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В глубь языка Python Для программистов |
Like many object-oriented languages, Python has exception handling via try...except blocks.
Python uses try...except to handle exceptions and raise to generate them. Java and C++ use try...catch to handle exceptions, and throw to generate them. |
If you already know all about exceptions, you can skim this section. If you've been stuck programming in a lesser language that doesn't have exception handling, or you've been using a real language but not using exceptions, this section is very important.
Exceptions are everywhere in Python; virtually every module in the standard Python library uses them, and Python itself will raise them in lots of different circumstances. You've already seen them repeatedly throughout this book.
In each of these cases, we were simply playing around in the Python IDE: an error occurred, the exception was printed (depending on your IDE, in an intentionally jarring shade of red), and that was that. This is called an unhandled exception; when the exception was raised, there was no code to explicitly notice it and deal with it, so it bubbled its way back to the default behavior built in to Python, which is to spit out some debugging information and give up. In the IDE, that's no big deal, but if that happened while your actual Python program was running, the entire program would come to a screeching halt.[6]
An exception doesn't have to be a complete program crash, though. Exceptions, when raised, can be handled. Sometimes an exception is really because you have a bug in your code (like accessing a variable that doesn't exist), but many times, an exception is something you can plan for. If you're opening a file, it might not exist; if you're connecting to a database, it might be unavailable, or you might not have the correct security credentials to access it. If you know a line of code may raise an exception, you should handle the exception using a try...except block.
Пример 3.21. Opening a non-existent file
>>> fsock = open("/notthere", "r") Traceback (innermost last): File "<interactive input>", line 1, in ? IOError: [Errno 2] No such file or directory: '/notthere' >>> try: ... fsock = open("/notthere") ... except IOError: ... print "The file does not exist, exiting gracefully" ... print "This line will always print" The file does not exist, exiting gracefully This line will always print
Exceptions may seem unfriendly (after all, if you don't catch the exception, your entire program will crash), but consider the alternative. Would you rather get back an unusable file object to a non-existent file? You'd have to check its validity somehow anyway, and if you forgot, your program would give you strange errors somewhere down the line that you would have to trace back to the source. I'm sure you've done this; it's not fun. With exceptions, errors occur immediately, and you can handle them in a standard way at the source of the problem.
There are lots of other uses for exceptions besides handling actual error conditions. A common use in the standard Python library is to try to import a module, then check whether it worked. Importing a module that does not exist will raise an ImportError exception. You can use this to define multiple levels of functionality based on which modules are available at run-time, or to support multiple platforms (where platform-specific code is separated into different modules).
Пример 3.22. Supporting platform-specific functionality
This code comes from the getpass module, a wrapper module for getting a password from the user. Getting a password is accomplished differently on UNIX, Windows, and Mac OS platforms, but this code encapsulates all of those differences.
# Bind the name getpass to the appropriate function try: import termios, TERMIOS except ImportError: try: import msvcrt except ImportError: try: from EasyDialogs import AskPassword except ImportError: getpass = default_getpass else: getpass = AskPassword else: getpass = win_getpass else: getpass = unix_getpass
Further reading
Footnotes
[6] Or, as some marketroids would put it, your program would perform an illegal action. Whatever.
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