<< Chapter < Page Chapter >> Page >

General background information

In Java, C#, and C++, once you define and compile a class, that class, (which is the blueprint for an object) , can only be modified by recompiling the class. In other words, once compiled, a class is intended to be stable and in generalcannot be modified at runtime.

Also in Java and C++, once you instantiate an object from a class, you cannot modify the structure of the object. You can modify the values of the data storedin the object but the structure of the object is stable and cannot be modified at runtime.

Along that line, the structure of an object in Java and C++ is explicitly tied to the blueprint provided by the class from which it was instantiated.There is a fixed relationship between the object and the class from which it was instantiated.

None of that is true in Python. You can modify the structure of a Python object at runtime after it is instantiated. By that I mean that you can add newinstance variables to an object such that the structure of the object is different from the blueprint provided by the class from which it wasinstantiated. Further, you can modify the blueprint provided by the class at runtime after the class has been used to instantiate one or more objects.

I'm not suggesting that this is either good or bad. I'm simply stating that itis true. If you are coming at Python from a Java or C++ background, you must significantly modify the way that you think about classes and objects to makethe transition. If you plan to progress from Python to either Java or C++, your will need to significantly modify the way that you think about classes andobjects when you make that transition.

Discussion and sample code

I will discuss three different short programs in this module. A complete listing of the first program is provided in Listing 9 . The output from that program is shown in Figure 5 .

I will break each program down and discuss it in fragments. The first fragment is shown in Listing 1 .

Empty objects

Listing 1 shows the definition of a class named TestClass that contains no variables and no methods. In other words, the class isempty.

Listing 1 . Empty objects.
class TestClass(object): passprint("Instantiate and display two objects of TestClass") ref01 = TestClass()print(ref01) ref02 = TestClass()print(ref02)

Instantiate two objects of the empty class

Listing 1 also shows the instantiation of two objects from the empty class named TestClass . Those object's references are stored in the variables named ref01 and ref02 .

The variables are also printed in Listing 1 producing the output shown in Figure 1 .

Figure 1 . Print the object's references.
Instantiate and display two objects of TestClass<__main__.TestClass object at 0x00274910><__main__.TestClass object at 0x005A25F0>

I told you in an earlier module that the variables named ref01 and ref02 contain information that somehow identifies the chunks of memory occupied by the objects. Thisis indicated by the two hexadecimal values at the right end of the last two lines of text in Figure 1 . This is apparently one thing that can't be changed about an object after it is instantiated. Even after modifying thecontents of an object, the hexadecimal identifier seems to remain constant.

Questions & Answers

if three forces F1.f2 .f3 act at a point on a Cartesian plane in the daigram .....so if the question says write down the x and y components ..... I really don't understand
Syamthanda Reply
hey , can you please explain oxidation reaction & redox ?
Boitumelo Reply
hey , can you please explain oxidation reaction and redox ?
Boitumelo
for grade 12 or grade 11?
Sibulele
the value of V1 and V2
Tumelo Reply
advantages of electrons in a circuit
Rethabile Reply
we're do you find electromagnetism past papers
Ntombifuthi
what a normal force
Tholulwazi Reply
it is the force or component of the force that the surface exert on an object incontact with it and which acts perpendicular to the surface
Sihle
what is physics?
Petrus Reply
what is the half reaction of Potassium and chlorine
Anna Reply
how to calculate coefficient of static friction
Lisa Reply
how to calculate static friction
Lisa
How to calculate a current
Tumelo
how to calculate the magnitude of horizontal component of the applied force
Mogano
How to calculate force
Monambi
a structure of a thermocouple used to measure inner temperature
Anna Reply
a fixed gas of a mass is held at standard pressure temperature of 15 degrees Celsius .Calculate the temperature of the gas in Celsius if the pressure is changed to 2×10 to the power 4
Amahle Reply
How is energy being used in bonding?
Raymond Reply
what is acceleration
Syamthanda Reply
a rate of change in velocity of an object whith respect to time
Khuthadzo
how can we find the moment of torque of a circular object
Kidist
Acceleration is a rate of change in velocity.
Justice
t =r×f
Khuthadzo
how to calculate tension by substitution
Precious Reply
hi
Shongi
hi
Leago
use fnet method. how many obects are being calculated ?
Khuthadzo
khuthadzo hii
Hulisani
how to calculate acceleration and tension force
Lungile Reply
you use Fnet equals ma , newtoms second law formula
Masego
please help me with vectors in two dimensions
Mulaudzi Reply
how to calculate normal force
Mulaudzi
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Itse 1359 introduction to scripting languages: python' conversation and receive update notifications?

Ask