Why use exceptions?
♦ Error handling
♦ Event notification
♦ Special-case handling
♦ Unusual control-flows
Exception topics
♦ The basics
♦ Exception idioms
♦ Exception catching modes
♦ Class exceptions
♦ Exception gotchas
♦ A high-level control flow device
♦ try statements catch exceptions
♦ raise statements trigger exceptions
♦ Exceptions are raised by Python or programs
Basic forms
● Python 2.5+: except/finally can now be mixed
● Python 2.5+: with/as context managers
● Python 3.X: “except E as X”, “raise from E”
♦ try/except/else
try:
<statements> # run/call actions
except <name>:
<statements> # if name raised during try block
except
<name>, <data>:
<statements> # if 'name' raised; get extra data
else:
<statements> # if no exception was raised
♦ try/finally
try:
<statements>
finally:
<statements> # always run 'on the way out'
♦ raise
raise <name> # manually trigger an exception
raise <name>, <data> # pass extra data to catcher too
♦ assert
assert <test>, <message>
# if not test: raise AssertionError, message
♦ with/as context managers (2.5+)
# alternative
to common try/finally idioms
from __future__ import with_statement # till 2.6
with open('/etc/passwd', 'r') as f:
# auto-closed after with
for line in
f: # even if
exception in block
print line
... more
processing code ...
lock = threading.Lock()
with lock: # auto
acquired, released
# critical section
of code # classes may define
managers
Builtin exceptions
♦ Python triggers builtin exceptions on errors
♦ Displays message at top-level if not caught
def kaboom(list, n):
print list[n] # trigger IndexError
try:
kaboom([0, 1, 2], 3)
except IndexError: # catch exception here
print 'Hello world!'
User-defined exceptions
♦ Python (and C) programs raise exceptions too
♦ User-defined exceptions are objects
MyError = "my error"
def stuff(file):
raise MyError
file = open('data', 'r') # open a file
try:
stuff(file) # raises exception
finally:
file.close() # always close file
♦ EOFError sometimes signals end-of-file
while 1:
try:
line = raw_input() # read from stdin
except EOFError:
break
else:
<process next ‘line’ here>
♦ Searches sometimes signal success by ‘raise’
Found = "Item found"
def searcher():
<raise Found or return>
try:
searcher()
except Found:
<success>
else:
<failure>
♦ Outer ‘try’ statements can be used to debug code
try:
<run program>
except: # all uncaught exceptions come here
import sys
print 'uncaught!', sys.exc_info()[:2] # type, value
♦ Try statements nest (are stacked) at runtime
♦ Python selects first clause that matches exception
♦ Try blocks can contain a variety of clauses
♦ Multiple excepts: catch 1-of-N exceptions
♦ Try can contain ‘except’ or ‘finally’, but not both
Try block clauses
|
Operation |
Interpretation |
|
except: |
catch
all exception types |
|
except
name: |
catch
a specific exception only |
|
except
name, value: |
2.X:
catch exception and its extra data |
|
except
name as value: |
3.X: catch exception and its instance |
|
except
(name1, name2): |
catch
any of the listed exceptions |
|
else: |
run
block if no exceptions raised |
|
finally: |
always
perform block, exception or not |
♦ Exceptions nest at run-time
● Runs most recent matching except clause
file: nestexc.py
def action2():
print 1 + [] # generate TypeError
def action1():
try:
action2()
except TypeError: # most recent matching try
print 'inner try'
try:
action1()
except TypeError: # here iff action1 re-raises
print 'outer try'
% python nestexc.py
inner try
♦ Catching 1-of-N exceptions
● Runs first match: top-to-bottom, left-to-right
● See manuals or reference text for a complete list
try:
action()
except NameError:
...
except IndexError
...
except KeyError:
...
except (AttributeError, TypeError, SyntaxError):
...
else:
...
♦ ‘finally’ clause executed on the way out
● useful for ‘cleanup’ actions: closing files,...
● block executed whether exception occurs or not
● Python propagates exception after block finishes
● but exception lost if finally runs a raise, return, or break
file: finally.py
def divide(x, y):
return x / y # divide-by-zero error?
def tester(y):
try:
print divide(8, y)
finally:
print 'on the way out...'
print '\nTest 1:'; tester(2)
print '\nTest 2:'; tester(0) # trigger error
% python finally.py
Test 1:
4
on the way out...
Test 2:
on the way out...
Traceback (innermost last):
File "finally.py", line 11, in ?
print 'Test 2:'; tester(0)
File "finally.py", line 6, in tester
print divide(8, y)
File "finally.py", line 2, in divide
return x / y # divide-by-zero error?
ZeroDivisionError: integer division or modulo
♦ Optional data
● Provides extra exception details
● Python passes None if no explicit data
file: helloexc.py
myException = 'Error' # string object
def raiser1():
raise myException, "hello" # raise, pass data
def raiser2():
raise myException # raise, None implied
def tryer(func):
try:
func()
except myException, extraInfo:
print 'got this:', extraInfo
% python
>>> from helloexc import *
>>> tryer(raiser1)
got this: hello
>>> tryer(raiser2)
got this: None
● Should use classes today: only option in 3.X, per BDFL
● Useful for catching categories of exceptions
● String exception match: same object (‘is’ identity)
● Class exception match: named class or subclass of it
● Class exceptions support exception hierarchies
General raise forms
raise string # matches same string object
raise string, data # optional extra data (default=None)
raise class, instance # matches class or its superclass
raise instance # = instance.__class__, instance
Example
file: classexc.py
class Super: pass
class Sub(Super): pass
def raiser1():
X = Super() # raise listed class instance
raise X
def raiser2():
X = Sub() # raise instance of subclass
raise X
for func in (raiser1, raiser2):
try:
func()
except Super: # match Super or a subclass
import sys
print 'caught:', sys.exc_info()[0]
% python classexc.py
caught: <class Super at 770580>
caught: <class Sub at 7707f0>
Example: numeric library
class NumErr: pass
class Divzero(NumErr): pass
class Oflow(NumErr): pass
raise DivZero()
import mathlib
try:
mathlib.func(…)
except mathlib.NumErr:
…report and recover…
Example: using raised instance
class MyBad:
def __init__(self, file, line):
self.file = file
self.line = line
def display(self):
print self.file * 2
def parser():
raise MyBad('spam.txt', 5)
try:
parser()
except MyBad, X:
print X.file, X.line
X.display()
# built-in file error numbers
def parser():
open('nonesuch')
try:
parser()
except IOError, X:
print X.errno, '=>', X.strerror
What to wrap in a try statement?
● Things that commonly fail: files, sockets, etc.
● Calls to large functions, not code inside the function
● Anything that shouldn’t kill your script
● Simple top-level scripts often should die on errors
● See also atexit module for shutdown time actions
Catching too much?
● Empty except clauses catch everything
● But may intercept error expected elsewhere
try:
[...]
except:
[...] # everything comes here: even sys.exit()!
Catching too little?
● Specific except clauses only catch listed exceptions
● But need to be updated if add new exceptions later
● Class exceptions would help here: category name
try:
[...]
except (myerror1, myerror2): # what if I add a myerror3?
[...] # non-errors
else:
[...] # assumed to be an error
Solution: exception protocol design
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