3.10. Handling exceptions

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.

Example 3.21. Opening a non-existent file

>>> fsock = open("/notthere", "r")      1
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
IOError: [Errno 2] No such file or directory: '/notthere'
>>> try:
...     fsock = open("/notthere")       2
... except IOError:                     3
...     print "The file does not exist, exiting gracefully"
... print "This line will always print" 4
The file does not exist, exiting gracefully
This line will always print
1 Using the built-in open function, we can try to open a file for reading (more on open in the next section). But the file doesn’t exist, so this raises the IOError exception. Since we haven’t provided any explicit check for an IOError exception, Python just prints out some debugging information about what happened and then gives up.
2 We’re trying to open the same non-existent file, but this time we’re doing it within a try...except block.
3 When the open method raises an IOError exception, we’re ready for it. The except IOError: line catches the exception and executes our own block of code, which in this case just prints a more pleasant error message.
4 Once an exception has been handled, processing continues normally on the first line after the try...except block. Note that this line will always print, whether or not an exception occurs. If you really did have a file called notthere in your root directory, the call to open would succeed, the except clause would be ignored, and this line would still be executed.

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).

You can also define your own exceptions by creating a class that inherits from the built-in Exception class, and then raise your exceptions with the raise command. This is beyond the scope of this section, but see the further reading section if you’re interested.

Example 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
      import termios, TERMIOS                     1
  except ImportError:
          import msvcrt                           2
      except ImportError:
              from EasyDialogs import AskPassword 3
          except ImportError:
              getpass = default_getpass           4
          else:                                   5
              getpass = AskPassword
          getpass = win_getpass
      getpass = unix_getpass
1 termios is a UNIX-specific module that provides low-level control over the input terminal. If this module is not available (because it’s not on your system, or your system doesn’t support it), the import fails and Python raises an ImportError, which we catch.
2 OK, we didn’t have termios, so let’s try msvcrt, which is a Windows-specific module that provides an API to lots of useful functions in the Microsoft Visual C++ runtime services. If this import fails, Python will raise an ImportError, which we catch.
3 If the first two didn’t work, we try to import a function from EasyDialogs, which is a Mac OS-specific module that provides functions to pop up dialogs of various types. Once again, if this import fails, Python will raise an ImportError, which we catch.
4 None of these platform-specific modules is available (which is possible, since Python has been ported to lots of different platforms), so we have to fall back on a default password input function (which is defined elsewhere in the getpass module). Notice what we’re doing here: we’re assigning the function default_getpass to the variable getpass. If you read the official getpass documentation, it tells you that the getpass module defines a getpass function. This is how it does it: by binding getpass to the right function for your platform. Then when you call the getpass function, you’re really calling a platform-specific function that this code has set up for you. You don’t have to know or care what platform your code is running on; just call getpass, and it will always do the right thing.
5 A try...except block can have an else clause, like an if statement. If no exception is raised during the try block, the else clause is executed afterwards. In this case, that means that the from EasyDialogs import AskPassword import worked, so we should bind getpass to the AskPassword function. Each of the other try...except blocks have similar else clauses to bind getpass to the appropriate function when we find an import that works.

Further reading


[6] Or, as some marketroids would put it, your program would perform an illegal action. Whatever.