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.
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.
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.
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
andexcept
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 thetry
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
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
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.
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!
KeyboardInterrupt
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
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.
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.