Feature extraction strategy in PyTorch

Introduction

Feature extraction helps us use important parts of data to teach a model faster and better. It saves time and improves results by focusing on useful information.

When you want to use a pre-trained model to get useful data features without training from scratch.

When you have limited data but want to use knowledge from a bigger dataset.

When you want to speed up training by freezing some parts of the model.

When you want to improve model accuracy by using strong features from another model.

When you want to reduce the size of input data by extracting key features.

Syntax

PyTorch

import torch
import torchvision.models as models

# Load a pre-trained model
model = models.resnet18(pretrained=True)

# Freeze all layers to prevent training
for param in model.parameters():
    param.requires_grad = False

# Replace the final layer to match your task
model.fc = torch.nn.Linear(model.fc.in_features, num_classes)

# Now only the final layer will be trained

Freezing layers means their weights won't change during training.

Replacing the final layer adapts the model to your specific problem.

Examples

Freeze all layers and change the last layer for 10 classes.

PyTorch

model = models.resnet18(pretrained=True)
for param in model.parameters():
    param.requires_grad = False
model.fc = torch.nn.Linear(model.fc.in_features, 10)

Freeze only feature layers in VGG16 and change classifier for 5 classes.

PyTorch

model = models.vgg16(pretrained=True)
for param in model.features.parameters():
    param.requires_grad = False
model.classifier[6] = torch.nn.Linear(4096, 5)

Sample Model

This code loads a pre-trained ResNet18, freezes all layers, replaces the last layer for 3 classes, and runs dummy data through it. It prints the output shape and how many parameters will be trained (should be 1 layer).

PyTorch

import torch
import torchvision.models as models
import torch.nn as nn

# Number of classes for new task
num_classes = 3

# Load pre-trained ResNet18
model = models.resnet18(pretrained=True)

# Freeze all layers
for param in model.parameters():
    param.requires_grad = False

# Replace final fully connected layer
model.fc = nn.Linear(model.fc.in_features, num_classes)

# Create dummy input (batch size 2, 3 color channels, 224x224 image)
dummy_input = torch.randn(2, 3, 224, 224)

# Get output predictions
output = model(dummy_input)

# Print output shape and requires_grad status of parameters
print(f"Output shape: {output.shape}")
trainable_params = [p for p in model.parameters() if p.requires_grad]
print(f"Number of trainable parameters: {len(trainable_params)}")

Important Notes

Freezing layers helps keep learned features and reduces training time.

Only the replaced final layer's parameters require gradients and will update during training.

Use dummy inputs with correct shape to test model output before training.

Summary

Feature extraction uses pre-trained models to get useful data features.

Freeze layers to keep their knowledge and train only new parts.

Replace the final layer to fit your specific task.

Practice

1. What is the main purpose of using a pre-trained model for feature extraction in PyTorch?

easy

A. To replace the optimizer with a new one

B. To use learned features from a large dataset and avoid training from scratch

C. To train all layers from random weights

D. To increase the size of the dataset automatically

Solution

1. Step 1: Understand feature extraction concept

   Feature extraction uses a model already trained on a large dataset to get useful features without training all layers again.

2. Step 2: Identify the main benefit

   This saves time and resources by reusing learned knowledge instead of starting from scratch.

3. Final Answer:

   To use learned features from a large dataset and avoid training from scratch -> Option B

4. Quick Check:

   Feature extraction = reuse learned features [OK]

Hint: Pre-trained means reuse, not retrain all layers [OK]

Common Mistakes:

• Thinking feature extraction means training all layers
• Confusing feature extraction with data augmentation
• Believing optimizer changes are part of feature extraction

2. Which PyTorch code snippet correctly freezes all layers of a pre-trained model except the final layer?

easy

A. for param in model.parameters(): param.requires_grad = True
model.fc = nn.Linear(512, 10)

B. model.fc.requires_grad = False
for param in model.parameters(): param.requires_grad = True

C. for param in model.parameters(): param.requires_grad = False
model.fc = nn.Linear(512, 10)

D. model.fc = nn.Linear(512, 10)
for param in model.parameters(): param.requires_grad = False

Solution

1. Step 1: Freeze all layers by setting requires_grad to false

   The loop disables gradient updates for all parameters to keep pre-trained weights fixed.

2. Step 2: Replace the final layer with a new one to train

   Assigning a new linear layer to model.fc allows training only this layer for the new task.

3. Final Answer:

   for param in model.parameters(): param.requires_grad = False model.fc = nn.Linear(512, 10) -> Option C

4. Quick Check:

   Freeze all except final layer = for param in model.parameters(): param.requires_grad = False model.fc = nn.Linear(512, 10) [OK]

Hint: Freeze first, then replace final layer [OK]

Common Mistakes:

• Not freezing layers before replacing final layer
• Freezing final layer instead of others
• Setting requires_grad true for all parameters

3. Given this PyTorch code for feature extraction, what will be the output shape of features?

import torch
import torchvision.models as models
model = models.resnet18(pretrained=True)
model.fc = torch.nn.Identity()
input_tensor = torch.randn(4, 3, 224, 224)
features = model(input_tensor)
print(features.shape)

medium

A. torch.Size([4, 512])

B. torch.Size([4, 1000])

C. torch.Size([4, 3, 224, 224])

D. torch.Size([4, 2048])

Solution

1. Step 1: Understand model modification

   Replacing model.fc with Identity removes the final classification layer, so output is the feature vector before classification.

2. Step 2: Know ResNet18 feature size

   ResNet18 outputs a 512-dimensional vector before the final fc layer for each input image.

3. Final Answer:

   torch.Size([4, 512]) -> Option A

4. Quick Check:

   ResNet18 features = 512 dims [OK]

Hint: Identity layer outputs feature vector size [OK]

Common Mistakes:

• Assuming output is 1000 classes without removing fc
• Confusing batch size with feature dimension
• Expecting 2048 features from ResNet18 (it's 512)

4. Identify the error in this feature extraction code snippet and select the fix:

model = models.resnet50(pretrained=True)
for param in model.parameters():
    param.requires_grad = False
model.fc = nn.Linear(2048, 5)
optimizer = torch.optim.SGD(model.parameters(), lr=0.01)

# Training loop here

medium

A. No error; code is correct

B. Set requires_grad=True for model.fc parameters after replacement

C. Use Adam optimizer instead of SGD

D. Remove freezing of parameters to train all layers

Solution

1. Step 1: Check freezing timing

   The loop freezes existing parameters before replacing model.fc, so the new fc layer's parameters are created with requires_grad=True by default.

2. Step 2: Verify optimizer behavior

   Optimizer only updates parameters where requires_grad=True, which are the new fc parameters; backbone remains frozen.

3. Final Answer:

   No error; code is correct -> Option A

4. Quick Check:

   New layer params unfrozen by default [OK]

Hint: New layers have requires_grad=True by default [OK]

Common Mistakes:

• Assuming freezing all parameters includes new layers
• Changing optimizer without fixing requires_grad
• Removing freezing unnecessarily

5. You want to use a pre-trained ResNet34 to classify 3 classes in your dataset. You freeze all layers except the last one. However, your training accuracy stays very low. What is the best next step to improve feature extraction performance?

hard

A. Reduce batch size to 1 to improve gradient estimates

B. Increase learning rate to 1.0 for faster training

C. Replace the optimizer with SGD without momentum

D. Unfreeze some deeper layers to fine-tune features for your task

Solution

1. Step 1: Understand freezing impact

   Freezing all but last layer may limit model's ability to adapt features to new classes, causing low accuracy.

2. Step 2: Fine-tune some deeper layers

   Unfreezing some layers closer to output allows the model to adjust features better for your specific dataset.

3. Final Answer:

   Unfreeze some deeper layers to fine-tune features for your task -> Option D

4. Quick Check:

   Fine-tune layers = better adaptation [OK]

Hint: Fine-tune layers if frozen model underperforms [OK]

Common Mistakes:

• Increasing learning rate too much causes instability
• Changing optimizer without addressing feature adaptation
• Reducing batch size unnecessarily