How to mock EC2 describe-instances tag filtering in Python

Simulate AWS EC2 describe-instances with tag-based filtering using a mock dataset and conditional list comprehension.

Easy Python 3.9+ Aug 9, 2026 Cloud + Python 15 views 0 copies

ec2 mock aws tags boto3

Python code

31 lines
Python 3.9+
Run it 
import json
from datetime import datetime, timezone


def mock_describe_instances(tag_key: str, tag_value: str) -> list[dict]:
    """Simulate EC2 describe-instances with tag filtering."""
    all_instances = [
        {"InstanceId": "i-0abc123", "State": "running", "Tags": [{"Key": "Name", "Value": "web-server"}, {"Key": "Env", "Value": "prod"}]},
        {"InstanceId": "i-0def456", "State": "stopped", "Tags": [{"Key": "Name", "Value": "db-server"}, {"Key": "Env", "Value": "dev"}]},
        {"InstanceId": "i-0ghi789", "State": "running", "Tags": [{"Key": "Name", "Value": "cache-server"}, {"Key": "Env", "Value": "prod"}]},
    ]

    filtered = [
        inst for inst in all_instances
        if any(tag["Key"] == tag_key and tag["Value"] == tag_value for tag in inst["Tags"])
    ]

    return [
        {
            "InstanceId": inst["InstanceId"],
            "State": inst["State"],
            "TagValue": next(tag["Value"] for tag in inst["Tags"] if tag["Key"] == tag_key),
            "FilteredAt": datetime.now(timezone.utc).isoformat(),
        }
        for inst in filtered
    ]


if __name__ == "__main__":
    result = mock_describe_instances("Env", "prod")
    print(json.dumps(result, indent=2))

Output

stdout 
[
  {
    "InstanceId": "i-0abc123",
    "State": "running",
    "TagValue": "prod",
    "FilteredAt": "2025-01-01T12:00:00.000000+00:00"
  },
  {
    "InstanceId": "i-0ghi789",
    "State": "running",
    "TagValue": "prod",
    "FilteredAt": "2025-01-01T12:00:00.000000+00:00"
  }
]

How it works

The function accepts a tag key and value, then loops through a mock list of EC2 instances. The any() generator checks if any tag matches both key and value, filtering only matching instances. A second list comprehension extracts only the fields you need, looking up the tag value with next() on the filtered tags. The datetime.now(timezone.utc) call adds a UTC timestamp, keeping the mock realistic. Returning a list of dicts keeps it easy to extend into a real boto3 response.

Common mistakes

  • Assuming every instance has the tag key, causing `next()` to raise StopIteration.
  • Comparing tag Key/Value with case sensitivity when AWS is case-insensitive.
  • Forgetting that `datetime.now(timezone.utc)` is naive if you omit timezone, creating ambiguous timestamps.

Variations

  1. Use a dict comprehension to build a lookup of tag key/value pairs for faster checks.
  2. Implement the same mock with a filter lambda, like `filter(lambda i: any(...), all_instances)`.

Real-world use cases

  • Unit-testing functions that rely on EC2 tag metadata without making live AWS calls.
  • Developing CI/CD scripts that need to target instances by environment tags locally.
  • Building reusable cloud resource inventory tools before deploying to real AWS APIs.

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