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8.3 Handling Exceptions

It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using Control-C or whatever the operating system supports); note that a user-generated interruption is signalled by raising the KeyboardInterrupt exception.

    >>> while True:
    ...     try:
    ...         x = int(raw_input("Enter a number: "))
    ...         break
    ...     except ValueError:
    ...         print "Not a valid number.  Try again..."
    ...     

The try statement works as follows.

• First, the try clause (the statement(s) between the try and except keywords) is executed.
• If no exception occurs, the except clause is skipped and execution of the try statement is finished.
• If an exception occurs during execution of the try clause, the rest of the clause is skipped. Then if its type matches the exception named after the except keyword, the except clause is executed, and then execution continues after the try statement.
• If an exception occurs which does not match the exception named in the except clause, it is passed on to outer try statements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above.

A try statement may have more than one except clause, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding try clause, not in other handlers of the same try statement. An except clause may name multiple exceptions as a parenthesized tuple, for example:

    ... except (RuntimeError, TypeError, NameError):
    ...     pass

The last except clause may omit the exception name(s), to serve as a wildcard. Use this with extreme caution, since it is easy to mask a real programming error in this way! It can also be used to print an error message and then re-raise the exception (allowing a caller to handle the exception as well):

    import sys
    
    try:
        f = open('myfile.txt')
        s = f.readline()
        i = int(s.strip())
    except IOError, (errno, strerror):
        print "I/O error(%s): %s" % (errno, strerror)
    except ValueError:
        print "Could not convert data to an integer."
    except:
        print "Unexpected error:", sys.exc_info()[0]
        raise

The try ... except statement has an optional else clause, which, when present, must follow all except clauses. It is useful for code that must be executed if the try clause does not raise an exception. For example:

    for arg in sys.argv[1:]:
        try:
            f = open(arg, 'r')
        except IOError:
            print 'cannot open', arg
        else:
            print arg, 'has', len(f.readlines()), 'lines'
            f.close()

The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn't raised by the code being protected by the try ... except statement.

When an exception occurs, it may have an associated value, also known as the exception's argument. The presence and type of the argument depend on the exception type.

The except clause may specify a variable after the exception name (or tuple). The variable is bound to an exception instance with the arguments stored in instance.args. For convenience, the exception instance defines __getitem__ and __str__ so the arguments can be accessed or printed directly without having to reference .args.

But use of .args is discouraged. Instead, the preferred use is to pass a single argument to an exception (which can be a tuple if multiple arguments are needed) and have it bound to the message attribute. One my also instantiate an exception first before raising it and add any attributes to it as desired.

    >>> try:
    ...    raise Exception('spam', 'eggs')
    ... except Exception, inst:
    ...    print type(inst)     # the exception instance
    ...    print inst.args      # arguments stored in .args
    ...    print inst           # __str__ allows args to
                                # printed directly
    ...    x, y = inst          # __getitem__ allows args
                                # to be unpacked directly
    ...    print 'x =', x
    ...    print 'y =', y
    ...
    <type 'instance'>
    ('spam', 'eggs')
    ('spam', 'eggs')
    x = spam
    y = eggs

If an exception has an argument, it is printed as the last part (`detail') of the message for unhandled exceptions.

Exception handlers don't just handle exceptions if they occur immediately in the try clause, but also if they occur inside functions that are called (even indirectly) in the try clause. For example:

    >>> def this_fails():
    ...     x = 1/0
    ... 
    >>> try:
    ...     this_fails()
    ... except ZeroDivisionError, detail:
    ...     print 'Handling run-time error:', detail
    ... 
    Handling run-time error: integer division or
     modulo by zero