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Karel as an adult



Blogaria

Just a collection of random rants that I can't really place anywhere else.

2017-09-05 His 1.00
2016-07-20 Fswatch 1.09
2016-05-04 Another fswatch update
2016-03-24 fswatch updated
2016-02-22 OTG Price Claimed
2016-02-01 File System Watcher fswatch
2016-01-05 Commandline flag parsing in Go
2015-11-19 Phoning the Dutch Vehicle Registration
2015-11-09 FE 1.05 released
2015-09-28 Bad Horse


2015-09-24 A generic attribute holder class in Python

A few days ago I was fumbling around with dictinaries for data storage. (A dictionary is the Python-term for a map or a hash.) This of course works really well, except that I wanted to pre-define what attributes a hash could hold and which not. As the typical object-oriented example, I wanted a type Person that could hold names in attributes like first and a last, but not nr_of_wheels.

Which is of course what classes are for, with specialized setters and getters. But (being lazy), I didn't want to create special classes with getters and setters for such trivial storage - too much typing. So: how do you create a generic class that allows some attributes, but disallows others? In code terms:

  p = Person()                             # new person
  p.first = 'Karel'                        # setter for first: ok
  p.last  = 'Kubat'                        # setter for last: ok
  print('The person is', p.first, p.last)  # getters: ok
  p.nr_of_wheels = 0                       # boom, this should be an error
Also, I want to be able to initialize an object using keyword arguments, as in:
  p = Person(first = 'Karel',              # initialization by keywords args
             last  = 'Kubat')
  q = Person(nr_of_wheels = 0)             # boom, this should give an error

Huub van E. sent me an alternative solution for this which is class-decorator based. Scroll down to see Huub's idea, which is very nice. My original solution (using a superclass) is here:
Of course, there's a way. Python's classes implement a number of 'magic' methods, such as the constructor __init__(self). But there are notably also:
  • __getattr__(self, attr): returns the value of attribute attr,
  • __setattr__(self, attr, val): sets attribute attr to value val;
  • __delattr(self, attr): removes attribute attr from the object.
So the challenge is just to implement these three, and to make sure that there's a check on attribute attr. Here's how. Below class Anyclass has a constructor that accepts a list of strings that are interpreted as the 'allowed attributes'. Furthermore, if there are keyword arguments, they are used as initializers. Easy huh?
"""
Generic attribute holder

A superclass designed to track what attributes are allowed and to throw
an AttributeError when an attempt is made to access a non-allowed
attribute.

Synopsis:
  import anyclass

  # Definition of a Person that may hold the attributes 'first' and 'last':
  class Person(anyclass.Anyclass):
    def __init__(self, **kw):
      super().__init__(['first', 'last'],         # list of valid attrs
                       **kw)                      # optional keyword inits

  # Use as follows:
  p = Person(first = 'Karel', last = 'Kubat')     # ok
  q = Person(answer = 42)                         # raises AttributeError

  p.first = 'Lerak'                               # ok
  p.answer = 42                                   # raises AttributeError
"""


# Dictionary of class types vs. allowed attributes. 
classlimits = {}

# Definition of the generic attribute-holder class.
class Anyclass:
    """ Generic class for a managed dictionary. """

    # Constructor: enters the attributes in the classlimits dict and
    # initializes the object from **kw.
    def __init__(self, attrs, **kw):
        """ The constructor expects a list of allowed attributes and
        allows a keyword list for initial fills. For example the following
        defines a class Person that may hold the attributes 'first' and
        'last'. These are passed to Anyclass' __init__():
          class Person(Anyclass):
            def __init__(self, **kw):
              super().__init__(['first', 'last'], **kw)
        The stanza
          super().__init__(attrs, **kw)
        must be repeated by every subclass in order to define allowed
        attributes using the list 'attrs'. The keyword arguments **kw
        allow automatic initialization, as in:
          p = Person(first = 'Karel', last = 'Kubat')
        """
        global classlimits
        classlimits[type(self)] = attrs
        for attr in kw:
            self.__setattr__(attr, kw[attr])

    # Getter: checks that it's an allowed attribute and returns the value
    def __getattr__(self, attr):
        """ Checks that attr is in the list of allowed attributes, then
        returns the attribute's value."""
        self._checkattr(attr)
        return self.__dict__.get(attr)

    # Setter: checks that it's an allowed attribute and sets the value
    def __setattr__(self, attr, val):
        """ Checks that attr is in the list of allowed attributes, then
        sets the attribute's value to val."""
        self._checkattr(attr)
        self.__dict__[attr] = val

    # Deleter: removes the attribute, without checking that it's allowed
    def __delattr__(self, attr):
        if attr in self.__dict__:
            del self.__dict__[attr]

    # Helper method to check that an attribute is allowed. When not,
    # raises an AttributeError.
    def _checkattr(self, attr):
        global classlimits
        if not attr in classlimits[type(self)]:
            raise AttributeError('attribute %s not allowed in class %s'
                                 % (attr, type(self)))

Good buddy Huub van E. recently sent me the following solution to the same problem. It is class-decorator based. Yay, very nice! It does all that it should except for the keyword-argument initialization that's implemented above.
def fixedAttributes(attrlist):
        def decorated(cls):
                cls._attrlist = attrlist
                def clsSetAttr(obj, attr, val):
                        if not attr in obj._attrlist:
                                raise AttributeError('attribute "%s" not allowed in @fixedAttributes class %s' % (attr, type(obj)))
                        obj.__dict__[attr] = val
                cls.__setattr__ = clsSetAttr
                
                return cls
        return decorated

if __name__ == "__main__":
        @fixedAttributes(['first','last'])
        class Persoon():
                pass

        huub = Persoon()
        huub.first = 'Huub'
        huub.last = 'van E.'
        print("huub.first = %s" % huub.first)
        print("huub.last = %s" % huub.last)
        huub.age = 55

2015-08-05 Autorotating jpegs according to EXIF tags
2015-04-15 0MQ in Python
2015-04-13 FE update
2015-02-23 Perl Course notes moved
2015-01-13 For my dad
2014-12-11 Peugeot 307 te koop
2014-11-20 Combining tornado.web StaticFileHandler with dynamic handlers
2014-11-05 Pushing scripts to servers using ssh
2014-11-03 Python3 argparse Mini HOWTO
2014-10-30 Handling secure httponly cookies with Python3 and tornado.web
2014-10-29 Overloading libc Calls
2014-10-27 Python Bootcamp Notes
2014-10-06 Custom Squid Proxy Authentication
2014-08-30 CyclicLog
2014-05-02 Playing with Python
2014-04-08 Kalk 1.38
2014-03-18 Closing C++ lecture
2014-02-26 Spring Cleanup
2014-02-26 Gravity error
2014-02-03 More on the knight tour
2014-01-28 The circular knight tour
2014-01-24 A Perl IBAN Checker
2013-10-16 Go2SEPA is live
2013-07-24 AJAXy Fileuploads with JQuery
2013-05-23 The Ticket
2013-04-24 Who is leeching my Facebook
2013-04-15 Time for a compact Coolpix
2013-04-15 Selling my Peavey amp
2013-03-28 encfs Helper Revisited
2013-03-15 c conf 1.17
2013-02-25 Flitsers op de Nederlandse wegen
2013-01-14 OTG Revisited
2013-01-11 Kalk revisited
2012-12-12 God is coming
2012-11-29 Strangely disturbing
2012-11-27 How to sneeze on a motorbike
2012-11-23 Een aanslag uit de toekomst
2012-11-22 More voluntary layoffs
2012-10-17 Browser fingerprint
2012-10-15 Site layout revamped
2012-10-14 Crossroads performing well
2012-10-08 The saddle seat
2012-09-30 Sudo Exploit
2012-09-26 Technomona is live
2012-09-19 A Perl to HTML Prettyprinter
2012-09-17 Playing with Moose
2012-08-22 How do you export photos from Adobe Photoshop Elements
2012-08-21 Security hole at the Empire State Building
2012-07-15 Matrix Sunglasses
2012-07-12 Two Morgans
2012-07-12 Review of the BMW R1200RT Motorbike
2012-06-13 Passwords: maak het hackers moeilijk
2012-06-13 Mijndomein foutje
2012-06-07 YOLNT
2012-06-06 Human Origins
2012-05-23 Me on Facebook
2012-05-16 Voluntary Layoffs
2012-05-07 RBS Bike Tour
2012-04-27 Room with a view
2012-04-23 Browser cookies and Javascript revisited
2012-02-17 Schrodingers Cat
2012-02-14 KPN heeft problemen
2012-01-12 Memebase forever
2012-01-11 Strange squares
2011-12-22 TVV Ondernemingsportaalnl.com zuigt ezel
2011-12-08 Dilbert vs Skype
2011-11-29 The uncanny resilience of bulshytt
2011-11-23 Another silly Trojan attempt
2011-10-29 ACTA is coming our way
2011-10-28 Burgernet in the Netherlands
2011-10-27 Facepalm art
2011-10-26 Do not drag this image
2011-10-22 Off The Grid Challenge
2011-10-12 PI like a boss
2011-10-07 Once upon a time
2011-07-13 Dutch eticket system for trains
2011-07-12 Is Hell exothermic or endothermic
2011-04-27 Optical Illusions
2011-04-19 Odd lyrics
2011-04-16 Band Revival at MON
2011-03-13 Protests in the Middle East and you
2011-03-10 Mac OSX Hotkey for locking your system
2011-02-12 dnspb 0.06 is out
2011-02-08 Would I buy this fridge
2011-02-06 InstaYouth
2011-02-05 The Thinker is back
2011-01-17 Math challenge
2011-01-11 Zero tolerance and zero intelligence
2011-01-05 My interest income in 1991
2011-01-01 Your horoscope by Eddie
2010-12-22 New York City Tours might be half price for you
2010-12-20 Weather Forecast
2010-12-14 World Economy Collapse explained in 3 minutes
2010-12-13 The Salvation Army and its choice of toys
2010-12-08 Elizabeth thinks highly of me
2010-12-06 Should I trust my government with my data
2010-12-05 Announcing dnspb
2010-12-03 Realistic piechart
2010-11-26 Crossroads 2.71 is out
2010-11-24 8 bit Starwars
2010-11-17 Six to eight black men
2010-11-16 Canada wants backdoors and data and everything
2010-11-11 Autumn storm over the Netherlands
2010-10-08 USA wants backdoors to everything
2010-10-05 Sudoku solver in Perl
2010-10-02 Finally wrote up a Syscheck page
2010-09-28 Neon sign fail
2010-09-27 The Renault Eco Team
2010-09-23 Crossroads 2.68 is out
2010-09-20 How to suppress Flash cookies
2010-09-15 Meanwhile on Facebook
2010-09-09 The Yes Men Fix The World
2010-09-07 ed is not dead
2010-08-26 Installing Perl modules in a non root environment
2010-08-22 Magic self leviation
2010-08-20 Google Chrome does not support offline Gmail
2010-08-19 The number 48
2010-08-12 Welsh trout mini HOWTO
2010-08-04 Fooling a NetCache proxy into fetching forbidden files
2010-07-30 The world will end on May 21, 2011
2010-07-28 Hiding or showing a textbox with image animation using JQuery
2010-07-27 Manipulating browser cookies using Javascript
2010-07-25 Survival of the fittest book
2010-07-23 Pastafarians in Spain
2010-07-22 You have two sheep
2010-07-09 Highway bank fire
2010-07-08 Setting up a remote git repository
2010-07-06 Bye bye trusted old Macbook
2010-06-28 John Cleese on Football
2010-06-23 ABN Amro and the Pathetic Customer Service Dept.
2010-06-22 Wally does not like criticism
2010-06-14 Soccermatch Netherlands vs Denmark
2010-06-13 Lazy Cat
2010-06-08 Reading public Buzz using the Google API
2010-06-07 A Personal Letter from Steve Martin
2010-06-05 Sushi Saturday
2010-06-04 Suppressing the Enter key with Javascript
2010-05-31 Temporal spacial anomaly on the Dutch highway
2010-05-23 Greenhost will not log your traffic
2010-05-10 Jarlsberg Webapp Exploits
2010-05-04 A Thought Experiment
2010-05-03 SafeEdit information updated
2010-05-01 Microproxy now supports ftp
2010-04-30 What could get Data angry
2010-04-29 Lego Mindstorm solving the Rubik Cube
2010-04-28 Crossroads 2.65 is out
2010-04-17 Goggomobil in its natural habitat
2010-04-14 Bacon Time
2010-04-11 104 More friends to connect with
2010-04-10 Bacteria infested radio reporter
2010-04-07 The Kubat STAR
2010-03-30 Homework Essay
2010-03-29 C++ mutexes again
2010-03-20 Weird Eyechart
2010-03-15 Microproxy 1.01
2010-03-05 Microproxy
2010-03-03 Sven Kramer and the wrong lane
2010-02-26 Endearing Babe Magnet
2010-02-17 Speed of light measured using chocolate and a microwave
2010-02-17 Never again expires after 65 years
2010-02-16 encfs on the Mac
2010-02-15 Hyves.nl and sexual predators
2010-02-10 Funny textbook
2010-02-09 DNS failing after sleep wake cycle
2010-02-06 Blast from the past
2010-01-28 Simple and straight Perl HTTP::Proxy
2010-01-15 Avatar the Movie
2010-01-08 Slightly NSFW Linux Ad
2010-01-07 WTF
2010-01-05 Stop Software Patents in the EU
2009-12-05 HammerServer 1.02
2009-11-28 Perls Automagical Autoloading
2009-10-07 Office Poster
2009-10-06 The nr 1 Nerdjoke
2009-10-04 WoW Startscript for my Mac
2009-09-27 HammerServer section is online
2009-09-26 The BING HQ
2009-09-26 Digging a WOW Tunnel
2009-06-29 Wee Todd
2009-06-23 The On Off Switch Revisited
2009-06-22 Meatspace
2009-05-30 My old houses
2009-05-11 LOLcats are funny
2009-05-11 Civic Duty WIN
2009-05-10 Vote for the baby, Sky Radio promo FAIL
2009-05-05 My secure data center
2009-02-15 My Valentine is sending me a dot exe
2009-02-05 MacPorts trash: .mp_123456 savefiles cleaning
2009-02-01 Truecrypt 6 on Linux and the ext3 filesystem
2009-01-28 www versus nl.youtube.com
2009-01-27 Songsmith and The Police
2009-01-25 My own Ministery of Silly Walks
2009-01-09 CoolIris Mini HOWTO
2008-11-04 UDP and DNS balancing
2008-11-02 Life in graphs
2008-11-01 Skeined yet?
2008-10-30 New Crossroads on the horizon
2008-10-28 Thread safe or not
2008-10-15 WOW patch 3 on a case sensitive MacOSX filesystem
2008-10-15 Surprising C++ optimizations
2008-10-14 Weird system message
2008-10-08 Data mining against terrorism does not work
2008-09-16 Crossroads at the top of Freshmeat.net
2008-09-09 Stupid spammers at Computable
2008-09-06 Spam prevention with Postfix and Postgrey
2008-09-03 The Gnomish Flying Machine
2008-08-27 Bank customer data on eBay
2008-08-26 Mutexes in C++ Threads
2008-08-22 4M dataloss in the UK last year
2008-08-21 Dropping spam with Postfix and Spamassassin
2008-08-18 Bayes and the War on Photography
2008-08-13 Good marital advice
2008-08-12 Squid proxy for personal usage
2008-08-11 Posix threads in C++
2008-08-09 Crossroads mailing list
2008-08-08 Crossroads 2.00 is out
2008-08-01 Fail Pics
2008-07-14 The Fish Dance
2008-07-01 Big Bother and Massive Data Storage
2008-06-30 MMV One of omitted Unix tools
2008-06-08 Even anonymous breadcrumbs can give you away
2008-05-29 Crossroads in Argentina
2008-05-20 The Party at the Company Outing
2008-05-19 Crossroads 1.80 is out
2008-05-18 Where does technical innovation really come from
2008-05-16 Corporate bs generator
2008-05-15 Even the Vatican has to adapt
2008-05-12 Big Brother is watching your dog
2008-05-09 666 all over the place
2008-04-17 Security and privacy are incompatible
2008-04-16 The Hallmark E Card
2008-04-15 Crosroads Solaris port is out
2008-04-04 Identity theft can cost you dearly
2008-04-03 Crossroads can already do that
2008-03-31 A dagerous safari
2008-03-28 Why some Java J2EE projects are inefficient
2008-03-26 The Hummingbird
2008-03-25 The Easter delusion
2008-03-18 McAfee detects mass hack of 200.000 webpages
2008-03-17 More predictive statistics
2008-03-10 Backwards conclusions even on Slashdot
2008-02-18 A fractal photograph
2008-02-15 Kaprekar revisited
2008-02-14 Kaprekar numbers
2008-02-12 A tale of the criminal ineptitude
2008-02-10 Irritating Selfregistered users in PHPBB
2008-02-08 B2B Spam in the Netherlands
2008-02-06 Surprising iSight Capture
2008-02-05 Breadcrumbs at WickedLasers.com
2008-01-29 iSight Capture Utility
2008-01-28 The Male Brain
2008-01-26 Searching for the next Uri Geller
2008-01-24 Opt in for b2b spam
2008-01-14 Bokito Revisited
2008-01-13 Top Crossroads User
2008-01-12 World of Warcraft Dancing
2008-01-12 Justice dispensed better late than never
2008-01-11 Jeremy Clarkson and Identity Theft
2008-01-10 Terrorism in the Netherlands
2007-12-07 The mind and bodysnatchers are among us
2007-12-05 Bruce Schneier and Hildo
2007-12-04 Bye bye, good Christian soul
2007-12-03 Confusing mail message
2007-11-30 Medion MD 85276 reviewed
2007-11-29 Recent cases of data exposure
2007-11-20 Bayes bites
2007-11-19 Japan starts fingerprinting foreigners
2007-11-14 Privacy, Yahoo and the Strange World
2007-11-14 Privacy, Fall through algorithms, and Securing data
2007-11-07 European airlines to retain data
2007-11-03 BloggEd
2007-10-30 Wilders and Marktplaats.nl
2007-10-28 The goldplated Mac
2007-10-26 More morons
2007-10-26 Dilbert nails it again
2007-10-23 Rough yet funny
2007-10-05 Another silly Trojan mail
2007-10-01 So ugly it is beautiful
2007-09-28 Here is a nickel kid
2007-09-23 Spy Shredder
2007-08-29 Web svn view 1.08
2007-08-24 Caught in THE Process
2007-08-21 Stupid Trojan attack
2007-08-21 Back in 1994
2007-08-20 A girly iPod
2007-08-17 Crossroads for RDP connections
2007-08-15 Firewall art
2007-08-14 jpeginfo
2007-08-13 Good People
2007-08-07 The Real Crossroads
2007-07-30 BBC Documentaries in the Netherlands
2007-07-12 No problems with Crossroads so far
2007-07-11 Politically correct ad nauseam
2007-07-02 Waka Waka Poem
2007-07-02 Voyage of the rubber ducks
2007-06-28 The On Off Switch
2007-06-27 No free lunch
2007-06-25 Crossroads web interface
2007-06-25 Blinkenlights
2007-06-21 There is no silver bullet
2007-06-18 Motto of the week
2007-06-18 Do not feed the troll
2007-06-17 Which programming language are you
2007-06-13 Crossroads support request
2007-06-12 Bokito glasses
2007-06-07 Apache mod_proxy balancer description
2007-06-05 A ticketnumber is not support
2007-06-05 403 Hammertime
2007-06-04 Playground Fun
2007-05-24 Ascii man
2007-05-07 Cannot find the damn server
2007-05-02 The BFG200
2007-04-27 Crossroads Top User
2007-03-30 Crossroads Usage
2007-03-25 The guy with the dark motorhelmet
2007-03-22 The Process and The Result
2007-03-21 Quotes attributed to Jos
2007-03-20 A really nice comment about Crossroads
2007-03-18 Kubat in the air