[newbie] Looking for a good introduction to object orientedprogramming with Python

Discussion in 'Python' started by Jean Dubois, Aug 4, 2012.

  1. Jean Dubois

    Jean Dubois Guest

    I'm looking for a good introduction to object oriented programming
    with Python. I am looking for an introduction which only refers to
    Python. I have seen introductions where the authors make comparisons
    to other languages such as C++ and Java, but as I don't know these
    languages that doesn't help me further much, it rather confuses me. I
    also found an introduction in which the author started by telling that
    "object oriented programming is weird", such a statement did stop me
    reading further as I think an author should at least believe in the
    topic he is going to present as being logical.
    If someone here has a link or title to such an intro, I'd appreciate
    that very much

    p.s. People who don't like my style of asking questions, please
    neglect this message
    Jean Dubois, Aug 4, 2012
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  2. Jean Dubois

    shearichard Guest

    One reason you may be having difficulty is that unlike some languages (C++/Java) object-orientation is not a be all and end all in Python, in fact youcould work with Python for a long time without really 'doing it' at all (well other than calling methods/properties on existing API's). Having said that here's what I would suggest ...

    Could do worse than this :


    and this


    read together.

    Judging by your question this is a probably a little advanced for now but you could bookmark it for the future:


    Here's the corresponding PDF to go with the video:

    http://assets.en.oreilly.com/1/event/45/Practical Python Patterns Presentation.pdf
    shearichard, Aug 5, 2012
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  3. Jean Dubois

    Jean Dubois Guest

    Thanks a lot for this information, I'll check it out the following

    best regards,
    Jean Dubois, Aug 5, 2012
  4. Jean Dubois

    Jean Dubois Guest

    This example seems to tell you need the concept of dictionaries to
    explain object oriented programming, is this really necessary? Unfortunately, the trouble with this explanation is exactly what made
    me ask the original question: it starts from concepts in c++ making it
    very hard to understand for someone who does not know that language
    Can someone here on this list give a trivial example of what object
    oriented programming is, using only Python?

    thanks in advance
    Jean Dubois, Aug 5, 2012
  5. Try this http://www.voidspace.org.uk/python/articles/OOP.shtml ???
    Mark Lawrence, Aug 5, 2012
  6. Jean Dubois

    Roy Smith Guest

    OOP seems to mean different things to different people. What OOP means
    to you is usually a strong function of whatever OOP language you learned
    first. That being said, I think the fundamental, universal, core
    principle of OOP is that an object contains some data, and some code
    that knows how to do something with that data.

    So, to give you a simple (but real-life) example, the system I'm working
    in now has User objects. A user is a pretty complicated class, but
    here's some simple methods from it:

    def __eq__(self, other):
    return isinstance(other, (User, AnonymousUser)) \
    and self.user_id == other.user_id

    def __unicode__(self):
    return self.username

    def __repr__(self):
    return '<User %d: %r>' % (self.user_id, self.username)

    This defines a few basic behaviors for User objects.

    First, it defines how to tell if something is equal to a given User
    object. The something must itself be a User (ignore the minor
    complication about AnonymousUser for the moment), and it must have the
    same user_id as this one. I could easily imagine lots of other possible
    ways two users could be considered equal (same username, for example),
    but we're using user_id. This means I can write:

    if user1 == user2:
    print "they're the same"

    and I don't have to worry about (or even know about) the details. In
    fact, sometime long after I've written that code, somebody could define
    some new kind of HighSecurityUser which tests for equality by comparing
    the scanned retina images for both of them. My code wouldn't have to
    change; it would magically just start enforcing retina matching.

    Likewise, I can write:

    print user


    logger.warning("%r did something interesting", user)

    and I don't have to know anything about how to print a User. The User
    knows how to print itself.
    Roy Smith, Aug 5, 2012
  7. On 05/08/2012 19:43, Ifthikhan Nazeem wrote:
    [top posting fixed]
    I'd forgotten about that so thanks for the reminder.
    Mark Lawrence, Aug 5, 2012
  8. Jean Dubois

    dncarac Guest

    I found Mark Lutz's book Learning Python had two or three chapters on object oriented programming from starting principles to more involved Python object programming. It helped me immensely.
    dncarac, Aug 5, 2012
  9. On 05/08/2012 20:46, lipska the kat wrote:

    Please no, that's the worst possible book for someone trying to learn
    OOD in Python. It's mostly if not completely irrelevant, jumping
    through hoops that you don't need in Python because of its dynamic
    nature. Start with the factory pattern and I hope you'll understand why
    I say this. Search for "design patterns alex martelli" and you'll get
    all you need and more.
    Mark Lawrence, Aug 5, 2012
  10. Then maybe you are asking the wrong question...

    Don't look for Object-Oriented Programming -- since the first widely
    popular OOP language was C++ (Smalltalk was earlier, but rather
    specialized, whereas C++ started as a preprocessor for C).

    Rather look for Object-Oriented Analysis and Design (OOAD). An OOAD
    textbook /should/ be language neutral and, these days, likely using the
    constructs/notation of UML [which derived from a merger of two or three
    separate proposals for OOAD tools]

    An OOAD text should cover, besides Classes, Use Cases, state
    diagrams, and lots of other things...

    The short view of a Class is that it is a means of encapsulating the
    methods (functions/operations) of an object along with the attributes
    (data) of a specific instance of the class.

    A Radio Class would define methods to change the volume, power
    state, tuning, and band. These methods are common to all radios. But an
    instance of a radio doesn't share its volume level with all other
    instances of the Radio class.

    OOP is more a style/philosophy of programming by using "objects".
    Ada 95 is an OOP language but didn't have the
    "object.method(parameters)" syntax (one had to use "method(object,
    parameters", but the language could determine which of similar named
    methods was meant based upon the type of "object" and the types of the
    Dennis Lee Bieber, Aug 5, 2012
  11. Good lord. I'd rather read C++ than UML. And I can't read C++.
    Steven D'Aprano, Aug 5, 2012
  12. Jean Dubois

    Roy Smith Guest

    +1 what Mark said. It's certainly the classic patterns book, but most
    of it is about clever ways to work around the C++/Java style of type
    bondage. Trying to learn OO from that book is like learning to paint by
    reading a textbook on the chemical properties of oil pigments.
    Roy Smith, Aug 5, 2012
  13. Jean Dubois

    Roy Smith Guest

    UML is under-rated. I certainly don't have any love of the 47 different
    flavors of diagram, but the basic idea of having a common graphical
    language for describing how objects and classes interact is pretty
    useful. Just don't ask me to remember which kind of arrowhead I'm
    supposed to use in which situation.
    Roy Smith, Aug 6, 2012
  14. Ask nicely and I'll lend you my copy of Martin Fowler's UML Distilled
    which covers "version 1.2 OMG UML standard". What's it up to now,
    version 17.38?
    Mark Lawrence, Aug 6, 2012
  15. That depends on how you define OOP, and in particular, which aspects of
    OOP your language supports.

    There are lots of differences between styles and implementations of OOP
    languages, and no two languages have exactly the same feature set, but in
    general most OOP languages involve most of the following features:

    - Dynamic dispatch when making method calls
    - Encapsulation, or multi-methods
    - Subtype polymorphism
    - Object inheritance, or delegation

    See here for more detail:


    But beyond those fundamentals, the details of OOP can differ greatly from
    language to language. Some languages treat classes as static, some as
    dynamic. Some languages treat classes as objects themselves, others treat
    them as non-objects. Syntax varies (although the dot operator has become
    almost completely ubiquitous, there are still some exceptions).

    In particularly, terminology varies -- I personally despise with the heat
    of a thousand suns the terms "instance variable" and "class variable" for
    attributes or members. Since a "string variable" is a variable holding a
    string, and a "float variable" is a variable holding a float, an instance
    variable should be a variable holding an instance, and a class variable
    should be a variable holding a class.

    Is this some sort of mystical "humans aren't objects, they're SPECIAL!!!"
    rubbish? Because it sure sounds like it.

    Can you give some non-religious reasons why you should not implement
    human beings or aliases as objects?

    If not, let me just say that I reject your prohibition and leave it at

    Incorrect. You don't even need classes to have objects. You can have
    class-based OOP, and prototype-based OOP, as in Javascript, Actionscript,
    Io, and the language which invented the term, Self.


    I think that's exactly why Python *is* a good language to learn OOP,
    because you can be productive even while learning. You can start off by
    just adding a little bit of OOP syntax to your programs:

    response = raw_input("What do you want to do? ")
    response = response.lower() # Look ma, I'm using OOP!

    while still being primarily procedural. Then you can gradually learn the
    terminology ("what's an instance?"), graduate to writing your own
    classes, and then finally move on to OOP techniques which some languages
    don't even allow, like metaclasses.

    And why do you think this is a problem?

    Classes are one possible solution to problems that actually matter. What
    matters is the solution, not the mechanism of the solution. "Writing
    classes" is just a means to an end (the solution), not the end itself.

    Two problems with "Design Patterns" is that many of them are excessively
    abstract, and that often they only exist to work around limitations in
    the source language (usually C++ or Java).

    The first problem means that any half-decent programmer has probably been
    using "Design Patterns" for years without realising it, or knowing the
    name. I remember writing an "Object Pool" in procedural Pascal in the
    1980s to recycle resources, long before the concept was given a name. Not
    that I claim to have invented the concept -- I merely copied it from
    someone else, who described the technique without giving it a name. Not
    that he invented it either.

    The emphasis on *names* is just jargon, design patterns are actually just
    problem-solving techniques. Sometimes it's useful to have jargon that
    everyone recognises, e.g. "factory function" and "singleton" are two I
    consistently remember (and I use the first *all the time* and the second
    *never*), but often Design Pattern proponents become side-tracked into
    finer and finer differences of greater and greater abstraction, at the
    expense of clarity.

    In my not-so-humble opinion, the popularity of Design Patterns has a lot
    to do with the fact that they are so abstract and jargon-ridden that they
    have become a badge of membership into an elite. Shorn of their excessive
    abstractness, they're not very special. People were writing helper
    functions to assemble complex data long before the Builder pattern was
    named, and a Facade is just an interface layer.

    As far as I am concerned, any language without an interactive interpreter
    is incomplete.
    Steven D'Aprano, Aug 6, 2012

  16. I frequently draw diagrams to understand the relationships between my
    classes and the problem I am trying to solve. I almost invariably use one
    type of box and one type of arrowhead. Sometimes if I'm bored I draw
    doodles on the diagram. If only I could remember to be consistent about
    what doodle I draw where, I too could be an UML guru.
    Steven D'Aprano, Aug 6, 2012
  17. Jean Dubois

    Dan Sommers Guest

    Dan Sommers, Aug 6, 2012
  18. Apparently only 2.4.1 http://www.omg.org/spec/UML/2.4.1/
    Dennis Lee Bieber, Aug 6, 2012
  19. On 06/08/2012 01:22, Steven D'Aprano wrote:

    [snipped to death]
    Design patterns being abstract and jargon ridden puts them alongside
    many other aspects of ICT, CS, call it what you like. Especially
    beloved by consultants as it means they can talk crap for hours and
    charge a fortune for it.
    Mark Lawrence, Aug 6, 2012
  20. I think it's an over-simplification of humans being "actors" who
    drive the software, and not something ("chess pieces") that the software
    should be driving. (OTOH: how many remember that surge in "up-shift"
    indicator lights on manual transmission cars, near the late 70s early
    80s? The few times I drove one of those, it was telling me to up-shift
    as soon as I let out the clutch from the previous up-shift).

    Though to me, that really means there should be no requirements
    phrased as "the operator shall <do something to the software>"; it
    should be "the software shall <respond to operator activity>".

    Taken literally, the prohibition would mean no modeling of Employees
    in a payroll or HR system -- since Employee represents a person working
    for the company.
    While I've probably used singletons (usually as sentinels in queues,
    I suspect), but can't say that I've ever used a "factory function"...
    Dennis Lee Bieber, Aug 6, 2012
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