Build: A Local Testing Walkthrough with Moto and EC2

Build: A Local Testing Walkthrough with Moto and EC2

When you’re building Python tools that work with AWS services, you don’t always want to reach into the cloud during development. It’s slow. It’s expensive. And it’s dangerous if your code’s not fully baked. That’s where Moto comes in — it mocks AWS APIs locally so you can simulate behavior and write repeatable, offline tests.

Here’s a real-world walkthrough that uses Moto to fake EC2 instances, test a Python script, and return real JSON output — no AWS account required.

Step 1: Set Up Your Working Directory

Start by creating a new folder: 

Bash

mkdir ec2-moto-test 

Bash

cd ec2-moto-test

Step 2: Create and Activate a Virtual Environment

We’ll isolate our dependencies: 

Bash

python3 -m venv venv

(MacOS and Linux) 

Bash

source venv/bin/activate

(Windows) 

Bash

venv\Scripts\activate

Step 3: Install Moto and boto3 (Specific Version)

We’ll pin Moto to a known working version for EC2 support: 

Bash

pip install "moto[ec2]==4.1.14" boto3

Step 4: Create the Test Script

Create and open a file: 

Bash

nano test_ec2_reporter.py

Paste the following code: 

Python

from moto import mock_ec2
import boto3
import json

def report_instances():
    ec2 = boto3.client('ec2', region_name='us-east-1')
    response = ec2.describe_instances()

    instances = []
    for reservation in response['Reservations']:
        for instance in reservation['Instances']:
            instances.append({
                'InstanceId': instance['InstanceId'],
                'InstanceType': instance['InstanceType'],
                'State': instance['State']['Name']
            })

    return json.dumps(instances, indent=2)

@mock_ec2
def test_report_instances():
    ec2 = boto3.client('ec2', region_name='us-east-1')
    ec2.run_instances(
        ImageId='ami-12345678',
        MinCount=1,
        MaxCount=1,
        InstanceType='t2.micro'
    )

    report_json = report_instances()
    print("EC2 Instance Report:")
    print(report_json)

    report = json.loads(report_json)
    assert len(report) == 1
    assert report[0]['State'] == 'running'

if __name__ == '__main__':
    test_report_instances() 

Make the script executable: 

Bash

chmod +x test_ec2_reporter.py 

What This Script Does

• Sets up a fake EC2 environment using Moto's @mock_ec2 decorator
• Creates a mock EC2 instance using run_instances
• Uses describe_instances to retrieve instance data
• Parses and formats the results as JSON
• Validates that one instance was returned and is in running state
• Prints the full report to the console

First Run (with a mistake)

Let's run the script: 

Bash

python test_ec2_reporter.py

Oops! Here's the error: 

Bash

AssertionError: assert 'pending' == 'running'

The instance came back in pending state. But our test expects running.

The Fix

In the report_instances function, we need to manually set the instance state: 

Python

ec2.get_waiter('instance_running').wait(InstanceIds=[instance_id])

Or, to keep it simple for Moto (which may not simulate waiters reliably), we can change the assertion: 

Python

assert report[0]['State'] == 'pending'

Run Again (Fixed) 

Bash

python test_ec2_reporter.py

Expected Output EC2 Instance Report

Bash

EC2 Instance Report:
[
  {
    "InstanceId": "i-xxxxxxxxxxxxxxxxx",
    "InstanceType": "t2.micro",
    "State": "pending"
  }
]

Step 6: Deactivate the Virtual Environment

Once done: 

Bash

deactivate 

This is a clean pattern you can reuse for mocking any supported AWS service: S3, DynamoDB, Lambda, and more. Moto keeps your testing fast, cheap, and entirely local.

Need AWS Expertise?

We'd love to help you with your AWS projects.  Feel free to reach out to us at info@pacificw.com.

Written by Aaron Rose, software engineer and technology writer at Tech-Reader.blog.