Python try/except examples

The basic patterns

# 1. the standard shape
try:
    age = int(user_input)
except ValueError:
    print("that wasn't a number")

# 2. use the error object
try:
    data = json.loads(text)
except json.JSONDecodeError as e:
    print(f"bad JSON at line {e.lineno}: {e.msg}")

# 3. different handling per error type
try:
    resp = requests.get(url, timeout=5)
except requests.Timeout:
    retry()                        # timeouts are retryable
except requests.ConnectionError:
    alert_ops()                    # network down is not

# 4. same handling for several types
except (ValueError, KeyError) as e:
    log(e)

else and finally — the forgotten clauses

try:
    f = open(path, encoding="utf-8")
except FileNotFoundError:
    print("no such file")
else:
    # runs ONLY if no exception — keeps the try block minimal,
    # so you don't accidentally catch errors from OTHER lines
    process(f.read())
    f.close()
finally:
    # runs NO MATTER WHAT — cleanup lives here
    release_lock()

Raising: your own errors, and passing them on

# refuse bad input loudly
def set_age(age):
    if age < 0:
        raise ValueError(f"age can't be negative, got {age}")

# log-and-rethrow: observe the error without swallowing it
try:
    charge_card(order)
except PaymentError:
    log_incident(order)
    raise                          # bare raise = same error continues up

# custom exception for your domain (it's just a class)
class QuotaExceeded(Exception): pass
raise QuotaExceeded("key used 10,000 calls today")

The retry loop (the pattern everyone eventually writes)

for attempt in range(3):
    try:
        result = flaky_api_call()
        break                      # success → exit loop
    except requests.Timeout:
        if attempt == 2:
            raise                  # out of retries → let it fail loudly
        time.sleep(2 ** attempt)   # 1s, 2s — backoff