Learning Python

Odds and Ends

Passing Optional Data

As we’ve seen, raise statements can pass an extra data item along with the exception for use in a handler. In general, the extra data allows you to send context information to a handler. In fact, every exception has the extra data; much like function results, it’s the special None object if nothing was passed explicitly. The following code illustrates:

myException = 'Error'              # string object

def raiser1():
    raise myException, "hello"     # raise, pass data

def raiser2():
    raise myException              # raise, None implied

def tryer(func):
    try:
        func()
    except myException, extraInfo:  # run func, catch exception + data
        print 'got this:', extraInfo

% python
>>> from helloexc import *
>>> tryer(raiser1)                  # gets explicitly passed extra data
got this: hello
>>> tryer(raiser2)              # gets None by default
got this: None

The assert Statement

As a special case, Python 1.5 introduced an assert statement, which is mostly syntactic shorthand for a raise. A statement of the form:

assert <test>, <data>          # the <data> part is optional

works like the following code:

if __debug__:
    if not <test>:
        raise AssertionError, <data>

but assert statements may be removed from the compiled program’s byte code if the -O command-line flag is used, thereby optimizing the program. Assertion-Error is a built-in exception, and the _ _debug__ flag is a built-in name which is automatically set to 1 unless the -O flag is used. Assertions are typically used to verify program conditions during development; when displayed, ...