>I want to make >sure you guys, especially Alan & Dave, are appreciated for taking their >own personal time to do this, for free.
Your bill will arrive shortly ;) Ramit Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology 712 Main Street | Houston, TX 77002 work phone: 713 - 216 - 5423 -- -----Original Message----- From: tutor-bounces+ramit.prasad=jpmorgan....@python.org [mailto:tutor-bounces+ramit.prasad=jpmorgan....@python.org] On Behalf Of Chris Kavanagh Sent: Monday, October 24, 2011 6:17 AM To: Alan Gauld Cc: tutor@python.org Subject: Re: [Tutor] Simple Question On A Method (in subclass) On 10/24/2011 4:40 AM, Alan Gauld wrote: > On 24/10/11 04:08, Chris Kavanagh wrote: > >> Thanks so much for the help Alan. . .I'm not trying to beat this >> question into the ground, LOL, but let me see if I can ask it a better >> way. > > Marc has already given a good answer, but I'll try a slightly different > approach to the same thing.... The differences are purely philosophical :-) > >> So we have {member.tell} as the last line of code. So trying to >> understand this piece of code, {member} the variable is considered an >> object? > > Almost. > > member is an object because it is an instance of a class. > We created instances of Teacher and Student (t and s) and put them in a > list (members) with this code: > > t = Teacher('Mrs. Shrividya', 40, 30000) > s = Student('Swaroop', 22, 75) > members = [t, s] > > Thus for member in members takes each object in the list and assigns it > to member. > > We can read {for member in members} as > > "Let member take on, in turn, each value in the collection members" > > And making it more explicit with a while loop: > > index = 0 > while index < len(members): > member = members[index] > member.tell() > index += 1 > > > Therefore we can combine it with a function {tell()} using dot > > notation?? Is this correct??? I haven't seen anything but an > > object combined with a function using dot notation. > > dot notation is how we access anything that is defined inside the class > definition. In this case there are only the __init__() and tell() > methods but there could be data attributes too. Furthermore we can add > data attributes to the objects by using the self parameter, as is done > in __init__() So we end up with the instance containing the attributes > defined at the class level (including those of its superclass) plus any > added by the __init__() method which is called when the object is > created (or instantiated). So for Teacher the attributes will be: > > __init__() > tell() > name - from SchoolMember.__init__() > age - ditto > salary - from Teacher.__init__() > > So we could have accessed any of those for the first iteration of the > loop because member was a Teacher instance (t) using: > > member.tell, member.name, member.age, member.salary > > But the last one would have broken for the second object in the list > since it was a student, which doesn't have a salary. So when dealing > with a list of objects we have to restrict the access via dot notation > to those attributes that are common to all objects in the collection - > usually the ones defined in the super class. > > Python doesn't care whether the attributes we access are data or > functions because in Python everything is an "object" (see Marc's post) > But by putting parentheses after the object name Python treats the named > object as a function (specifically a "callable" object) > > You can see this diffence at work in the interpreter if you > type this: > > >>> def f(): > ... return "Called f" > ... > >>> print f > <function f at 0x3180c88> > >>> print f() > Called f > > See the difference the () made? Without it we print the function object > called f. With it we print the result of calling the function f. > Thats quite a subtle concept and usually hard for beginners to grasp but > it is a very powerful concept and you will find it being used in > more advanced programs, especially GUI development. > > > So I'm trying to figure out how we can >> combine the variable {member} with the function {tell}. Hope this >> question makes sense to you, LOL. Thanks again. > > How we combine it is done inside Python as part of the magic of classes > and instantiation. Basically you can call any function defined inside a > class and Python will automatically assign the first parameter of that > function to the object instace that you are referencing at the time. > > Thus when we do > > for member in members: > member.tell() > > Python recognises that first time round member refers to the object t > which is an instanmce of Teacher. It then calls the tell() method within > the Teacher class definition and assigns t as the self parameter. > We could write it explicitly as: > > Teacher.tell(member) # where member is t > > On the second time round member is assigned to s and Python recognizes > that s in an instance of Student. So it calls Student.tell(member) where > member is s. > > The member.tell() notation is just a convenient way for us to write it > and allows Python to do the hard work of figuring out what kind of > object member is at any given time. > > [Aside: In true OOP speak we describe the process as "sending a message > to the object. Thus member.tell() sends the message "tell" to the object > "member" and the OOP framework invokes the corresponding method tell() > in the corresponding class. In some OOP languages you can use different > names for the messages and corresponding methods but in Python they are > the same so we just talk about calling the method.] > > HTH, > Yes, both of you gave great answers. I just couldn't get it straight in my head for some reason. And I do get it now. I just wasn't seeing {member} as an instance of a class. I guess because I'm used to seeing simple straightforward examples of classes & objects so far. Thank you guys so much for the help. You don't know how nice it is to be able to ask someone who is an expert a question, and get an answer! Thanks to everyone who contributes, I've learned a lot by reading others questions. And THANKS for providing this awesome service! _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor This email is confidential and subject to important disclaimers and conditions including on offers for the purchase or sale of securities, accuracy and completeness of information, viruses, confidentiality, legal privilege, and legal entity disclaimers, available at http://www.jpmorgan.com/pages/disclosures/email. _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
