 to the initializer of Net , there would be an extra line of output, something like: (x): 0 . But that didn't happen. So who decides which part of Net gets printed?

Answer 1

From the Python3 documentation

repr(object)
Return a string containing a printable representation of an object. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a __repr__() method.

Since your class inherits the nn.Module class, it uses its repr method