Chapter 6: Files and Exceptions. COSC 1436, Spring 2017 Hong Sun 3/6/2017

Transcription

1 Chapter 6: Files and Exceptions COSC 1436, Spring 2017 Hong Sun 3/6/2017

2 Function Review: A major purpose of functions is to group code that gets executed multiple times. Without a function defined, you would have to copy and paste this code each time def hello(): print('howdy!') def hello(name): ## def statement with parameter print('howdy!, name) def get_name(): name=input( Enter your name: ) return name

3 Function Review: Position argument and keyword argument def remainder(number, divisor): return number % divisor remainder(20, 7) # position argument remainder(number=20, divisor=7) #keyword argument remainder(divisor=7, number=20) #keyword argument

4 Function Review: LOCAL AND GLOBAL SCOPE Scopes matter for several reasons: o Code in the global scope cannot use any local variables. o However, a local scope can access global variables. o Code in a function s local scope cannot use variables in any other local scope. o You can use the same name for different variables if they are in different scopes. That is, there can be a local variable named spam and a global variable also named spam.

5 Function Review: LOCAL AND GLOBAL SCOPE globvar = 0 def set_globvar_to_one(): # Needed to modify global copy of globvar global globvar globvar = 1 def print_globvar(): # No need for global declaration to read value of globvar print(globvar) set_globvar_to_one() print_globvar() # Prints 1

6 Function Review RANDOM random.randint(1,10) #generate random interger random.randrange(0,101,10) random.random() ##gives you a random floating point number in the range [0.0, 1.0) (so including 0.0, but not including 1.0). random.uniform(a, b) ##gives you a random floating point number in the range [a, b], (where rounding may end up giving you b). random.seed(a) ##This is the seed for the next random number random.seed( 10 ) print ("Random number with seed 10 : ", random.random()) # It will generate same random number random.seed( 10 ) print( "Random number with seed 10 : ", random.random())

7 Introduction to File Input and Output Concept: When a program needs to save data for later use, it writes the data in a file. The data can be read from the file a later time. The process of retrieving data from a file is known as reading data from the file. The term input file is used to describe a file that data is read from. Three steps that must be taken when a file is used by a program. Open the file : Opening a file create a connection between the file and the program. Opening an input file allows the program to read data from the file. Opening an output file usually creates the file on the disk and allows the program to write data to it. Process the file: In this step data is either written to the file ( if it is an output file) or read from the file (if it is an input file). Close the file : When the program is finished using the file, the file must be closed. Closing a file disconnects the file from the program.

8 Two Types of Files 1. Text file: A text file contains data that has been encoded as a text, using a scheme such as ASCII or Unicode. Even if the file contains numbers, those numbers are stored in the file as a series of characters. The file may be opened and viewed in a text editor. 2. Binary file: A binary file contains data that has not been converted to text. The data that is stored in a binary file is intended onl for a program to read. As a consequence, you cannot view the contents of a binary file with a text editor.

9 File Access Methods, Filenames and File Objects Two ways to access data stored in a file: Sequential access: access the data from the beginning of the file to the end of the file. Direct access file (also known as a random access file): you can jump directly to any piece of daa in the file without reading the data that comes from it. Filenames: Each of operating system has its own rules for naming files. Many system support the use of filename extensions, which are short sequences of characters that appear at the end of a filename preceded by a period (which is known as a dot ) File Object: is an object that is associated with a specific file and provides a way for the program to work with that file. In the program, a variable references the file object.

10 Opening a File Open function The open function creates a file object and associates it with a file on the disk. file_variable = open(filename, mode) file_variable is the name of the variable that will reference the file object. Filename is a string specifying the name of the file. Mode is a string specifying the mode (reading, writing ) in which the file will be opened (see table below) Mode r w a Description Open a file for reading only. The file cannot be changed or written to Open a file for writing. If the file already exists, erase its contents. If it does not exist, create it. Open a file to be written to. All data written to the file will be appended to the end. If the file does not exist, create it.

11 Specifying the Location of a File The same location as that of the program Pass a file name that does not contain a path as an argument to the open function, the Python interpreter assumes that the file s location is the same as that of the program. test_file = open( test.txt, w ) Specify a path as well as a filename and prefix the string with letter r. test_file = open( r C:\users\blake\temp\test.txt, w )

12 Writing Data to a file and Close a file file_variable.write(string) file_variable is a variable that references a file object, string is a string that will be written to the file. The file must be opened for writing(using the w or a mode) file_variable.close() Once a program is finished working with a file, it should close the file.

13 Code Example # This program writes three lines of data # to a file. def main(): # Open a file named philosophers.txt. outfile = open('philosophers.txt', 'w') # Write the names of three philosphers # to the file. outfile.write('john Locke\n') outfile.write('david Hume\n') outfile.write('edmund Burke\n') # Close the file. outfile.close() # Call the main function. main()

14 Appending Data to an Existing File a mode when open an output file in append mode. If the file already exists, it will not be erased. If the file does not exist, it will be created. When data is written to the file, it will be written at the end of the file s current contents

15 Reading Data From a File read() The read() method reads the file s contents. readline() readline() method reads a line form the file readlines() returns a list containing all the lines in the file

16 Reading Data From a File Use the file object s read method to read its entire contents into memory. Code example: # This program reads and displays the contents # of the philosophers.txt file. def main(): # Open a file named philosophers.txt. infile = open('philosophers.txt', 'r') # Read the file's contents. file_contents = infile.read() # Close the file. infile.close() # Print the data that was read into # memory. print(file_contents) # Call the main function. main()

17 readline method Initial read position John Locke\nDavid Hume\nEdmund Brurke\n Read Position Read position advanced to the next line John Locke\nDavid Hume\nEdmund Brurke\n Read Position Read position advanced to the end of the file John Locke\nDavid Hume\nEdmund Brurke\n Read Position

18 Concatenating a Newline to a String # This program gets three names from the user # and writes them to a file. def main(): # Get three names. print('enter the names of three friends.') name1 = input('friend #1: ') name2 = input('friend #2: ') name3 = input('friend #3: ') # Open a file named friends.txt. myfile = open('friends.txt', 'w') # Write the names to the file. myfile.write(name1 + '\n') myfile.write(name2 + '\n') myfile.write(name3 + '\n') # Close the file. myfile.close() print('the names were written to friends.txt.') # Call the main function. main()

19 Strip Characters in Python Python strings have the strip(), lstrip(), rstrip() methods for removing any character from both ends of a string. If the characters to be removed are not specified then white-space will be removed strip() #removes from both ends lstrip() #removes leading characters (Left-strip) rstrip() #removes trailing characters (Right-strip)

20 Strip Characters in Python strip() #removes from both ends spacious = " xyz " print spacious.strip() lstrip() #removes leading characters (Left-strip) spacious = " xyz " print spacious.lstrip() rstrip() #removes trailing characters (Right-strip) spacious = "xyz " print spacious.rstrip()

21 Reading a String and Stripping the rstrip method Name = Joanne Manchester\n Name = Name.rstrip( \n ) Code example # This program reads the contents of the # philosophers.txt file one line at a time. def main(): # Open a file named philosophers.txt. infile = open('philosophers.txt', 'r') # Read three lines from the file line1 = infile.readline() line2 = infile.readline() line3 = infile.readline() # Strip the \n from each string. line1 = line1.rstrip('\n') line2 = line2.rstrip('\n') line3 = line3.rstrip('\n') # Close the file. infile.close() # Print the data that was read into # memory. print(line1) print(line2) print(line3) # Call the main function. main() Newline from it

22 Writing and Reading Numeric Data str method to convert numeric data to string str(number) # This program demonstrates how numbers # must be converted to strings before they # are written to a text file. def main(): # Open a file for writing. outfile = open('numbers.txt', 'w') # Get three numbers from the user. num1 = int(input('enter a number: ')) num2 = int(input('enter another number: ')) num3 = int(input('enter another number: ')) # Write the numbers to the file. outfile.write(str(num1) + '\n') outfile.write(str(num2) + '\n') outfile.write(str(num3) + '\n') # Close the file. outfile.close() print('data written to numbers.txt') # Call the main function. main()

23 Using Loops to Process Files Use a loop to process the data in file. Open the file Use readline to read the first line from the file Did readline return an empty string? No (false) Process the item that was just read from the file Use readline to read the next line from the file Yes (true) Close the file

24 While Loop and For Loop while line!= for line in file Code example read_sales.py (p255) Read_sales2.py (p266)

25 Exceptions An exception is an error that occurs while a program is running, causing the program to abruptly halt. Use the try/except statement to gracefully handle exceptions. Try: statement statement etc Except ExceptionName: statement statement etc Code example : gross_pay2.py p275 Multiple Exception handling Try: except ExceptionName: except ExceptionName: except ExceptionName: Code example: sales_report1.py p279

26 Exceptions

27 Exceptions

28 Exceptions

29 Exceptions

30 Exceptions

31 Exception - continue One except clause to catch all Exceptions try: statement statement except: statement Exception s Default error Message try: statement except Exception as err: statement else clause try: statement except Exception as err: statement else: statement finally clause try: statement except Exception as err: statement Finally: statement etc

32 Exception - continue try: x = float(raw_input("your number: ")) inverse = 1.0 / x except ValueError: print ("You should have given either an int or a float" ) except ZeroDivisionError: print "Infinity" finally: print("there may or may not have been an exception.")

33 Exception - continue try: x = float(raw_input("your number: ")) inverse = 1.0 / x except ValueError: print ("You should have given either an int or a float" ) except ZeroDivisionError: print "Infinity" finally: print("there may or may not have been an exception.")

34 Lab and assignment Lab: programming Exercises #1,#2,#3,#5 Assignment: Programming Exercises #8, #9,#10 Due After midterm test.