#11: File manipulation Reading: Chapter 7

Transcription

1 CS 130R: Programming in Python #11: File manipulation Reading: Chapter 7

2 Contents File manipulation Text ASCII files Binary files - pickle Exceptions

3 File manipulation Electronic files Files store useful data in an meaningful easy way to find them (organized). How to work with files in Python???

4 File manipulation Files purpose is to store large amount of data in an organized way for efficient retrieval. Make data persistent. Types of files: Text - ASCII Binary Images, Movies, Databases Html, Zip, Etc Text ASCII file - contains only ASCII characters. Why text files? Text files are cross-platform (same on Windows, Linux, Mac) Text files are easy to use

5 I/O Text File As far as Python is concerned, a file is just a string (often very large!) stored on your file system, that you can read or write, gradually or all together. The first thing to do when you are working with files in Python is to open the file. When you open the files, you can specify with parameters how you want to open them: r is for reading w for writing a for appending When you open a file the reading cursor is positioned on the first character of the file so you can read data from the beginning of the file.

6 I/O Text File r is for reading file1 = open( filename_here, r ) w for writing file2 = open( filename_here, w ) a for appending file3 = open( filename_here, a ) It is very important to close the file after you are done working with it. Use close to close the file when the program doesn t need it more. file1.close()

7 I/O Text File Functions available for reading the files: read, readline and readlines. Read this function reads all characters as a big string, unless you specify otherwise by giving the number of characters file1 = open( filename_here, r ) content1 = file1.read() # reads entire file content2 = file1.read(10) # reads first 10 characters Readline - this function reads a single line from the file file1 = open( filename_here, r ) content3 = file1.readline() # reads entire 1 st line from the file content4 = file1.readline(3) # reads entire 3 rd line from the file

8 I/O Text File Readlines - this function returns a list containing all the lines of data in the file. It reads all rows (lines) and retains the newlines character that is at the end of every row (line). file1 = open( filename_here, r ) listlines = file1.readlines() # reads entire file into a list of lines [ Line 1\n, Line 2\n, Line 3\n ] Looping through a file directly file1 = open( filename_here, r ) listlines = file1.readlines() for line in listlines: print(line) Or file1 = open( filename_here, r ) for line in file1: print(line) file1.close()

9 I/O Text File Functions available for writing the files: write and writelines. Write this function writes a fixed sequence of characters to a file file1 = open( filename_here, w ) file1.write( Hello World!\n ) file1.write( Hello Pythonizers!\n ) file1.close() #in the file Hello World! Hello Pythonizers! The write method doesn t automatically put the newline character at the end of the string it writes. These need to be put manually.

10 I/O Text File Writelines this function writes a list of strings to a file fh = open("hello.txt", "w") lines_of_text = ["a line of text\n", "another line of text\n", "a third line"] fh.writelines(lines_of_text) fh.close() #in the file hello.txt a line of text another line of text a third line The writelines method doesn t automatically put the newline character at the end of the string it writes. These need to be put manually.

11 I/O Binary File -Pickles Storing complex data in files Use pickle function to preserve the data (object, list, dictionary, tuple) and save it in its entirety to a file. The object can then be restored, or unpickled, later on. # import the pickle module import pickle # lets create a list to be pickled picklelist = ['one',2,'three','four',5,'can you count?'] # now create a file file = open('filename', 'w') # now let's pickle picklelist pickle.dump(picklelist,file) # object to pickle, and file # close the file, and your pickling is complete file.close()

12 I/O Binary File -Pickles Reading complex data in files Use pickle.load() function to unpickle the data. # import the pickle module import pickle # now open a file for reading unpicklefile = open('filename', 'r') # now load the list that we pickled into a new object unpickledlist = pickle.load(unpicklefile) # close the file, just for safety unpicklefile.close() # Try out using the list for item in unpickledlist: print(item)

13 import pickle data = (1.4,42) output = open('data.pkl', 'w') pickle.dump(data, output) output.close() #file data.pkl has the following input (F I42 tp0. Pickling #This file can easily be read in again: import pickle f = open("data.pkl") data = pickle.load(f) print(data) ( , 42) And there are more efficient ways to work with complex data (shelve, databases, XML), but that s it for now

14 Handling Exceptions An exception is an error that happens during the execution of a program. Different types of errors (syntactical) result in different types of exceptions. Error handling is generally resolved by saving the state of execution at the moment the error occurred and interrupting the normal flow of the program to execute a special function or piece of code, which is known as the exception handler. Depending on the kind of error ("division by zero", "file open error" and so on) which had occurred, the error handler can "fix" the problem and the program can be continued afterwards with the previously saved data.

15 Handling Exceptions Syntax: try: statements except [(Exception1, Exception2, )]: statement # example of error n = int(input("please enter a number: ")) Please enter a number: 23.5 Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: invalid literal for int() with base 10: '23.5'

16 Handling Exceptions # handling error by specifying the exception type try: n = int(input("please enter a number: ")) except ValueError: #print executes only if error occurs print( That was not an integer! ) Please enter a number: 23.5 That was not an integer!

17 Handling Exceptions while True: try: n = input("please enter an integer: ") n = int(n) break except ValueError: print("no valid integer! Please try again...") print("great, you successfully entered an integer! )

18 Error types: Handling Exceptions IOError I/O files when they do not exist IndexError for indices out of limits KeyError when a dictionary key is not found NameError when a name (variable, function) is not found SyntaxError for syntax errors ValueError inappropriate value ZeroDivisionError

19 Handling Exceptions When to handle errors? - Every time there is input from user, - I/O to from files import sys try: f = open('integers.txt') s = f.readline() i = int(s.strip()) except IOError : print("i/o error ) except ValueError: print("no valid integer in line." ) except: print("unexpected error: )

20 Importing modules Next lecture