Lecture 3 - Overview. Object-oriented programming and classes Operators Very briefly about naming conventions Graphical user interfaces (GUIs)

Transcription

1 Lecture 3 - Overview Object-oriented programming and classes Operators Very briefly about naming conventions Graphical user interfaces (GUIs)

2 Object-oriented philosophy Object = Data + Algorithm An object is a self-contained unit The object has a state Hiding internal structure Origin: The software is a model

3 Terminology Attribute: An objects public interface. In Python this can be both public instance variables and methods. Argument: Is here used for the variables that are sent to a method or function. Parameter: Equivalent to argument (in parameter) and return value (out parameter) Instance variables: Variables that are connected to a certain object. In Python these are (almost) always public, which makes them to attributes automatically. Class variable: Variables which are connected to a certain class, not an instance of a class. Can exists without any instances of the class.

4 A class with instance methods Methods are defined the same way as functions but inside a class definition. The first argument to a method is always a reference to the object that has called the method. The (strong) convention is to call this argument self. Instance variables are created with: self.variablename. # classes_02.py class SimpleClass: def assign_name(self, name): self.name = name def print_name(self): print "I'm %s." % self.name girl = SimpleClass() girl.assign_name("abby") girl.print_name() boy = SimpleClass() boy.assign_name("daniel") boy.print_name() $> python classes_02.py I'm Abby. I'm Daniel.

5 Constructors Constructors have the reserved name init (double underscores in the beginning and the end). The constructor can take arguments just as an ordinary method. # classes_03.py class SimpleClass: def init (self, name="unknown"): self.name = name $> python classes_03.py I'm Abby. I'm Unknown. def print_name(self): print "I'm %s." % self.name girl = SimpleClass("Abby") girl.print_name() unknown = SimpleClass() unknown.print_name()

6 Destructors Destructors have the reserved name del and are called when an object is deleted. # classes_04.py class SimpleClass: def del (self): print "Object 0x%08X is closing down." \ % id(self) print "Creating an object." obj_1 = SimpleClass() obj_1 = 1 # Implicit deletion print "Creating another one." obj_2 = SimpleClass() del(obj_2) # Explicit deletion $> python classes_04.py Creating an object. Object 0x00AD3C88 is closing down. Creating another one. Object 0x00AD3C88 is closing down.

7 Class variables Class variables are used to handle information that belongs to the class rather than specific instances of the class. The classic example is a variable that keeps track of how many instances of a class that exists at a given time point.

8 Class variables, example # classes_05.py class SimpleClass: # Class variable instances = 0 def init (self): self. class.instances += 1 print "Creating object %08X." \ " instances = %i" \ % (id(self), self. class.instances) def del (self): self. class.instances -= 1 print "Object 0x%08X is closing down." \ " instances = %i" \ % (id(self), self. class.instances) Creating object 023D3490. instances = 1 Creating object 023D34B8. instances = 2 Creating object 023D34E0. instances = 3 Creating object 023D3508. instances = 4 Creating object 023D3530. instances = 5 Now there are 5 instances. Object 0x023D3530 is closing down. instances = 4 Object 0x023D3508 is closing down. instances = 3 Object 0x023D34E0 is closing down. instances = 2 Object 0x023D34B8 is closing down. instances = 1 Object 0x023D3490 is closing down. instances = 0 Now there are 0 instances. objects = [] for i in range(5): objects.append(simpleclass()) print "Now there are %i instances." % objects[0].instances del objects print "Now there are %i instances." % SimpleClass.instances

9 Redefining classes # classes_06.py class SimpleClass: def init (self): print "Creating an instance of the first version." def print_info(self): print "I'm an instance of the first version." obj_1 = SimpleClass() obj_1.print_info() class SimpleClass: def init (self): print "Creating an instance of the second version." def print_info(self): print "I'm an instance of the second version." obj_2 = SimpleClass() obj_2.print_info() obj_1.print_info() We can redefine a class at run-time Objects of the old class keeps its methods. $> python classes_06.py Creating an instance of the first version. I'm an instance of the first version. Creating an instance of the second version. I'm an instance of the second version. I'm an instance of the first version.

10 Private instance variables Private instance variables and methods are created with two underscores in the beginning of the name It's still possible to access the function # private_01.py class SimpleClass: def init (self): self. secret_data = 19 def secret(self): print "Secret data: %i" \ % self. secret_data def public_method(self): self. secret() obj = SimpleClass() obj.public_method() print obj. secret_data obj. secret() $> python private_01.py Secret data: 19 Traceback (most recent call last): File "private_01.py", line 17, in <module> print obj. secret_data AttributeError: SimpleClass instance has no attribute ' secret_data'

11 Regarding get and set methods In Java get and set methods are often used to handle private instance variables. This is seen as needless in Python, even though it's possible. The philosophy is that it's up to the programmer to take responsibility over the code and what it's doing. If you want to access some inner part of an object shouldn't the language stop that. Documentation becomes more important when objects aren't as protected.

12 Inheritance A hierarchical class structure can reduce the code base and makes the relationship between different classes clearer. Through inheritance can a new class reuse existing methods and instance variables from an existing class. Constructors in base classes aren't called automatically.

13 Example of inheritance # -*- coding: utf-8 -*- # car_01.py class Car: def init (self, distance=0): self.distance = distance def repr (self): return "Travelled %i miles." \ % self.distance Car: Travelled 10 miles. Taxi: Travelled 13 miles. The trip costs 455 kr. class Taxi(Car): def init (self, distance=0, fare=10): Car. init (self, distance) self.fare = fare def total_fare(self): return self.distance * self.fare def repr (self): return Car. repr (self) + \ "\nthe trip costs %i kr." \ % self.total_fare() car = Car(10) taxi = Taxi(13, 35) print "Car:\n", car print "Taxi:\n", taxi

14 Operators Operators are characters with special meaning in the language. These include the standard mathematical operators as +, -, * and /, but also more programming oriented operators as [...] (for both getting and setting a value). Overloaded operators make it possible to use normal mathematical syntax for the builtin and own classes.

15 Operatorer are syntactical sugar Operators work in Python as special methods >>> a = "Hello " >>> b = "world!" >>> a + b 'Hello world!' >>> a. add (b) # Equivalent 'Hello world!' Note that a and b have different parts in the addition. a becomes the called object and b the argument. In other words: It's the add -method of a that will be called. A consequence of this is that addition not necessarily is a symmetric operation in Python. a + b can't be assumed to be the same as b + a.

16 Example 1: Addition with a class An overloaded operator can carry out anything. There is no requirement that operators shall do something that resembles the mathematical equivalent, even if it's strongly recommended. # -*- coding: utf-8 -*- # operators_01.py class ClassA: def init (self, text = "Default string"): self.text = text $> python operators_01.py Default stringdefault string def add (self, other): return ClassA(self.text + other.text) obj_1 = ClassA() obj_2 = ClassA() obj_3 = obj_1 + obj_2 print obj_3.text

17 Example 2: Different classes # -*- coding: utf-8 -*- # operators_02.py class ClassA: def init (self, text="instance of ClassA"): self.text = text def str (self): return self.text def add (self, other): return ClassA(str(self) + " + " + str(other)) class ClassB: def init (self, text="instance of ClassB"): self.text = text def str (self): return self.text def add (self, other): return "Addition not allowed with ClassB first." obj_a = ClassA() obj_b = ClassB() print obj_a + obj_a # => Instance of ClassA + Instance of ClassA print obj_a + obj_b # => Instance of ClassA + Instance of ClassB print obj_b + obj_a # => Addition not allowed with ClassB first.

18 Example 3: Indexing To read values getitem is used. To set values setitem is used. # -*- coding: utf-8 -*- # operators_03.py import random class ColorRepo: colors = ["Yellow", "White", "Black", "Pink", "Brown"] def getitem (self, index): return self. class.colors[index % len(self. class.colors)] cr = ColorRepo() for i in range(5): print cr[random.randint(0, 30000)]

19 Mathematical operators add (self,other) Addition, + sub (self,other) Subtraction, - mul (self,other) Multiplication, * div (self,other) Division, / floordiv (self,other) Integer division, // mod (self, other) Rest, % pow (self, other) Power of, ** or pow and (self, other) Logical and, and or (self, other) Logical or, or xor (self, other) Logical exclusive or, xor

20 Formatting and naming Variable names can't begin with a number. Conventions (PEP-8): 4 spaces for indentation most common Never mix spaces and tabs Limit all lines to a maximum of 79 characters. Classes begin with a capital letter Functions with lower case Multiple words in function names should preferably be separated with _: function_name

21 GUI: Graphical User Interface

22 Overview What is a GUI? Why do we need it? How to code in Python (one way) Difference in programming style: Event-driven prog. vs linear prog.

23 GUI examples The graphical frontend visible to the user

24 Why have a GUI? Increase the usability Easier to learn Not always the best solution, it's easier to make scripts for command line programs for example. Is perceived as more user friendly

25 GUI-library TkInter De facto standard, comes with almost all standard installations. Stands for Tk interface and can be seen as a connection library between Tk GUI toolkit and Python (i.e. an adapter or wrapper) One of several ways of creating graphics in Python (other GUI library: wxpython, 3D graphics: VPython)

26 What objects does a GUI contain? Window Color area Frame Label Button Button

27 A first code example # first_example.py from Tkinter import * root = Tk() my_widget = Label(root, text="hello world") my_widget.pack() root.mainloop()

28 Breakdown of the program # first_example.py from Tkinter import * root = Tk() my_widget = Label(root, text="hello world") my_widget.pack() root.mainloop() The program follows the standard way in Tk to create widgets: 1. Create a main container, in this case a Tk-object. 2. Create a widget of the type you want, here a Label-object. 3. Define the characteristics of the widget, for example color and form. 4. Call the pack-method to add the widget in the interface. 5. Repeat step 2-4 for each widget. 6. Call the mainloop-method for the main container to show it and all the underlying widgets on the screen.

29 Packing widgets The packer is called for each widget with a request of how the widget should look and act. The requests are partly regarding placement and extension in a container, and partly about how the widget should change when the container changes size. The packer tries to meet all requests. If two requests are in conflict, the packer will take regard to the packing order, that is in which order pack has been called.

30 Placement A widget's placement is governed by the argument to the pack-method. Sides in the container are denoted with side and placement along the sides with anchor. How the widget should adapt to changes in the main container is set with expand and fill.

31 Example on placements # graphics02.py from Tkinter import * Button(text="A button").pack() mainloop() # graphics03.py from Tkinter import * Button(text="A button").pack(anchor=ne) mainloop() # graphics04.py from Tkinter import * Button(text="A button").pack(expand=yes, fill=both) mainloop()

32 Several widgets # graphics07.py from Tkinter import * Button(text="Button 1").pack(side=TOP, fill=x) Button(text="Button 2").pack(side=RIGHT, fill=y) Button(text="Button 3").pack(expand=YES, fill=both) mainloop()

33 Common widgets Label. Text label. Button. Clickable button. Radiobutton. Choose between of many. Checkbutton. Choose any number of alternatives. Entry. Read a shorter string. Frame. Arrange different groups of widget.

34 Multiple frames from Tkinter import * from tkmessagebox import * class TestGUI: def init (self, parent): # Create frames for the GUI frametop = Frame(parent) frametop.pack() framebottom=frame(parent, bg="blue") framebottom.pack(expand=yes, fill=both) frametopleft=frame(frametop) frametopleft.pack(side=left) frametopright=frame(frametop) frametopright.pack(side=right) # Add buttons Button(frameTopLeft, text="launch window", \ command=self.show_message).pack() Entry(frameTopRight).pack() Button(frameBottom, text="exit", \ command=parent.destroy).pack() def show_message(self): showinfo("message title", "Message text") if name == ' main ': root = Tk() root.title("test window") app = TestGUI(root) root.mainloop()

35 More advanced widgets Menu. Ordinary menus. Menubutton. Buttons that open a menu. Canvas. Painting area for composed graphics. Text. Text box for longer texts. Dialog boxes (not really widgets).

36 Classes and inheritance in TkInter Each type of widget in TkInter is a class. Each widget in a GUI is an object. The main program can be created as subclasses to TkInter-classes, for example Tk or Toplevel

37 Event handling The purpose of the GUI is to communication efficiently with the user, so it must be able to take input from the user and act according to this. Input can either be through the mouse, keyboard or other similar equipment. The focus on events change the program structure and the design. Suitable for modularisation and object oriented programming.

38 Linear or event driven program In a linear program: Things are carried out in a predetermined order. All input is done at predetermined occasions. In an event driven program: There is often a central waiting loop (main loop) Program pieces called event manager are executed depending on which event that occur. Note that event driven programs doesn't necessarily have to be dependent on a GUI.

39 Control variables Used to smoothly handle data from widgets. Can connect several widgets which in that way are kept synchronized. The content can either be changed by the user or by the program through the control variable.

40 Example with control variables # events07.py from Tkinter import * root = Tk() t = StringVar() # Create control variable t.set('this value is given to both objects.') # Set start value # The button's and the text box text is connected to the same variable. Button(root,textvariable=t).pack(fill=X) Entry(root, textvariable=t).pack(fill=x) root.mainloop()

41 Check boxes and control variables # checkbutton01.py from Tkinter import * def printform(): if haslicence.get(): print 'The person has a driver's license.' if hasowncar.get(): print 'The person has a car.' import sys sys.exit() root = Tk() haslicence = IntVar() Checkbutton(root,text = 'Driver's license', variable =haslicence).pack(anchor=w) hasowncar = IntVar() Checkbutton(root,text = 'Car',variable = hasowncar).pack(anchor=w) Button(root,text = 'Save',command = printform).pack(anchor=s) root.mainloop()

42 Group radio buttons # radiobutton01.py from Tkinter import * def onmealclick(): print mealvar.get() def ondrinkclick(): print drinkvar.get() root = Tk() mealvar = StringVar() meals = ['Breakfast','Lunch','Dinner'] for meal in meals: Radiobutton(root,command = onmealclick, text = meal, value = meal, variable = mealvar).pack(anchor=w) drinkvar = StringVar() drinks = ['Milk','Water','Soft drink'] for drink in drinks: Radiobutton(root,command = ondrinkclick, text = drink, value = drink, variable = drinkvar).pack(anchor=w) root.mainloop()