From Python Basics to Production Scripts

Prerequisites

Complete functions, exceptions, files, and modules in fundamentals, then the file-processing lab. This bridge runs locally without AWS credentials.

Set up a project environment

Create a project directory and run python -m venv .venv. Activate it with .venv\Scripts\Activate.ps1 in PowerShell or source .venv/bin/activate in bash. Alternatively, call .venv\Scripts\python.exe on Windows or .venv/bin/python on Unix directly. Confirm python -c "import sys; print(sys.executable)" points inside the environment.

For the later AWS labs, install boto3 with python -m pip install boto3. Record tested dependencies with python -m pip freeze > requirements.txt, and recreate them with python -m pip install -r requirements.txt. Keep .venv/ out of version control. See the Python virtual environment tutorial.

Sample input and task

Turn a list of server states into a command-line report. Save the solution as report.py. Accept a status filter, log a count to stderr, print matching names to stdout, and return zero on success.

Run python report.py --status critical. Expected stdout: web-02. Stderr includes INFO matched=1. An unsupported status must produce an argument error and a nonzero exit code.

Hint

Keep filtering separate from argument parsing and logging so tests can pass records directly.

Show solution: report.py

import argparse
import logging
import os


def select_names(records, status):
    return [row["name"] for row in records if row["status"] == status]


def main():
    parser = argparse.ArgumentParser(description="Filter server states")
    parser.add_argument("--status", choices=["healthy", "critical"],
                        default="critical")
    args = parser.parse_args()
    level = os.environ.get("REPORT_LOG_LEVEL", "INFO").upper()
    if level not in {"DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"}:
        parser.error("REPORT_LOG_LEVEL must be a logging level")
    logging.basicConfig(level=level, format="%(levelname)s %(message)s")
    records = [{"name": "web-01", "status": "healthy"},
               {"name": "web-02", "status": "critical"}]
    names = select_names(records, args.status)
    logging.info("matched=%d", len(names))
    for name in names:
        print(name)
    return 0


if __name__ == "__main__":
    raise SystemExit(main())

Test the behavior

Save this beside the script as test_report.py and run python -m unittest -v.

import unittest
from report import select_names


class ReportTests(unittest.TestCase):
    def test_filters_without_changing_input(self):
        rows = [{"name": "web-01", "status": "healthy"},
                {"name": "web-02", "status": "critical"}]
        self.assertEqual(select_names(rows, "critical"), ["web-02"])
        self.assertEqual(len(rows), 2)

    def test_empty_and_no_match(self):
        self.assertEqual(select_names([], "critical"), [])
        self.assertEqual(select_names(
            [{"name": "web-01", "status": "healthy"}], "critical"), [])

Before adding an API

Concern	Apply it to your script
Configuration	Use CLI flags for the current run and environment variables for deployment defaults; validate both.
Credentials	Use the SDK credential chain and a role or configured profile.
Logging	Send diagnostics to stderr and report data to stdout or a file; exclude secrets.
Timeouts	Bound connection and read waits; a read timeout is not a deadline for a whole paginated job.
Retries	Use the SDK’s bounded retry policy for supported transient failures.
Exit codes	Return zero only on success so CI and schedulers can detect failures.

For boto3, use Config(connect_timeout=5, read_timeout=20, retries={"mode": "standard", "total_max_attempts": 4}). The attempt limit includes the original request. See the SDK retry documentation.

Completion checklist and challenge

Run both status filters and the invalid-status case.
Pass the unit tests without calling a network service.
Explain the difference between logs and report output.
Add a CSV input argument and a test for a missing required column.

Continue to the EC2 report lab and then the inventory capstone.