This course is an introduction to computer science and programming in Python. Upon successful completion of this course, you will be able to: 1. Take a new computational problem and develop a plan to solve it through problem understanding and decomposition. 2. Follow a design creation process that includes specifications, algorithms, and testing. 3. Code, test, and debug a program in Python, based on your design. Important computer science concepts such as problem solving (computational thinking), problem decomposition, algorithms, abstraction, and software quality are emphasized throughout. The Python programming language and video games are used to demonstrate computer science concepts in a concrete and fun manner. However, a learner can take the knowledge and skills from this course and apply them to non-game problems, other programming languages, and other computer science courses. You do not need any previous programming, Python, or video game experience. However, some computer skills (e.g., mouse, keyboard, document editing), knowledge of algebra, attention to detail (as with many technical subjects), and a “just give it a try” spirit will be keys to your success. Despite the use of video games for all the programming examples, PVG is not about computer games. PVG will still provide valuable knowledge and skills for non-game computational problems. The interactive learning objects (ILO) of the course provide automatic, context-specific guidance and feedback, like a virtual teaching assistant, as you develop problem descriptions, algorithms, and functional test plans. The course forums will be supported by the creators of the course, to help you succeed. All videos, assessments, and ILOs are available free of charge. There is an optional certificate available for a fee.
Python User-defined Methods and Self
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來自 University of Alberta 的課程
Problem Solving, Programming, and Video Games
5 個評分
從本節課中
Module 10: Poke the Dots Version 4
In Module 10, you will modify your game design using a new form of control abstraction called user-defined methods. User-defined methods allow you to restrict access to the attributes of a class to improve data abstraction. You will employ user-defined methods to implement Poke the Dots Version 4.
與講師見面
Duane SzafronProfessor
Computing Science
Paul LuProfessor
Computing Science
This video introduces user defined methods.
A function that is defined inside a class is called a method.
Every method must be applied to an object
whose type is the class where the method is defined.
Recall from the method calls lesson that the method lower is defined in the string class.
So, it can be applied to a string object to return a similar string object,
but with all uppercase letters converted to lowercase letters.
Recall the string method find returns the index
of a strain where the first occurrence of a substring starts.
In this case, the first occurrence of the substring
'on' is at index three of the string 'Edmonton'.
The object that the method is applied to is a special argument,
in addition to its normal arguments.
The special argument of the method lower is 'R2D2'.
The special argument of the method find is 'Edmonton'.
But find also has an ordinary argument 'on'.
A user defined method is created by placing
a function definition inside the suite of the class that defines it.
One extra parameter is added to the start of the function definition,
and this parameter is bound to the special argument when the method is called.
Consider the program from the class definition lesson that checks to
see if two rectangles intersect or overlap.
Each rectangle object has four attributes: x, y, width,
and height, where x and y are
integer coordinates of the top-left corner of the rectangle.
Here is a new version of the program,
where the create rectangle and rectangle's intersect functions have been replaced by
the methods underscore underscore init underscore underscore and intersects respectively.
I will explain why I have used underscores later.
I will use this new rectangle program to show you how user defined methods are defined.
When the program starts,
the local block is the main module,
whose namespace is both the local and global namespace.
Although the prebound identifier print is bound in this namespace,
I'll leave it out to simplify the diagram.
The first statement in the main module is a function definition.
So, the interpreter applies function definition semantics.
It creates a function object with its own namespace.
It adds the identifier main to the local namespace and binds main to the function object.
It then binds a reference to the main module namespace in the main function namespace.
But I'll leave this global reference out to simplify the diagram.
The next statement in the main module is a class definition.
So, the interpreter applies class definition semantics.
In step one, the interpreter creates a new empty namespace as
the local namespace and uses the current global namespace to evaluate the suite.
The suite consists of two function definition statements.
So, function definition semantics are used to evaluate each function definition.
Notice that the function names are added to the local namespace,
which is the new local namespace that will be used by the class.
Once again, I won't add a global reference to simplify the diagram.
Returning to step two of the semantics for class definition,
a new class object is created,
and its namespace is the new local namespace.
So, when a function definition appears inside a class,
the function name is added to the namespace of the class object.
This is what makes a function a method.
Any function whose name is bound inside the namespace of
a class object is created as a method object instead of a normal function object.
So, both init and intersects are
method objects since they are defined in the rectangle class.
In step three, the class name rectangle is added to the original local namespace,
the namespace of the main module.
In step four, rectangle is bound in the original local namespace to the new class object.
The last statement in the main module is a function call to main.
So, the interpreter applies the function call semantics.
The identifier main is dereferenced in
the local namespace to obtain the main function object.
The main function code is evaluated to obtain a result object.
The local block is now the main function,
and the local namespace is the main function namespace.
The first statement is
an assignment statement whose expression is a function call to rectangle,
which is a class object.
In step one of the function call semantics,
the identifier rectangle is dereferenced in
the global namespace to obtain a class object.
Since every class object is a function,
no semantic error occurs.
In step two, the four argument expressions, 5, 10,
15, and 20, are evaluated and put in an argument list.
In step three, rectangle is not a method object.
So, no special argument is added to the argument object list.
In step four, the argument list length is four.
The function object is the rectangle class instead of a normal function.
How many parameters does a class object have when it is used as a function?
I must generalize the function call semantics to support calls to
functions which are class objects by adding a new step three.
If the function object is a class,
create a new object whose type is that class,
then apply its init method to the new object,
and return the new object as the result object.
I will now backup and apply this new step three.
The rectangle object is a class object.
So, a new rectangle object is created,
and its init method is applied to the new rectangle.
In step four, since the definition of init is inside the rectangle class,
the init function object is a method.
Because init is being applied to the new rectangle,
the new rectangle is added to the argument list of init.
In step five, the argument list length is five,
and the parameter list length is five.
So, no error is reported.
In step six, the interpreter adds to the parameters special, corner_x, corner_y, width,
and height to init's namespace,
binds each parameter to its corresponding argument,
and evaluates init's code.
The local block is now the init method,
and the local namespace is the init method's namespace.
Each of the four statements in init adds an attribute to
the rectangle object and binds the attribute to an argument object.
There is no return statement in init.
So, you might think that the non-object is returned by the rectangle class function.
In fact, init does return the non-object.
However, step three of the revised function calls semantics indicates that
the created rectangle object is returned as
the result object of the call to the rectangle class function.
The identifier rect1 is bound to this rectangle object.
The next assignment statement in the main function is interpreted similarly.
So, the identifier rect2 is bound to a second rectangle object.
The next statement in the main function is
an if statement whose condition matches the syntax of a function call.
So, function call semantics are applied.
In step one, the function expression is the attribute reference rect1.intersects.
The interpreter applies the semantic rule for attribute reference.
The interpreter dereferences rect1 to obtain
the first rectangle object and looks for
the attribute intersects in the namespace of this first rectangle.
Intersects is not in the namespace of the first rectangle.
So, an attribute error should be reported.
However, no error is reported.
When the class definition was evaluated,
its method names were added to the namespace of the class.
The names were not added to the namespace of any object from that class.
In fact, no objects from that class even existed when the class definition was evaluated,
and the class was created.
Method names are put in the classes' namespace and are shared with all of its objects.
An object uses the shared method names from its class.
When an attribute is not found in an object,
the interpreter searches for that attribute in its class.
I will generalize our simplified semantics of attributes reference to reflect this.
If the attribute is in the namespace of the object,
return the object it is bound to.
Otherwise, if the attribute is in the namespace of the object's class,
return the object it is bound to.
Since the interpreter does not find the method name
intersects in the namespace of the rectangle object,
it searches in the namespace of
the rectangle class and dereferences it to obtain the intersects method object.
Normal function call semantics continue.
So, the argument list is created by evaluating
rect2 to obtain a one-element argument object list,
and the special argument is added to the start of this argument list.
Intersects parameters are added to its namespace and bound to the two argument objects.
The intersects method is evaluated and returns true.
So, the if statement suite is evaluated and displays overlap in the shell.
The main function ends,
and so does the program.
This program uses special as the name
of the parameter that is bound to the special object.
Any name can be used for the initial parameter of a method.
However, Python has a convention that self is used as the initial parameter name.
All changed the name to self in the program to follow this convention.
The previous rectangle program did not have
an underscore underscore init underscore underscore method in the class definition.
So, step three of the function call semantics doesn't make sense.
If the class definition does not contain
an underscore underscore init underscore underscore method,
Python applies a default init method that doesn't do anything.
The underscores must be used so that Python can find your init method.
This video introduced user defined methods.
