Part VII, Exceptions and Tools

See Test Your Knowledge: Part VII Exercises in Chapter 35 for the exercises.

 

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  1. try/except. My version of the oops function (file oops.py) follows. As for the noncoding questions, changing oops to raise a KeyError instead of an IndexError means that the try handler won’t catch the exception (it “percolates” to the top level and triggers Python’s default error message). The names KeyError and IndexError come from the outermost built-in names scope. Import builtins (__builtin__ in Python 2.6) and pass it as an argument to the dir function to see for yourself:

    def oops():
        raise IndexError()

    def doomed():
        try:
            oops()
        except IndexError:
            print('caught an index error!')
        else:
            print('no error caught...')

    if __name__ == '__main__': doomed()

    % python oops.py
    caught an index error!

  2. Exception objects and lists. Here’s the way I extended this module for an exception of my own:

    class MyError(Exception): pass

    def oops():
        raise MyError('Spam!')

    def doomed():
        try:
            oops()
        except IndexError:
            print('caught an index error!')
        except MyError as data:
            print('caught error:', MyError, data)
        else:
            print('no error caught...')

    if __name__ == '__main__':
        doomed()

    % python oops.py
    caught error: <class '__main__.MyError'> Spam!

    Like all class exceptions, the instance comes back as the extra data; the error message shows both the class (<...>) and its instance (Spam!). The instance must be inheriting both an __init__ and a __repr__ or __str__ from Python’s Exception class, or it would print like the class does. See Chapter 34 for details on how this works in built-in exception classes.
  3. Error handling. Here’s one way to solve this one (file safe2.py). I did my tests in a file, rather than interactively, but the results are about the same.

    import sys, traceback

    def safe(entry, *args):
        try:
            entry(*args)                       # Catch everything else
        except:
            traceback.print_exc()
            print('Got', sys.exc_info()[0], sys.exc_info()[1])

    import oops
    safe(oops.oops)

    % python safe2.py
    Traceback (innermost last):
      File "safe2.py", line 5, in safe
        entry(*args)                           # Catch everything else
      File "oops.py", line 4, in oops
        raise MyError, 'world'
    hello: world
    Got hello world

  4. Here are a few examples for you to study as time allows; for more, see follow-up books and the Web:

    # Find the largest Python source file in a single directory

    import os, glob
    dirname = r'C:\Python30\Lib'

    allsizes = []
    allpy = glob.glob(dirname + os.sep + '*.py')
    for filename in allpy:
        filesize = os.path.getsize(filename)
        allsizes.append((filesize, filename))

    allsizes.sort()
    print(allsizes[:2])
    print(allsizes[-2:])


    # Find the largest Python source file in an entire directory tree

    import sys, os, pprint
    if sys.platform[:3] == 'win':
        dirname = r'C:\Python30\Lib'
    else:
        dirname = '/usr/lib/python'

    allsizes = []
    for (thisDir, subsHere, filesHere) in os.walk(dirname):
        for filename in filesHere:
            if filename.endswith('.py'):
                fullname = os.path.join(thisDir, filename)
                fullsize = os.path.getsize(fullname)
                allsizes.append((fullsize, fullname))

    allsizes.sort()
    pprint.pprint(allsizes[:2])
    pprint.pprint(allsizes[-2:])


    # Find the largest Python source file on the module import search path

    import sys, os, pprint
    visited  = {}
    allsizes = []
    for srcdir in sys.path:
        for (thisDir, subsHere, filesHere) in os.walk(srcdir):
            thisDir = os.path.normpath(thisDir)
            if thisDir.upper() in visited:
                continue
            else:
                visited[thisDir.upper()] = True
            for filename in filesHere:
                if filename.endswith('.py'):
                    pypath  = os.path.join(thisDir, filename)
                    try:
                        pysize = os.path.getsize(pypath)
                    except:
                        print('skipping', pypath)
                    allsizes.append((pysize, pypath))

    allsizes.sort()
    pprint.pprint(allsizes[:3])
    pprint.pprint(allsizes[-3:])


    # Sum columns in a text file separated by commas

    filename = 'data.txt'
    sums = {}

    for line in open(filename):
        cols = line.split(',')
        nums = [int(col) for col in cols]
        for (ix, num) in enumerate(nums):
            sums[ix] = sums.get(ix, 0) + num

    for key in sorted(sums):
        print(key, '=', sums[key])


    # Similar to prior, but using lists instead of dictionaries for sums

    import sys
    filename = sys.argv[1]
    numcols  = int(sys.argv[2])
    totals   = [0] * numcols

    for line in open(filename):
        cols = line.split(',')
        nums = [int(x) for x in cols]
        totals = [(x + y) for (x, y) in zip(totals, nums)]

    print(totals)


    # Test for regressions in the output of a set of scripts

    import os
    testscripts = [dict(script='test1.py', args=''),       # Or glob script/args dir
                   dict(script='test2.py', args='spam')]

    for testcase in testscripts:
        commandline = '%(script)s %(args)s' % testcase
        output = os.popen(commandline).read()
        result = testcase['script'] + '.result'
        if not os.path.exists(result):
            open(result, 'w').write(output)
            print('Created:', result)
        else:
            priorresult = open(result).read()
            if output != priorresult:
                print('FAILED:', testcase['script'])
                print(output)
            else:
                print('Passed:', testcase['script'])


    # Build GUI with tkinter (Tkinter in 2.6) with buttons that change color and grow

    from tkinter import *                                  # Use Tkinter in 2.6
    import random
    fontsize = 25
    colors = ['red', 'green', 'blue', 'yellow', 'orange', 'white', 'cyan', 'purple']

    def reply(text):
        print(text)
        popup = Toplevel()
        color = random.choice(colors)
        Label(popup, text='Popup', bg='black', fg=color).pack()
        L.config(fg=color)

    def timer():
        L.config(fg=random.choice(colors))
        win.after(250, timer)

    def grow():
        global fontsize
        fontsize += 5
        L.config(font=('arial', fontsize, 'italic'))
        win.after(100, grow)

    win = Tk()
    L = Label(win, text='Spam',
              font=('arial', fontsize, 'italic'), fg='yellow', bg='navy',
              relief=RAISED)
    L.pack(side=TOP, expand=YES, fill=BOTH)
    Button(win, text='press', command=(lambda: reply('red'))).pack(side=BOTTOM,fill=X)
    Button(win, text='timer', command=timer).pack(side=BOTTOM, fill=X)
    Button(win, text='grow', command=grow).pack(side=BOTTOM, fill=X)
    win.mainloop()


    # Similar to prior, but use classes so each window has own state information

    from tkinter import *
    import random

    class MyGui:
        """
        A GUI with buttons that change color and make the label grow
        """
        colors = ['blue', 'green', 'orange', 'red', 'brown', 'yellow']

        def __init__(self, parent, title='popup'):
            parent.title(title)
            self.growing = False
            self.fontsize = 10
            self.lab = Label(parent, text='Gui1', fg='white', bg='navy')
            self.lab.pack(expand=YES, fill=BOTH)
            Button(parent, text='Spam', command=self.reply).pack(side=LEFT)
            Button(parent, text='Grow', command=self.grow).pack(side=LEFT)
            Button(parent, text='Stop', command=self.stop).pack(side=LEFT)

        def reply(self):
            "change the button's color at random on Spam presses"
            self.fontsize += 5
            color = random.choice(self.colors)
            self.lab.config(bg=color,
                    font=('courier', self.fontsize, 'bold italic'))

        def grow(self):
            "start making the label grow on Grow presses"
            self.growing = True
            self.grower()

        def grower(self):
            if self.growing:
                self.fontsize += 5
                self.lab.config(font=('courier', self.fontsize, 'bold'))
                self.lab.after(500, self.grower)

        def stop(self):
            "stop the button growing on Stop presses"
            self.growing = False

    class MySubGui(MyGui):
        colors = ['black', 'purple']           # Customize to change color choices

    MyGui(Tk(), 'main')
    MyGui(Toplevel())
    MySubGui(Toplevel())
    mainloop()


    # Email inbox scanning and maintenance utility

    """
    scan pop email box, fetching just headers, allowing
    deletions without downloading the complete message
    """

    import poplib, getpass, sys

    mailserver = 'your pop email server name here'                 # pop.rmi.net
    mailuser   = 'your pop email user name here'                   # brian
    mailpasswd = getpass.getpass('Password for %s?' % mailserver)

    print('Connecting...')
    server = poplib.POP3(mailserver)
    server.user(mailuser)
    server.pass_(mailpasswd)

    try:
        print(server.getwelcome())
        msgCount, mboxSize = server.stat()
        print('There are', msgCount, 'mail messages, size ', mboxSize)
        msginfo = server.list()
        print(msginfo)
        for i in range(msgCount):
            msgnum  = i+1
            msgsize = msginfo[1][i].split()[1]
            resp, hdrlines, octets = server.top(msgnum, 0)         # Get hdrs only
            print('-'*80)
            print('[%d: octets=%d, size=%s]' % (msgnum, octets, msgsize))
            for line in hdrlines: print(line)

            if input('Print?') in ['y', 'Y']:
                for line in server.retr(msgnum)[1]: print(line)    # Get whole msg
            if input('Delete?') in ['y', 'Y']:
                print('deleting')
                server.dele(msgnum)                                # Delete on srvr
            else:
                print('skipping')
    finally:
        server.quit()                                  # Make sure we unlock mbox
    input('Bye.')                                      # Keep window up on Windows


    # CGI server-side script to interact with a web browser

    #!/usr/bin/python
    import cgi
    form = cgi.FieldStorage()                          # Parse form data
    print("Content-type: text/html\n")                 # hdr plus blank line
    print("<HTML>")
    print("<title>Reply Page</title>")                 # HTML reply page
    print("<BODY>")
    if not 'user' in form:
        print("<h1>Who are you?</h1>")
    else:
        print("<h1>Hello <i>%s</i>!</h1>" % cgi.escape(form['user'].value))
    print("</BODY></HTML>")


    # Database script to populate and query a MySql database

    from MySQLdb import Connect
    conn = Connect(host='localhost', user='root', passwd='darling')
    curs = conn.cursor()
    try:
        curs.execute('drop database testpeopledb')
    except:
        pass                                           # Did not exist

    curs.execute('create database testpeopledb')
    curs.execute('use testpeopledb')
    curs.execute('create table people (name char(30), job char(10), pay int(4))')

    curs.execute('insert people values (%s, %s, %s)', ('Bob', 'dev', 50000))
    curs.execute('insert people values (%s, %s, %s)', ('Sue', 'dev', 60000))
    curs.execute('insert people values (%s, %s, %s)', ('Ann', 'mgr', 40000))

    curs.execute('select * from people')
    for row in curs.fetchall():
        print(row)

    curs.execute('select * from people where name = %s', ('Bob',))
    print(curs.description)
    colnames = [desc[0] for desc in curs.description]
    while True:
        print('-' * 30)
        row = curs.fetchone()
        if not row: break
        for (name, value) in zip(colnames, row):
            print('%s => %s' % (name, value))

    conn.commit()                                      # Save inserted records


    # Database script to populate a shelve with Python objects

    # see also Chapter 27 shelve and Chapter 30 pickle examples

    rec1 = {'name': {'first': 'Bob', 'last': 'Smith'},
            'job':  ['dev', 'mgr'],
            'age':  40.5}

    rec2 = {'name': {'first': 'Sue', 'last': 'Jones'},
            'job':  ['mgr'],
            'age':  35.0}

    import shelve
    db = shelve.open('dbfile')
    db['bob'] = rec1
    db['sue'] = rec2
    db.close()


    # Database script to print and update shelve created in prior script

    import shelve
    db = shelve.open('dbfile')
    for key in db:
        print(key, '=>', db[key])

    bob = db['bob']
    bob['age'] += 1
    db['bob'] = bob
    db.close()