Lab — Python YAML Config Validator¶

Lab Overview¶

Purpose: Load infrastructure YAML safely and return automation-friendly exit codes.

Scenario: A PR adds deploy.yaml. CI should fail fast if the file is missing keys or is not valid YAML, without applying anything.

Expected outcome: A CLI that validates one or more YAML files and exits 0 (ok) or 2 (usage / invalid).

This is a lab, not a tutorial

Apply File Handling — pathlib, JSON, YAML, CSV and Error Handling and Exceptions.

Business Scenario¶

Platform requires every service repo to ship deploy.yaml with at least name (non-empty string) and replicas (integer ≥ 1). Your job is a local validator teammates can run before push — and CI can run without kubectl.

Learning Objectives¶

Install PyYAML in a venv and pin a version
Use yaml.safe_load (never unsafe load)
Validate required keys and types
Map validation failures to exit code 2
Print human errors on stderr

Prerequisites¶

Knowledge¶

Software¶

Tool	Notes
Python 3.12+	`python3 --version`
PyYAML	pin e.g. `PyYAML>=6.0,<7`

Estimated cost: £0.

Architecture¶

deploy.yaml  -->  safe_load  -->  schema checks  -->  exit 0 | 2
                      |
                 stderr messages

Environment¶

mkdir -p ~/rebash-lab-python/configs
cd ~/rebash-lab-python
python3 -m venv .venv
source .venv/bin/activate
pip install 'PyYAML>=6.0,<7'

Initial State¶

cat > configs/deploy-ok.yaml << 'EOF'
name: payments-api
replicas: 2
EOF

cat > configs/deploy-bad.yaml << 'EOF'
name: payments-api
replicas: zero
EOF

cat > configs/not-yaml.yaml << 'EOF'
::: this is not yaml
EOF

Task¶

Step 1 – Validator skeleton¶

Create validate_config.py:

argparse: positional path (file)
sys.exit(0) on success
sys.exit(2) on any validation failure

Step 2 – Load safely¶

from pathlib import Path
import yaml

text = Path(path).read_text(encoding="utf-8")
data = yaml.safe_load(text)

Treat None / non-dict roots as invalid.

Step 3 – Schema checks¶

Require:

name: str, stripped length ≥ 1
replicas: int, ≥ 1

Collect all errors (do not stop at the first) and print each on stderr.

Step 4 – Multi-file mode (stretch)¶

Accept multiple paths; exit 2 if any file fails.

Validation¶

python validate_config.py configs/deploy-ok.yaml; echo $?   # 0
python validate_config.py configs/deploy-bad.yaml; echo $?  # 2
python validate_config.py configs/not-yaml.yaml; echo $?    # 2
python validate_config.py missing.yaml; echo $?             # 2

Valid file exits 0
Bad replicas / broken YAML / missing file exit 2
Errors go to stderr; success may be silent or one line on stdout
Code uses safe_load only

Troubleshooting¶

Symptom	Fix
`YAMLLoadWarning`	You used unsafe `load` — switch to `safe_load`
`replicas: "2"` passes	Coerce or reject strings; require `isinstance(..., int)` and not `bool`
Relative path fails	Resolve with `Path(path).expanduser().resolve()`

Cleanup¶

deactivate 2>/dev/null || true
# keep shared ~/rebash-lab-python if used by other labs, or:
rm -rf ~/rebash-lab-python

Production Discussion¶

Prefer JSON Schema or a typed model (Pydantic) for larger contracts. Keep validators pure (no cluster calls) so CI stays fast and credential-free.

Stretch Goals¶

Validate optional image matches repo:tag
Add --strict to forbid unknown keys
Return a JSON report for CI annotations