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*.


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
       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.


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 <module>
   ZeroDivisionError: integer division or modulo by zero
   >>> 4 + spam*3
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   NameError: name 'spam' is not defined
   >>> '2' + 2
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   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 provides detail based on the type of exception
and what caused it.

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

Built-in Exceptions lists the built-in exceptions and their meanings.


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 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

Note that the parentheses around this tuple are required, because
"except ValueError, e:" was the syntax used for what is normally
written as "except ValueError as e:" in modern Python (described
below). The old syntax is still supported for backwards compatibility.
This means "except RuntimeError, TypeError" is not equivalent to
"except (RuntimeError, TypeError):" but to "except RuntimeError as
TypeError:" which is not what you want.

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 as e:
       print "I/O error({0}): {1}".format(e.errno, e.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 "__str__()" so the arguments can be printed directly
without having to reference ".args".

One may also instantiate an exception first before raising it and add
any attributes to it as desired.

   >>> try:
   ...     raise Exception('spam', 'eggs')
   ... except Exception as inst:
   ...     print type(inst)     # the exception instance
   ...     print inst.args      # arguments stored in .args
   ...     print inst           # __str__ allows args to be printed directly
   ...     x, y = inst.args
   ...     print 'x =', x
   ...     print 'y =', y
   ...
   <type 'exceptions.Exception'>
   ('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 as detail:
   ...     print 'Handling run-time error:', detail
   ...
   Handling run-time error: integer division or modulo by zero


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 <module>
   NameError: HiThere

The sole argument to "raise" indicates the exception to be raised.
This must be either an exception instance or an exception class (a
class that derives from "Exception").

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 <module>
   NameError: HiThere


8.5. User-defined Exceptions
============================

Programs may name their own exceptions by creating a new exception
class (see Classes for more about Python classes).  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 as 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 <module>
   __main__.MyError: 'oops!'

In this example, the default "__init__()" of "Exception" has been
overridden.  The new behavior simply creates the *value* attribute.
This replaces the default behavior of creating the *args* attribute.

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 that 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:
           expr -- input expression in which the error occurred
           msg  -- explanation of the error
       """

       def __init__(self, expr, msg):
           self.expr = expr
           self.msg = msg

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

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

       def __init__(self, prev, next, msg):
           self.prev = prev
           self.next = next
           self.msg = msg

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 Classes.


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 <module>
   KeyboardInterrupt

A *finally clause* is always executed before leaving the "try"
statement, whether an exception has occurred or not. When an exception
has occurred in the "try" clause and has not been handled by an
"except" clause (or it has occurred in an "except" or "else" clause),
it is re-raised after the "finally" clause has been executed.  The
"finally" clause is also executed “on the way out” when any other
clause of the "try" statement is left via a "break", "continue" or
"return" statement.  A more complicated example (having "except" and
"finally" clauses in the same "try" statement works as of Python 2.5):

   >>> def divide(x, y):
   ...     try:
   ...         result = x / y
   ...     except ZeroDivisionError:
   ...         print "division by zero!"
   ...     else:
   ...         print "result is", result
   ...     finally:
   ...         print "executing finally clause"
   ...
   >>> divide(2, 1)
   result is 2
   executing finally clause
   >>> divide(2, 0)
   division by zero!
   executing finally clause
   >>> divide("2", "1")
   executing finally clause
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "<stdin>", line 3, in divide
   TypeError: unsupported operand type(s) for /: 'str' and 'str'

As you can see, the "finally" clause is executed in any event.  The
"TypeError" raised by dividing two strings is not handled by the
"except" clause and therefore re-raised after the "finally" clause has
been executed.

In real world applications, the "finally" clause is useful for
releasing external resources (such as files or network connections),
regardless of whether the use of the resource was successful.


8.7. Predefined Clean-up Actions
================================

Some objects define standard clean-up actions to be undertaken when
the object is no longer needed, regardless of whether or not the
operation using the object succeeded or failed. Look at the following
example, which tries to open a file and print its contents to the
screen.

   for line in open("myfile.txt"):
       print line,

The problem with this code is that it leaves the file open for an
indeterminate amount of time after the code has finished executing.
This is not an issue in simple scripts, but can be a problem for
larger applications. The "with" statement allows objects like files to
be used in a way that ensures they are always cleaned up promptly and
correctly.

   with open("myfile.txt") as f:
       for line in f:
           print line,

After the statement is executed, the file *f* is always closed, even
if a problem was encountered while processing the lines. Other objects
which provide predefined clean-up actions will indicate this in their
documentation.
