Lecture 3 - Overview. More about functions Operators Very briefly about naming conventions Graphical user interfaces (GUIs)

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1 Lecture 3 - Overview More about functions Operators Very briefly about naming conventions Graphical user interfaces (GUIs)

2 Function parameters Passed by reference, but the standard implication that the outside value is always changed when the parameter is altered inside the function doesn't hold. Immutables (such as integers) can't change so it won't change outside either Mutables (such as lists) are changed in the outside scope as well.

3 Parameter example # parameter_test.py def set_zero(x): print "id(x): ", id(x) if type(x) == list: x[0] = 0 elif type(x) == int: x = 0 $> python paremter_test.py id(a): id(b): id(x): id(x): [0, 1, 1] 1 a = [1,1,1] b = 1 print "id(a): ", id(a) print "id(b): ", id(b) set_zero(a) set_zero(b) print a print b

4 Functions - optional and named parameters def function_name(parameter_name=default_value): function_code_block When a function defines a parameter with a default value, that parameter and all afterwards are optional (and must also have default values). It is possible to pass only a later parameter

5 Optional parameter example # function_05.py def greeting(name="unknown", age=20): print "Hi %s. You are %i years old.""" % (name, age) greeting() greeting("carl") # Still works greeting(name="carl") # Equivalent greeting(age=45) # Gives the right result greeting("sara", 33) greeting(name = "Sara", age = 33) # Equivalent as previous line greeting(24) $> python function_05.py Hi Unknown. You are 20 years old. Hi Carl. You are 20 years old. Hi Carl. You are 20 years old. Hi Unknown. You are 45 years old. Hi Sara. You are 33 years old. Hi Sara. You are 33 years old. Hi 24. You are 20 years old.

6 Functions with many return values You can return several values If several variables are assigned the output, the will be defined consecutively The values are passed via a tuple # return_values_03.py def min_max(seq): return min(seq),max(seq) a = [3, 573, 234, 24] minimum, maximum = min_max(a) print minimum, maximum result = min_max(a) print result print result. class $> python return_values_03.py (3, 573) <type 'tuple'>

7 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.

8 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.

9 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

10 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.

11 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)]

12 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

13 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

14 Graphical user interfaces

15 Graphical user interfaces (GUIs) Outline: Why are there graphical user interface? How are they built? Create graphics in Python. A few examples. Event handling.

16 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

17 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)

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

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

20 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.

21 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.

22 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.

23 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()

24 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()

25 Several widgets # graphics08.py from Tkinter import * Button(text="Button 2").pack(side=RIGHT, fill=y) Button(text="Button 1").pack(side=TOP, fill=x) Button(text="Button 3").pack(expand=YES, fill=both) mainloop()

26 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.

Multiple frames from Tkinter import * from tkmessagebox import * class TestGUI: def init (self, parent): # Create frames for the GUI frametop = Frame(parent) frametop.

27 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()

28 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).

29 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

30 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.

31 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.

32 A first event handler # events01.py from Tkinter import * def onleftclick(event): print event. class label.configure(text="you clicked on the left mouse button.") def onrightclick(event): label.configure(text="you clicked on the right mouse button.") root = Tk() label = Label(root, text="") label.pack(fill=x) button = Button(root, text="click on any mouse button") button.bind('<button-1>', onleftclick) # Connect left click button.bind('<button-3>', onrightclick) # Connect right click button.pack(fill=x) root.mainloop()

33 About the event handler Can be a function or a method Takes one argument, an Event-object that contain information about the event that took place and the widget that generated it. The same event handler can handle events from several different widgets. When the event handles is closed the program returns to the wait state again.

34 Defining an event An event is defined by a sequence of identifiers together with one or more modifiers. Identifiers are for example Button-n, Activate, FocusIn, FocusOut. Modifiers are for example Alt, Control och Shift. <Control-ButtonRelease-1> denotes that the left mouse button have been clicked and released, while the control-key was also pressed.

35 More common events Activate/Deactivate. The widget change state. Configure. The widget change size. Double-n. Mouse button n has been double-clicked. KeyPress-key. key (for example Alt) has been pressed. KeyRelease-key. key has been released. Enter. The mouse pointer is over the widget. Leave. The mouse pointer has left the widget.

36 The event object Other usable attributes for the event object are: time, serial, widget, x and y. # events05.py from Tkinter import * def onmouseclick(event): print 'You clicked with button', event.num root = Tk() button = Button(text="Click on me") button.bind('<any-button>', onmouseclick) button.pack(fill=x) root.mainloop()

37 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.

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.

38 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()

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.

39 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()

40 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()

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

Lecture 3 - Overview. Object-oriented programming and classes Operators Very briefly about naming conventions Graphical user interfaces (GUIs) Lecture 3 - Overview Object-oriented programming and classes Operators Very briefly about naming conventions Graphical user interfaces (GUIs) Object-oriented philosophy Object = Data + Algorithm An object