# exceptions.py

print '''
Unlike other demo code, this script is not executable; it 
uses pseudo-code.
'''
raise SystemExit


# Simple, standard case

try:
    x = 1/0
except ZeroDivisionError:
    x = None
    # Alternatively:
    # x = 0


# Don't use bare except

try:
    db.connect()
    db.execute(sql)
except:
    '''
    What do we do here?  We don't know what kind of 
    exception we have, plus we might be catching things
    we don't want, like KeyboardInterrupt
    '''
db.close()

# Use this instead:

try:
    db.connect()
    db.execute(sql)
except DBError:
    db.rollback()
db.close()


# try/finally example
# Make sure that lock gets cleaned up
# Note that exception gets propagated past finally
# until caught by except (or program terminates)

try:
    lock.acquire()
    processData()
finally:
    lock.release()


# To handle exception *and* clean up, use try/finally
# wrapped around try/except:

try:
    try:
        lock.acquire()
        db.connect()
        db.execute(sql)
    except DBError:
        db.rollback()
finally:
    db.close()
    lock.release()


# Multiple exceptions

try:
    a,b = getInput()
    return int(a/b)
except (ValueError, ArithmeticError):
    return None

try:
    db.connect()
    db.execute(sql)
except DBError:
    db.rollback()
except KeyboardError:
    db.close()
    raise
db.close()


# Exception info

try:
    page = urllib.urlopen(url).read()
except IOError, error:
    print "Download failed due to %s", error


# User-defined exception

class MyException(Exception):
    pass

def f():
    for result in getInput():
        if result is None:
            raise MyException, "No valid input"
        process(result)

try:
    f()
except MyException, error:
    print error
    cleanup()