Errors, exceptions and context managers

def parse_amount(text):
    try:
        value = float(text)
    except ValueError:
        print("  except: bad value", repr(text))
        return None
    else:
        print("  else: parsed", value)
        return value
    finally:
        print("  finally: always runs")

for t in ["1198.00", "abc"]:
    print(t, "->", parse_amount(t))

  else: parsed 1198.0
  finally: always runs
1198.00 -> 1198.0
  except: bad value 'abc'
  finally: always runs
abc -> None

class DataQualityError(Exception):
    """raised when a batch fails a hard check"""

def check_batch(rows):
    bad = [r for r in rows if r["amount"] <= 0]
    if bad:
        raise DataQualityError(f"{len(bad)} rows <= 0")

try:
    check_batch([{"amount": 10}, {"amount": -50}])
except DataQualityError as e:
    print("blocked load:", e)

try:
    int("12a")
except ValueError as e:
    raise RuntimeError("could not parse order_id") from e

blocked load: 1 rows <= 0
RuntimeError: could not parse order_id

class Connection:
    def __enter__(self):
        print("open connection")
        return self
    def __exit__(self, exc_type, exc, tb):
        name = exc_type.__name__ if exc_type else None
        print("close connection, error:", name)
        return False                 # do not swallow errors

try:
    with Connection() as conn:
        print("loading rows")
        raise TimeoutError("warehouse timeout")
except TimeoutError:
    print("caught after cleanup")

open connection
loading rows
close connection, error: TimeoutError
caught after cleanup