REST API inference in PyTorch - ML Experiment: Train & Evaluate

Experiment - REST API inference

Problem:You have a trained PyTorch image classification model. You want to serve it so users can send images via a REST API and get predictions back.

Current Metrics:Model accuracy on test set: 88%. No API implemented yet.

Issue:No REST API exists to serve the model for real-time inference.

Your Task

Create a REST API using FastAPI that loads the trained PyTorch model and returns predictions for input images. The API should accept image files and respond with predicted class labels.

Use FastAPI for the REST API.

Load the PyTorch model once at startup.

Accept images as file uploads.

Return JSON with predicted class label.

Do not retrain or change the model.

Hint 1

Hint 2

Hint 3

Hint 4

Solution

PyTorch

import io
from fastapi import FastAPI, File, UploadFile
from fastapi.responses import JSONResponse
from PIL import Image
import torch
from torchvision import transforms

app = FastAPI()

# Load the trained model (assume model.pth exists and model architecture is known)
class SimpleCNN(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.conv = torch.nn.Conv2d(3, 16, 3, 1)
        self.fc = torch.nn.Linear(16*30*30, 10)  # example for 32x32 input
    def forward(self, x):
        x = torch.relu(self.conv(x))
        x = x.view(x.size(0), -1)
        x = self.fc(x)
        return x

model = SimpleCNN()
model.load_state_dict(torch.load('model.pth', map_location=torch.device('cpu')))
model.eval()

# Define image preprocessing
transform = transforms.Compose([
    transforms.Resize((32, 32)),
    transforms.ToTensor(),
    transforms.Normalize(mean=[0.485, 0.456, 0.406],
                         std=[0.229, 0.224, 0.225])
])

class_names = [f'class_{i}' for i in range(10)]

@app.post('/predict')
async def predict(file: UploadFile = File(...)):
    image_bytes = await file.read()
    image = Image.open(io.BytesIO(image_bytes)).convert('RGB')
    input_tensor = transform(image).unsqueeze(0)  # add batch dimension
    with torch.no_grad():
        outputs = model(input_tensor)
        _, predicted = torch.max(outputs, 1)
        predicted_class = class_names[predicted.item()]
    return JSONResponse(content={'predicted_class': predicted_class})

# To run: uvicorn filename:app --reload

Created a FastAPI app to serve the model.

Loaded the PyTorch model once at startup with map_location to ensure CPU compatibility.

Added an endpoint '/predict' to accept image uploads.

Preprocessed images with torchvision transforms.

Returned JSON response with predicted class label.

Results Interpretation

Before: No API, model only usable offline.

After: REST API accepts images and returns predictions in JSON, enabling real-time inference.

Serving a PyTorch model via a REST API allows easy integration of ML models into applications for real-time predictions.

Bonus Experiment

Add a batch prediction endpoint that accepts multiple images and returns predictions for all.

💡 Hint

Modify the API to accept a list of files, preprocess all, stack tensors, and return a list of predicted classes.

Practice

1. What is the main purpose of using a REST API for PyTorch model inference?

easy

A. To train the model faster using multiple GPUs

B. To send data to the model and receive predictions over the web

C. To visualize the model architecture in a browser

D. To save the model weights to disk

Solution

1. Step 1: Understand REST API role

   A REST API allows communication over the web by sending requests and receiving responses.

2. Step 2: Connect REST API with model inference

   In PyTorch, a REST API is used to send input data and get predictions from the model remotely.

3. Final Answer:

   To send data to the model and receive predictions over the web -> Option B

4. Quick Check:

   REST API = Web data exchange for predictions [OK]

Hint: REST API = send input, get prediction over web [OK]

Common Mistakes:

• Confusing inference with training
• Thinking REST API is for saving models
• Assuming REST API visualizes models

2. Which of the following is the correct way to disable gradient calculation during inference in a Flask REST API using PyTorch?

easy

A. with torch.no_grad():

B. model.train()

C. torch.enable_grad()

D. model.backward()

Solution

1. Step 1: Identify how to disable gradients

   PyTorch uses torch.no_grad() context to turn off gradient tracking during inference.

2. Step 2: Check other options

   model.train() enables training mode, torch.enable_grad() enables gradients, and model.backward() is invalid.

3. Final Answer:

   with torch.no_grad(): -> Option A

4. Quick Check:

   Disable gradients = torch.no_grad() [OK]

Hint: Use torch.no_grad() to save memory during inference [OK]

Common Mistakes:

• Using model.train() instead of model.eval()
• Not disabling gradients causing slow inference
• Calling backward() during inference

3. Given this Flask route for inference, what will be the output if the input JSON is {'input': [1.0, 2.0]} and the model returns tensor([3.0])?

from flask import Flask, request, jsonify
import torch
app = Flask(__name__)
model = torch.nn.Linear(2, 1)
model.eval()

@app.route('/predict', methods=['POST'])
def predict():
    data = request.get_json()
    x = torch.tensor(data['input']).unsqueeze(0).float()
    with torch.no_grad():
        output = model(x)
    return jsonify({'prediction': output.item()})

medium

A. {"prediction": "tensor([3.0])"}

B. {"prediction": [3.0]}

C. Error: model not loaded

D. {"prediction": 3.0}

Solution

1. Step 1: Understand input and output processing

   The input JSON is converted to a tensor with shape (1, 2). The model outputs a tensor with shape (1, 1).

2. Step 2: Check output conversion

   The output tensor's single value is extracted using output.item(), which returns a float, then wrapped in JSON.

3. Final Answer:

   {"prediction": 3.0} -> Option D

4. Quick Check:

   output.item() returns float for JSON [OK]

Hint: Use output.item() to get scalar from tensor for JSON [OK]

Common Mistakes:

• Returning tensor object directly in JSON
• Forgetting to convert input to tensor
• Not using torch.no_grad() causing slow inference

4. You wrote this Flask REST API code for inference but get an error: 'RuntimeError: expected scalar type Float but found Double'. What is the likely fix?

@app.route('/predict', methods=['POST'])
def predict():
    data = request.get_json()
    x = torch.tensor(data['input'])
    with torch.no_grad():
        output = model(x)
    return jsonify({'prediction': output.item()})

medium

A. Convert input tensor to float using .float() before passing to model

B. Call model.train() before inference

C. Remove torch.no_grad() context

D. Change model output to int

Solution

1. Step 1: Understand the error cause

   The model expects float tensors but input tensor is double (default from Python floats).

2. Step 2: Fix input tensor type

   Convert input tensor to float type using .float() before passing to the model.

3. Final Answer:

   Convert input tensor to float using .float() before passing to model -> Option A

4. Quick Check:

   Tensor dtype mismatch fixed by .float() [OK]

Hint: Always convert input to float tensor before model call [OK]

Common Mistakes:

• Not converting input data type
• Calling model.train() during inference
• Removing torch.no_grad() causing slow inference

5. You want to deploy a PyTorch model as a REST API that accepts images, preprocesses them, and returns class predictions. Which sequence of steps is best practice inside your Flask route?

hard

A. Load model globally, call model.eval(), preprocess input image to numpy array, call model backward(), return JSON

B. Load model inside route on every request, call model.train(), preprocess input, get prediction, return JSON

C. Load model once globally, set model.eval(), preprocess input image to tensor, use torch.no_grad(), get prediction, return JSON

D. Preprocess input image after prediction, load model globally, call torch.enable_grad(), return raw tensor

Solution

1. Step 1: Model loading and mode

   Load the model once globally to avoid overhead and set it to evaluation mode with model.eval().

2. Step 2: Input preprocessing and inference

   Preprocess the input image to a tensor, use torch.no_grad() to disable gradients, get prediction, and return JSON response.

3. Final Answer:

   Load model once globally, set model.eval(), preprocess input image to tensor, use torch.no_grad(), get prediction, return JSON -> Option C

4. Quick Check:

   Efficient inference = global load + eval + no_grad + preprocess [OK]

Hint: Load model once, eval mode, no_grad, preprocess input tensor [OK]

Common Mistakes:

• Loading model on every request causing slow response
• Using model.train() during inference
• Not preprocessing input to tensor
• Calling backward() during inference