Fundamentals of Programming. Functions Redux. Event-based Programming. File and Web IO. November 4th, 2014

Transcription

1 Fundamentals of Programming Functions Redux. Event-based Programming. File and Web IO. November 4th, 2014

2 Today Briefly show file and web IO. Revisit functions. Learn a bit more about them. Event-based programming in Tkinter.

3 File and Web Input/Output

4 File I/O 3 Steps 1. Opening a file Associate file on hard disk with an object in your program. 2. Operations to manipulate the file object - read the information in the file - write new information to the file 3. Closing the file Any bookkeeping to maintain the interaction file on disk file object in program is ended.

5 File I/O reading and writing fin = open( funnyquotes.txt, r ) text = fin.read() fin.close() fout = open( funnyquotes.txt, w ) text = Laugh and the world laughs with you, snore and you sleep alone. fout.write(text) fout.close()

6 Functions for reading and writing def readfile(filename, mode="rt"): # rt = "read text" with open(filename, mode) as fin: return fin.read() text = readfile( funnyquotes.txt ) def writefile(filename, contents, mode="wt"): # wt = "write text" with open(filename, mode) as fout: fout.write(contents) s = Laugh and the world laughs with you, snore and you sleep alone. writefile( funnyquotes.txt, s) (open the file, then close it, even if an exception occurs)

7 Function for reading a webpage import urllib import contextlib def readwebpage(url): assert(url.startswith(" with contextlib.closing(urllib.urlopen(url)) as fin: return fin.read() html = readwebpage( ) html contains the HTML code for the webpage. You need to extract the information you want.

8 Functions Revisited

9 Functions: First class objects Functions are first-class citizens: Can use them like you use any other object. (in Python, pretty much everything is an object) - Can pass functions as arguments to other functions - Functions can be return values for other functions - Functions can be assigned to other variables, or can be stored in data structures (e.g. lists)

10 Functions: First class objects def f(x): return x g = f print g(5) print dir(f) [' call ', ' class ', ' closure ', ' code ', ' defaults ', ' delattr ', ' dict ', ' doc ', ' format ', ' get ', ' getattribute ', ' globals ', ' hash ', ' init ', ' module ', ' name ', ' new ', ' reduce ', ' reduce_ex ', ' repr ', ' setattr ', ' sizeof ', ' str ', ' subclasshook ', 'func_closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals', 'func_name']

11 Functions: First class objects def testsort(sortfn, n): a = [random.randint(0, 2**31) for i in xrange(n)] start = time.time() sortfn(a) end = time.time() return end - start sortfunctions = [selectionsort, bubblesort, mergesort] n = 2**12 for sortfn in sortfunctions: testsort(sortfn, n)

12 Functions: Default argument values def myprint(x, times=1): for i in xrange(times): print x myprint( Hello ) myprint( Hi, 5) Hello Hi Hi Hi Hi Hi

13 Functions: Default argument values print cmp( fred, wilma ) print cmp( wilma, fred ) print cmp( fred, fred ) # -1 # 1 # 0 names = [ fred, wilma, barney, betty, dino ] print sorted(names) # [ barney, betty, dino, fred, wilma ] # sorted uses the default cmp function as a default argument def comparelengths(a, b): return len(a) - len(b) print sorted(names, comparelengths) # [ fred, dino, wilma, betty, barney ]

14 Functions: Default argument values Need to be careful with default argument values. def f(x, a=[]): a.append(x) print a f(1) f(2) # expect: [1] # expect: [2] reality: [1] reality: [1, 2] Default argument is evaluated once when function is defined.

15 Functions: Default argument values Need to be careful with default argument values. def f(x, a=[]): a.append(x) print a f(1) f(2) # expect: [1] # expect: [2] reality: [1] reality: [1, 2] defaulta > [] if given as input, use that. if not given as input, use an alias of defaulta. Default argument is evaluated once when function is defined.

16 Functions: Keyword arguments def f(x, y, z): print x, y, z def f(x, y=0, z=0): print x, y, z f(1, 2, 3) f(1, z=3, y=2) canvas.create_rectangle(0, 0, 50, 50, fill= green, outline= red, width=3)

17 Functions: Variable length argument list def longestword(*args): * packs arguments into one tuple if (len(args) == 0): return None result = args[0] for word in args: if (len(word) > len(result)): result = word return result print longestword( this, is, really, nice ) args = ( this, is, really, nice )

18 Functions: Variable length argument list def longestword(*args): if (len(args) == 0): return None result = args[0] for word in args: if (len(word) > len(result)): result = word return result print longestword( this, is, really, nice ) words = ( this, is, really, nice ) print longestword(words) args = (( this, is, really, nice ), )

19 Functions: Variable length argument list def longestword(*args): if (len(args) == 0): return None result = args[0] for word in args: if (len(word) > len(result)): result = word return result print longestword( this, is, really, nice ) words = ( this, is, really, nice ) print longestword(words[0], words[1], words[2], words[3])

20 Functions: Variable length argument list def longestword(*args): if (len(args) == 0): return None result = args[0] for word in args: if (len(word) > len(result)): result = word return result print longestword( this, is, really, nice ) words = ( this, is, really, nice ) (a, b, c, d) = words print longestword(a, b, c, d)

21 Functions: Variable length argument list def longestword(*args): if (len(args) == 0): return None result = args[0] for word in args: if (len(word) > len(result)): result = word return result print longestword( this, is, really, nice ) words = ( this, is, really, nice ) print longestword(*words) * unpacks the tuple

22 Functions: Documentation string help(max) print max. doc max(iterable[, key=func]) -> value max(a, b, c,...[, key=func]) -> value With a single iterable argument, return its largest item. With two or more arguments, return the largest argument. print longestword. doc None

23 Functions: Documentation string def longestword(*args): Returns the longest argument if (len(args) == 0): return None result = args[0] for word in args: if (len(word) > len(result)): result = word return result print longestword. doc Returns the longest argument

24 Functions: Anonymous functions Motivational example: class Book(object): def init (self, title, author, year): self.title = title self.author = author self.year = year def getyear(book): return book.year b = Book( Hamlet, Shakespeare, 1603) Creates an object of type Book print getyear(book( It, Stephen King, 1986)) Didn t assign it to a variable first

25 Functions: Anonymous functions Motivational example: class Book(object): def init (self, title, author, year): self.title = title self.author = author self.year = year def getyear(book): return book.year b = Book( Hamlet, Shakespeare, 1603) Creates an object of type Book library = [] b.append(book( It, Stephen King, 1986))

26 Functions: Anonymous functions Can - sort of - create and use functions similarly with lambda expressions. f = lambda x,y: x+y Creates an object of type function

27 Functions: Anonymous functions Can - sort of - create and use functions similarly with lambda expressions. f = lambda x,y: x+y inputs Creates an object of type function

28 Functions: Anonymous functions Can - sort of - create and use functions similarly with lambda expressions. f = lambda x,y: x+y Creates an object of type function an expression (the value is returned)

29 Functions: Anonymous functions Can - sort of - create and use functions similarly with lambda expressions. f = lambda x,y: x+y Creates an object of type function f = lambda x,y: print x+y # Crashes names = [ fred, wilma, barney, betty, dino ] print sorted(names, lambda a,b: len(a) - len(b)) Didn t assign it to a variable first

30 Functions: Nested functions Can be used to avoid polluting the global space. def f(a): def evens(a): return [value for value in a if (value % 2) == 0] return list(reversed(evens(a))) print f(range(10)) print evens(range(10)) # Crashes

31 Functions: Nested functions Can be used to change function signature. def nqueens(n): def solve(n, m, constraints): return solve(n, n, [])

32 Functions: Nested functions Can be used to change function signature. Suppose you have a math function f(x, y) For each fixed variable: f y (x) y, f(x, y) defines a function in one Example: f(x, y) =x + y f 0 (x) =x f 1 (x) =x +1 f 2 (x) =x +2 f(x,y) is like a collection of functions in one variable. How can we generate these functions in Python?

33 Functions: Nested functions How to do this in Python: def f(y): def g(x): return x + y return g f_1 = f(1) print f_1(5) f_2 = f(2) print f_2(5) y is called a non-local variable. For each y, this returns a different function Returned value: g packaged together with a y value Closure: a function bound together with a value

34 Functions: Nested functions How to do this in Python: def f(y): return lambda x: x + y f_1 = f(1) print f_1(5) f_2 = f(2) print f_2(5)

35 Functions: Nested functions How to do this in Python: def makeadderfn(delta): def g(x): return x + delta return g add3 = makeadderfn(3) add4 = makeadderfn(4) print add3(5) print add4(5)

36 Functions: Nested functions How to do this in Python: def makeadderfn(delta): def g(x): delta = delta + 1 return x + delta return g Error add3 = makeadderfn(3) add4 = makeadderfn(4) print add3(5) print add4(5)

37 Functions: Nested functions Another example: def f(canvas): def fstripped(canvas): def g(): return f(canvas) return g h = fstripped(canvas) # Now h is like f canvas

38 Event based programming in Tkinter

39 The high level structure Asynchronous: Flow of the program is determined by events e.g. mouse clicks, key presses 3 Parts: - Event handler functions - Binding events to correct event-handler functions - Main loop: - listens for events - triggers a callback function when event is detected. (events are added to an event queue)

40 from Tkinter import * def mousepressed(event): def keypressed(event): def timerfired(): def redrawall(): def init(): def run(): # Create the root and the canvas global canvas root = Tk() canvas = Canvas(root, width=300, height=200) canvas.pack() # Set up canvas data and call init class Struct: pass canvas.data = Struct() init() # Bind events to correct event handler functions root.bind("<button-1>", mousepressed) root.bind("<key>", keypressed) timerfired() # Main loop root.mainloop()

41 from Tkinter import * def mousepressed(canvas, event): def keypressed(canvas, event): def timerfired(canvas): def redrawall(canvas): def init(canvas): def run(): # Create the root and the canvas root = Tk() canvas = Canvas(root, width=300, height=200) canvas.pack() # Set up canvas data and call init class Struct: pass canvas.data = Struct() init() # Bind events to correct event handler functions root.bind("<button-1>", mousepressed) root.bind("<key>", keypressed) timerfired() # Main loop root.mainloop()

42 from Tkinter import * def mousepressed(canvas, event): def keypressed(canvas, event): def timerfired(canvas): def redrawall(canvas): def init(canvas): def run(): # Create the root and the canvas root = Tk() canvas = Canvas(root, width=300, height=200) canvas.pack() # Set up canvas data and call init class Struct: pass canvas.data = Struct() init(canvas) # Bind events to correct event handler functions root.bind("<button-1>", mousepressed(canvas)) root.bind("<key>", keypressed(canvas)) timerfired(canvas) # Main loop root.mainloop() Not a reference to a function

43 def run(): # Create the root and the canvas root = Tk() canvas = Canvas(root, width=300, height=200) canvas.pack() # Set up canvas data and call init class Struct: pass canvas.data = Struct() init(canvas) # Bind events to correct event handler functions def f(event): mousepressed(canvas, event) root.bind("<button-1>", f) root.bind("<key>", keypressed(canvas)) timerfired(canvas) # Main loop root.mainloop() f > mousepressed canvas

44 def run(): # Create the root and the canvas root = Tk() canvas = Canvas(root, width=300, height=200) canvas.pack() # Set up canvas data and call init class Struct: pass canvas.data = Struct() init(canvas) # Bind events to correct event handler functions def f(event): mousepressed(canvas, event) root.bind("<button-1>", f) root.bind("<key>", lambda event: keypressed(canvas, event)) timerfired(canvas) # Main loop root.mainloop()

45 def timerfired(canvas): redrawall(canvas) delay = 250 # milliseconds def f(): timerfired(canvas) canvas.after(delay, f) f > timerfired canvas Wants a reference to a function

46 eventbasedanimationclass.py eventbasedanimationdemo.py

47 Event list Event <Button-1> <Key> <Return> <B1-Motion> <ButtonRelease-1> <Double-Button-1> <Enter> <Leave> <Configure> Description Mouse left button click Any key press Enter key press Drag: mouse move with left button down Mouse left button release Double click (with left button) Mouse pointer enters the widget Mouse pointer leaves the widget The widget changes size

48 Event object properties Attribute Description type The event type x, y The current mouse position char The character code (for keyboard events) keysym The key symbol (for keyboard events) num The button number (for mouse button events) width, height widget The new size of the widget (for configure events) The widget that the event occurred on (as a reference)