Errors, Files & Config

Real programs fail. The difference between a demo and a system is how you handle it — files, config, and clean failure paths.

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What you'll learn

try / except / finally
Custom exceptions
Context managers (with)
pathlib — modern paths
JSON files
Env vars & .env

Remember this

Catch specific exceptions, re-raise with bare raise, build domain exception trees
with guarantees cleanup; pathlib replaces string paths; JSON ↔ dict via loads/dumps
Config and secrets come from env vars; .env is dev-only and gitignored

try / except / finally

Catch specific exceptions; bare except: is a smell.
except (A, B) as e: · finally: always runs.
Log then raise (bare) — don't swallow.

Custom exceptions

class AppError(Exception): ...
class ConfigError(AppError): ...
raise ConfigError("missing key") from original

Domain trees let callers catch broadly or narrowly.

Context managers

with X() as x: ≡ C# using — cleanup on every exit path.
Roll your own: contextlib.contextmanager decorator.

pathlib

p = Path("data") / "raw" / "x.json"
text = p.read_text(encoding="utf-8")
Path("out").mkdir(parents=True, exist_ok=True)
list(Path("data").glob("*.csv"))

JSON

json.loads(text) → dict · json.dumps(obj, indent=2) → text.
json.load(f) / json.dump(obj, f) for file objects.
Raw dicts don't validate → typed parsing (Pydantic, Reel PY-09).

Env vars & .env

os.environ["KEY"] (strict) · os.getenv("KEY", default).
.env + python-dotenv for dev; gitignore it.
Secrets in git history = rotate immediately.

Code: Errors, files & config — production-shaped

import json, os
from pathlib import Path
from dotenv import load_dotenv

load_dotenv()                       # reads .env into os.environ
API_KEY = os.environ["OPENAI_API_KEY"]   # fail fast if missing

# --- custom exceptions ----------------------------------------
class AppError(Exception): pass
class ConfigError(AppError): pass

# --- config from JSON with real errors -------------------------
CONFIG_PATH = Path("config") / "app.json"

def load_config() -> dict:
    try:
        return json.loads(CONFIG_PATH.read_text(encoding="utf-8"))
    except FileNotFoundError as e:
        raise ConfigError(f"missing config: {CONFIG_PATH}") from e
    except json.JSONDecodeError as e:
        raise ConfigError(f"invalid JSON at line {e.lineno}") from e

# --- with = guaranteed cleanup ---------------------------------
with open("data/users.csv", encoding="utf-8") as f:
    header = f.readline().strip()

# --- pathlib everywhere ----------------------------------------
out = Path("output") / "report.json"
out.parent.mkdir(parents=True, exist_ok=True)
out.write_text(json.dumps({"rows": 42}, indent=2), encoding="utf-8")

csvs = sorted(Path("data").glob("*.csv"))