Exceptions

Objects and errors5 min readLesson 2 of 3

An exception stops the line it happened on and looks for a handler.

python

try:
    value = int("not a number")
except ValueError:
    value = 0
except (TypeError, KeyError) as error:
    print("bad input:", error)
else:
    print("parsed", value)
finally:
    print("always runs")

Catch the narrowest exception you can handle. A bare except hides real bugs, including the ones you would want to see. Raise your own with a clear message:

python

def average(values):
    if not values:
        raise ValueError("average() needs at least one value")
    return sum(values) / len(values)

Use raise ... from error when translating one error into another, so the original cause stays visible.