第785页 | Learning Python | 阅读 ‧ 电子书库

Exceptions Aren’t Always Errors

In Python, all errors are exceptions, but not all exceptions are errors. For instance, we saw in Chapter 9 that file object read methods return an empty string at the end of a file. In contrast, the built-in input function (which we first met in Chapter 3 and deployed in an interactive loop in Chapter 10) reads a line of text from the standard input stream, sys.stdin, at each call and raises the built-in EOFError at end-of-file. (This function is known as raw_input in Python 2.6.)

Unlike file methods, this function does not return an empty string—an empty string from input means an empty line. Despite its name, the EOFError exception is just a signal in this context, not an error. Because of this behavior, unless the end-of-file should terminate a script, input often appears wrapped in a try handler and nested in a loop, as in the following code:

while True:
    try:
        line = input()           # Read line from stdin
    except EOFError:
        break                    # Exit loop at end-of-file
    else:
        ...process next line here...

Several other built-in exceptions are similarly signals, not errors—calling sys.exit() and pressing Ctrl-C on your keyboard, respectively, raise SystemExit and KeyboardInterrupt, for example. Python also has a set of built-in exceptions that represent warnings rather than errors; some of these are used to signal use of deprecated (phased out) language features. See the standard library manual’s description of built-in exceptions for more information, and consult the warnings module’s documentation for more on warnings.