CSC1018F: Object Orientation, Exceptions and File Handling (Tutorial)

1. CSC1018F: Object Orientation, Exceptions and File Handling(Tutorial) Diving into Python Ch. 5&6 Think Like a Comp. Scientist Ch. 11-13

2. Mini-Test Answers (1) • If you want access to the contents of a module “abra” but do not want to bring it into the current namespace (i.e., all functions of the module must be qualified with “module.function”) then you would call: • from abra import * • from abra import function1, function2, … • import abra • Which of the following statements concerning classes is NOT correct: • Data attributes of a class must be declared in the __init__ method • __init__ methods are optional • Class attributes can be accessed from both a class and instance as “class.attrib” or “instance.attrib” • Python does not support multiple inheritance • The “__data__” notation indicates a private method or attribute

3. Mini-Test Answers (2) • The following python code: try: fsock = open("/program.py") except IOError: print "The file does not exist" except AttributeError: print "open() not available" print "Program opened" will: • Always print “Program opened” • Print “Program opened” only if no IOError occurs • Print “Program opened” if no IOError and no AttributeError occur • Fail to execute because it contains a syntax error

4. Mini-Test Answers (3) • Assuming that a file called “f” of length 256 has been newly opened, you could move the current read position to 128 by calling: • f.seek(-128, 2) • f.seek(-128, 1) • f.seek(128, 0) • f.seek(128, 1) • f.seek(-128, 0) • Calling “from module import *” twice in succession will cause a syntax error because of namespace conflicts: • True • False

5. Mini-Test Answers (4) • The following python code executed on a windows system: >>> import os >>> os.path.join("c:\\documents", "photo.jpg") will output: • "c:\\documentsphoto.jpg" • "c:\\documents\\photo.jpg" • "c:\documents\photo.jpg"

6. Revision Exercise • Create an image class with the following methods: • __init__ - creates a white image with a user specified x, y resolution • CheckValidity() - which checks if all pixel rows have the same length • PadTrunc(l) - which appends white pixels to rows < length l or truncates rows > length l • Threshold(t) - set all pixels <= t to 0 and the rest to 255 • Resize(x, y) - alters the resolution of an image by making calls to PadTrunc and/or dropping rows

7. Revision Solution (1) class Image: """A greyscale image class consisting of a rectangular matrix of integer pixels where 0 represents black and 255 white """ def __init__(self, x = 100, y = 100): self.__image = [] self.__width = x self.__height = y for i in range(y): self.__image.append([255]*x) def CheckValidity(self): for row in self.__image: if(len(row) != self.__width): return False return True def PadTrunc(self, length): self.__image = [(len(row) < length and row.extend([255]*(length-len(row))) or row[:length]) for row in self.__image]

8. Revision Solution (2) def Resize(self, x, y): if not self.CheckValidity(): print "Warning: Correcting Invalid Image" self.PadTrunc(self.__width) # why do we resize y before x? if y < self.__height: self.__image = self.__image[:y] elif y > self.__height: for i in range(y-self.__height): self.__image.append([255]*self.__width) self.__height = y if x != self.__width: self.PadTrunc(x) self.__width = x def __str__(self): return "resolution (" + str(self.__width) + "," + str(self.__height) + "):\n" + str(self.__image)