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8. Errors and Exceptions

Until now error messages haven't been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions.

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8.1 Syntax Errors

Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python:

    >>> while True print 'Hello world'
      File "<stdin>", line 1, in ?
        while True print 'Hello world'
                       ^
    SyntaxError: invalid syntax

The parser repeats the offending line and displays a little `arrow' pointing at the earliest point in the line where the error was detected. The error is caused by (or at least detected at) the token preceding the arrow: in the example, the error is detected at the keyword `print`, since a colon ("`:`") is missing before it. File name and line number are printed so you know where to look in case the input came from a script.

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8.2 Exceptions

Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here:

    >>> 10 * (1/0)
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    ZeroDivisionError: integer division or modulo by zero
    >>> 4 + spam*3
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    NameError: name 'spam' is not defined
    >>> '2' + 2
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    TypeError: cannot concatenate 'str' and 'int' objects

The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are `ZeroDivisionError`, `NameError` and `TypeError`. The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords).

The rest of the line is a detail whose interpretation depends on the exception type; its meaning is dependent on the exception type.

The preceding part of the error message shows the context where the exception happened, in the form of a stack backtrace. In general it contains a stack backtrace listing source lines; however, it will not display lines read from standard input.

The Python Library Reference lists the built-in exceptions and their meanings.

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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("Please enter a number: "))
    ...         break
    ...     except ValueError:
    ...         print "Oops! That was no 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 rest of the try clause is skipped, 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 list, 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 list). 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`.

    >>> 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

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8.4 Raising Exceptions

The `raise` statement allows the programmer to force a specified exception to occur. For example:

    >>> raise NameError, 'HiThere'
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    NameError: HiThere

The first argument to `raise` names the exception to be raised. The optional second argument specifies the exception's argument.

If you need to determine whether an exception was raised but don't intend to handle it, a simpler form of the `raise` statement allows you to re-raise the exception:

    >>> try:
    ...     raise NameError, 'HiThere'
    ... except NameError:
    ...     print 'An exception flew by!'
    ...     raise
    ...
    An exception flew by!
    Traceback (most recent call last):
      File "<stdin>", line 2, in ?
    NameError: HiThere

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8.5 User-defined Exceptions

Programs may name their own exceptions by creating a new exception class. Exceptions should typically be derived from the `Exception` class, either directly or indirectly. For example:

    >>> class MyError(Exception):
    ...     def __init__(self, value):
    ...         self.value = value
    ...     def __str__(self):
    ...         return repr(self.value)
    ...
    >>> try:
    ...     raise MyError(2*2)
    ... except MyError, e:
    ...     print 'My exception occurred, value:', e.value
    ...
    My exception occurred, value: 4
    >>> raise MyError, 'oops!'
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    __main__.MyError: 'oops!'

Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception. When creating a module which can raise several distinct errors, a common practice is to create a base class for exceptions defined by that module, and subclass that to create specific exception classes for different error conditions:

    class Error(Exception):
        """Base class for exceptions in this module."""
        pass

    class InputError(Error):
        """Exception raised for errors in the input.

        Attributes:
            expression -- input expression in which the error occurred
            message -- explanation of the error
        """

        def __init__(self, expression, message):
            self.expression = expression
            self.message = message

    class TransitionError(Error):
        """Raised when an operation attempts a state transition that's not
        allowed.

        Attributes:
            previous -- state at beginning of transition
            next -- attempted new state
            message -- explanation of why the specific transition is not allowed
        """

        def __init__(self, previous, next, message):
            self.previous = previous
            self.next = next
            self.message = message

Most exceptions are defined with names that end in ``Error,'' similar to the naming of the standard exceptions.

Many standard modules define their own exceptions to report errors that may occur in functions they define. More information on classes is presented in chapter 9, ``Classes.''

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8.6 Defining Clean-up Actions

The `try` statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:

    >>> try:
    ...     raise KeyboardInterrupt
    ... finally:
    ...     print 'Goodbye, world!'
    ...
    Goodbye, world!
    Traceback (most recent call last):
      File "<stdin>", line 2, in ?
    KeyboardInterrupt

A finally clause is executed whether or not an exception has occurred in the try clause. When an exception has occurred, it is re-raised after the finally clause is executed. The finally clause is also executed ``on the way out'' when the `try` statement is left via a `break` or `return` statement.

The code in the finally clause is useful for releasing external resources (such as files or network connections), regardless of whether or not the use of the resource was successful.

A `try` statement must either have one or more except clauses or one finally clause, but not both.