Control Flow — Conditionals and Loops

Overview

Branch and iterate confidently with if/match, for/while, and loop control so inventory, health-check, and remediation scripts stay readable and correct.

Automation is mostly “for each host/resource, if unhealthy then alert.” This module makes that structure explicit before functions and data structures.

Complete Python Basics first. Diagrams use Excalidraw only.

This is a core tutorial in Module 3 · Control Flow of the REBASH Academy Python for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

Required

• Python Basics — Types and I/O
• Project venv from Module 1–2

Learning Objectives

By the end of this tutorial, you will be able to:

• Write if / elif / else branches for ops decisions
• Use match for multi-way status or action dispatch (3.10+)
• Iterate with for over sequences and range
• Use while for retry-style loops carefully
• Apply break, continue, and pass intentionally

Architecture

This topic’s control points and relationships are shown below.

Theory

if / elif / else

status = "degraded"
if status == "healthy":
    action = "noop"
elif status == "degraded":
    action = "page_secondary"
else:
    action = "page_primary"

Keep conditions boolean-clear. Prefer early returns in functions (Module 4) over deep nesting.

match (structural pattern matching)

Useful for status codes and command verbs:

code = 503
match code:
    case 200 | 204:
        level = "ok"
    case 404:
        level = "missing"
    case 500 | 502 | 503 | 504:
        level = "upstream"
    case _:
        level = "other"

Requires Python 3.10+. This course assumes 3.12+.

for loops

hosts = ["web-a", "web-b", "web-c"]
for host in hosts:
    print(f"check {host}")

for i in range(3):
    print(f"attempt {i + 1}")

Iterate dict keys/items when you reach Module 5:

# for name, ip in inventory.items():
#     ...

while loops

Use for retries with a hard cap:

attempts = 0
while attempts < 5:
    attempts += 1
    # try connect...
    break  # on success
else:
    # while-else: ran out without break
    raise SystemExit("error: retries exhausted")

Unbounded while True without a break condition hangs CI and cron jobs.

break, continue, pass

Keyword	Meaning
break	Leave the loop
continue	Skip to next iteration
pass	Empty body placeholder

for host in hosts:
    if host.startswith("lab-"):
        continue
    if host == "bastion":
        break

Hands-on Lab

Focus: practise the core workflow for Control Flow — Conditionals and Loops

mkdir -p ~/rebash-python/module-03
cd ~/rebash-python/module-03

python3 -m venv .venv
source .venv/bin/activate

Step 1 – Health triage script

cd ~/rebash-python/module-03
source .venv/bin/activate

cat > triage.py << 'EOF'
#!/usr/bin/env python3
from __future__ import annotations

import sys

HOSTS = [
    ("web-a", "healthy"),
    ("web-b", "degraded"),
    ("web-c", "down"),
    ("lab-1", "healthy"),
]


def action_for(status: str) -> str:
    match status:
        case "healthy":
            return "noop"
        case "degraded":
            return "warn"
        case "down":
            return "page"
        case _:
            return "unknown"


def main() -> int:
    worst = 0
    for host, status in HOSTS:
        if host.startswith("lab-"):
            continue
        act = action_for(status)
        print(f"host={host} status={status} action={act}")
        if act == "page":
            worst = max(worst, 2)
        elif act == "warn":
            worst = max(worst, 1)
    return worst


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

python triage.py; echo "exit=$?"

Step 2 – Retry with while

cat > retry_demo.py << 'EOF'
#!/usr/bin/env python3
from __future__ import annotations

import sys


def main() -> int:
    target_success_on = 3
    attempt = 0
    while attempt < 5:
        attempt += 1
        print(f"attempt={attempt}", file=sys.stderr)
        if attempt == target_success_on:
            print("ok")
            return 0
    print("error: retries exhausted", file=sys.stderr)
    return 1


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

python retry_demo.py

Step 3 – range inventory numbering

python - <<'PY'
for idx, name in enumerate(["db-a", "db-b"], start=1):
    print(f"{idx}. {name}")
PY

Validation

• Lab commands run under ~/rebash-python/module-03/
• You can explain each Theory section in your own words
• You used modern tooling where it applies to this topic
• You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Control Flow — Conditionals and Loops always combines:

1. Inspect before you change (status, plan, logs, dry-run)
2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
3. Capture evidence (command output, pipeline logs) for handovers
4. Prefer current tools and APIs over legacy shortcuts
5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

• Treat credentials and tokens for python as privileged — never commit them
• Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
• Validate blast radius before apply/deploy/delete operations
• Restrict who can approve production changes
• Collect audit logs; limit who can read sensitive traces

Common Mistakes

Skipping fundamentals for Control Flow — Conditionals and Loops

Validate assumptions against the Theory section and official docs before changing production.

Treating lab defaults as production-ready for Control Flow — Conditionals and Loops

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

• Encode Control Flow — Conditionals and Loops changes as code and review them in pull requests
• Pin versions (images, modules, actions, provider plugins)
• Separate environments with clear promotion gates
• Alert on symptoms with runbooks attached
• Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom	Likely cause	What to do
Infinite loop	while True without break	Add max attempts
SyntaxError on match	Python < 3.10	Upgrade interpreter
Nested if soup	Too many branches	Use match or dict dispatch

Summary

• Branch with if/match; iterate with for; retry with capped while
• continue/break keep inventory loops clear
• Exit codes should reflect worst severity for CI

Interview Questions

1. How does Control Flow — Conditionals and Loops show up when operating Cloud or production platforms?
2. What would you check first if this area misbehaves in production?
3. Which modern tools or APIs replace older equivalents here?
4. What security control should accompany this capability?
5. How would you automate verification of this topic in CI?

Sample answer — question 2

Start with blast radius and recent changes, gather evidence (logs, status, plan/diff), then fix forward with a known rollback path — not guesswork.

Related Tutorials

• Course overview
• • Functions — Parameters and Scope
• Data Structures

References

• Control flow tutorial
• match statement