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What is a state_dict in PyTorch

In PyTorch, the learnable parameters (i.e. weights and biases) of a torch.nn.Module model are contained in the model’s parameters (accessed with model.parameters()). A state_dict is simply a Python dictionary object that maps each layer to its parameter tensor.

Introduction

A state_dict is an integral entity if you are interested in saving or loading models from PyTorch. Because state_dict objects are Python dictionaries, they can be easily saved, updated, altered, and restored, adding a great deal of modularity to PyTorch models and optimizers. Note that only layers with learnable parameters (convolutional layers, linear layers, etc.) and registered buffers (batchnorm’s running_mean) have entries in the model’s state_dict. Optimizer objects (torch.optim) also have a state_dict, which contains information about the optimizer’s state, as well as the hyperparameters used. In this recipe, we will see how state_dict is used with a simple model.

Setup

Before we begin, we need to install torch if it isn’t already available.

pip install torch

Steps

  1. Import all necessary libraries for loading our data

  2. Define and intialize the neural network

  3. Initialize the optimizer

  4. Access the model and optimizer state_dict

1. Import necessary libraries for loading our data

For this recipe, we will use torch and its subsidiaries torch.nn and torch.optim.

import torch
import torch.nn as nn
import torch.optim as optim

2. Define and intialize the neural network

For sake of example, we will create a neural network for training images. To learn more see the Defining a Neural Network recipe.

class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.conv1 = nn.Conv2d(3, 6, 5)
        self.pool = nn.MaxPool2d(2, 2)
        self.conv2 = nn.Conv2d(6, 16, 5)
        self.fc1 = nn.Linear(16 * 5 * 5, 120)
        self.fc2 = nn.Linear(120, 84)
        self.fc3 = nn.Linear(84, 10)

    def forward(self, x):
        x = self.pool(F.relu(self.conv1(x)))
        x = self.pool(F.relu(self.conv2(x)))
        x = x.view(-1, 16 * 5 * 5)
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x

net = Net()
print(net)

3. Initialize the optimizer

We will use SGD with momentum.

optimizer = optim.SGD(net.parameters(), lr=0.001, momentum=0.9)

4. Access the model and optimizer state_dict

Now that we have constructed our model and optimizer, we can understand what is preserved in their respective state_dict properties.

# Print model's state_dict
print("Model's state_dict:")
for param_tensor in net.state_dict():
    print(param_tensor, "\t", net.state_dict()[param_tensor].size())

print()

# Print optimizer's state_dict
print("Optimizer's state_dict:")
for var_name in optimizer.state_dict():
    print(var_name, "\t", optimizer.state_dict()[var_name])

This information is relevant for saving and loading the model and optimizers for future use.

Congratulations! You have successfully used state_dict in PyTorch.

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