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No exception type(s) specified

The function divide simply divides a by b. To avoid invalid calculations (e.g., a division by zero), a try-except block is added. This is valid and ensures that the function always returns a result. However, by securing your code with the try clause, you might hide actual programming errors, e.g., that you pass a string or an object as b, instead of a number. By not specifying an exception type, you do not only hide this error but you loose also information about the error itself.

Anti-pattern

def divide(a, b):

    try:
        result = a / b
    except:
        result = None

    return result

Best practice

Handle exceptions with Python’s built in exception types.

def divide(a, b):

    result = None

    try:
        result = a / b
    except ZeroDivisionError:
        print("Type error: division by 0.")
    except TypeError:
        # E.g., if b is a string
        print("Type error: division by '{0}'.".format(b))
    except Exception as e:
        # handle any other exception
        print("Error '{0}' occured. Arguments {1}.".format(e.message, e.args))
    else:
        # Excecutes if no exception occured
        print("No errors")
    finally:
        # Executes always
        if result is None:
            result = 0

    return result

With this pattern, you are able to handle exceptions based on their actual exception-type. The first exception type that matches the current error is handled first. Thus, it is recommended to handle specific exception types first (e.g,. ZeroDivisionError) and generic error types (e.g., Exception) towards the end of the try-except block.

Cleanup actions (optional): The else-clause executes only, if no exception occurred. It is useful to log the success of your code. The finally-block executes under all circumstances — no matter if an error occured or not. It is useful to clean up the try-except block.

Implement user defined exceptions

In addition to Python’s standard exceptions, you can implement your own exception classes.

class DivisorTooSmallError(StandardError):
    def __init__(self, arg):
        self.args = arg


def divide(a, b):
    if b < 1:
        raise DivisorTooSmallError
    return a / b


try:
    divide(10, 0)
except DivisorTooSmallError:
    print("Unable to divide these numbers!")

References

  • PyLint W0702, bare-except
  • Python Built-in Exceptions<https://docs.python.org/2/library/exceptions.html#exceptions.BaseException>
  • Python Errors and Exceptions<https://docs.python.org/2/tutorial/errors.html>