Python Files

T his chapter will cover all the basic I/O functions available in Python. For more functions, please refer to standard Python documentation. Printing to the Screen: T he simplest way to produce output is using the print statement where you can pass zero or more expressions separated by commas. T his function converts the expressions you pass into a string and writes the result to standard output as follows: #!/usr/bin/python print "Python is really a great language,", "isn't it?"; T his would produce the following result on your standard screen: Python is really a great language, isn't it? Reading Keyboard Input: Python provides two built-in functions to read a line of text from standard input, which by default comes from the keyboard. T hese functions are: raw_input input The raw_input Function: T he raw_input([prompt]) function reads one line from standard input and returns it as a string (removing the trailing newline). #!/usr/bin/python str = raw_input("Enter your input: "); print "Received input is : ", str T his would prompt you to enter any string and it would display same string on the screen. When I typed "Hello Python!", its output is like this: Enter your input: Hello Python Received input is : Hello Python The input Function: T he input([prompt]) function is equivalent to raw_input, except that it assumes the input is a valid Python expression and returns the evaluated result to you. #!/usr/bin/python str = input("Enter your input: "); print "Received input is : ", str T his would produce the following result ag ainst the entered input: Enter your input: [x*5 for x in range(2,10,2)] Recieved input is : [10, 20, 30, 40] Opening and Closing Files: Until now, you have been reading and writing to the standard input and output. Now, we will see how to play with actual data files. Python provides basic functions and methods necessary to manipulate files by default. You can do your most of the file manipulation using a file object. The open Function: Before you can read or write a file, you have to open it using Python's built-in open() function. T his function creates a file object, which would be utilized to call other support methods associated with it. Syntax: file object = open(file_name [, access_mode][, buffering]) Here is paramters' detail: file_name: T he file_name arg ument is a string value that contains the name of the file that you want to access. ac c ess_mode: T he access_mode determines the mode in which the file has to be opened, i.e., read, write, append, etc. A complete list of possible values is g iven below in the table. T his is optional parameter and the default file access mode is read (r). buffering : If the buffering value is set to 0, no buffering will take place. If the buffering value is 1, line buffering will be performed while accessing a file. If you specify the buffering value as an integ er g reater than 1, then buffering action will be performed with the indicated buffer size. If neg ative, the buffer size is the system default(default behavior). Here is a list of the different modes of opening a file: Modes Desc ription r Opens a file for reading only. T he file pointer is placed at the beg inning of the file. T his is the default mode. rb Opens a file for reading only in binary format. T he file pointer is placed at the beg inning of the file. T his is the default mode. r+ Opens a file for both reading and writing . T he file pointer will be at the beg inning of the file. rb+ Opens a file for both reading and writing in binary format. T he file pointer will be at the beg inning of the file. w Opens a file for writing only. Overwrites the file if the file exists. If the file does not exist, creates a new file for writing . wb Opens a file for writing only in binary format. Overwrites the file if the file exists. If the file does not exist, creates a new file for writing . w+ Opens a file for both writing and reading . Overwrites the existing file if the file exists. If the file does not exist, creates a new file for reading and writing . wb+ Opens a file for both writing and reading in binary format. Overwrites the existing file if the file exists. If the file does not exist, creates a new file for reading and writing . a Opens a file for appending . T he file pointer is at the end of the file if the file exists. T hat is, the file is in the append mode. If the file does not exist, it creates a new file for writing . ab Opens a file for appending in binary format. T he file pointer is at the end of the file if the file exists. T hat is, the file is in the append mode. If the file does not exist, it creates a new file for writing . a+ Opens a file for both appending and reading . T he file pointer is at the end of the file if the file exists. T he file opens in the append mode. If the file does not exist, it creates a new file for reading and writing . ab+ Opens a file for both appending and reading in binary format. T he file pointer is at the end of the file if the file exists. T he file opens in the append mode. If the file does not exist, it creates a new file for reading and writing . The file object attributes: Once a file is opened and you have one file object, you can g et various information related to that file. Here is a list of all attributes related to file object: Attribute Desc ription file.closed Returns true if file is closed, false otherwise. file.mode Returns access mode with which file was opened. file.name Returns name of the file. file.softspace Returns false if space explicitly required with print, true otherwise. Example: #!/usr/bin/python # Open a file fo = open("foo.txt", "wb") print "Name of the file: ", fo.name print "Closed or not : ", fo.closed print "Opening mode : ", fo.mode print "Softspace flag : ", fo.softspace T his would produce the following result: Name of the file: foo.txt Closed or not : False Opening mode : wb Softspace flag : 0 The close() Method: T he close() method of a file object flushes any unwritten information and closes the file object, after which no more writing can be done. Python automatically closes a file when the reference object of a file is reassig ned to another file. It is a g ood practice to use the close() method to close a file. Syntax: fileObject.close(); Example: #!/usr/bin/python # Open a file fo = open("foo.txt", "wb") print "Name of the file: ", fo.name # Close opend file fo.close() T his would produce the following result: Name of the file: foo.txt Reading and Writing Files: T he file object provides a set of access methods to make our lives easier. We would see how to use read() and write() methods to read and write files. The write() Method: T he write() method writes any string to an open file. It is important to note that Python string s can have binary data and not just text. T he write() method does not add a newline character ('\n') to the end of the string : Syntax: fileObject.write(string); Here, passed parameter is the content to be written into the opened file. Example: #!/usr/bin/python # Open a file fo = open("/tmp/foo.txt", "wb") fo.write( "Python is a great language.\nYeah its great!!\n"); # Close opend file fo.close() T he above method would create foo.txt file and would write g iven content in that file and finally it would close that file. If you would open this file, it would have following content. Python is a great language. Yeah its great!! The read() Method: T he read() method reads a string from an open file. It is important to note that Python string s can have binary data and not just text. Syntax: fileObject.read([count]); Here, passed parameter is the number of bytes to be read from the opened file. T his method starts reading from the beg inning of the file and if count is missing , then it tries to read as much as possible, maybe until the end of file. Example: Let's take a file foo.txt, which we have created above. #!/usr/bin/python # Open a file fo = open("/tmp/foo.txt", "r+") str = fo.read(10); print "Read String is : ", str # Close opend file fo.close() T his would produce the following result: Read String is : Python is File Positions: T he tell() method tells you the current position within the file; in other words, the next read or write will occur at that many bytes from the beg inning of the file. T he seek(offset[, from]) method chang es the current file position. T he offset arg ument indicates the number of bytes to be moved. T he from arg ument specifies the reference position from where the bytes are to be moved. If from is set to 0, it means use the beg inning of the file as the reference position and 1 means use the current position as the reference position and if it is set to 2 then the end of the file would be taken as the reference position. Example: Let's take a file foo.txt, which we have created above. #!/usr/bin/python # Open a file fo = open("/tmp/foo.txt", "r+") str = fo.read(10); print "Read String is : ", str # Check current position position = fo.tell(); print "Current file position : ", position # Reposition pointer at the beginning once again position = fo.seek(0, 0); str = fo.read(10); print "Again read String is : ", str # Close opend file fo.close() T his would produce the following result: Read String is : Python is Current file position : 10 Again read String is : Python is Renaming and Deleting Files: Python os module provides methods that help you perform file-processing operations, such as renaming and deleting files. T o use this module you need to import it first and then you can call any related functions. The rename() Method: T he rename() method takes two arg uments, the current filename and the new filename. Syntax: os.rename(current_file_name, new_file_name) Example: Following is the example to rename an existing file test1.txt: #!/usr/bin/python import os # Rename a file from test1.txt to test2.txt os.rename( "test1.txt", "test2.txt" ) The remove() Method: You can use the remove() method to delete files by supplying the name of the file to be deleted as the arg ument. Syntax: os.remove(file_name) Example: Following is the example to delete an existing file test2.txt: #!/usr/bin/python import os # Delete file test2.txt os.remove("text2.txt") Directories in Python: All files are contained within various directories, and Python has no problem handling these too. T he os module has several methods that help you create, remove and chang e directories. The mkdir() Method: You can use the mkdir() method of the os module to create directories in the current directory. You need to supply an arg ument to this method which contains the name of the directory to be created. Syntax: os.mkdir("newdir") Example: Following is the example to create a directory test in the current directory: #!/usr/bin/python import os # Create a directory "test" os.mkdir("test") The chdir() Method: You can use the chdir() method to chang e the current directory. T he chdir() method takes an arg ument, which is the name of the directory that you want to make the current directory. Syntax: os.chdir("newdir") Example: Following is the example to g o into "/home/newdir" directory: #!/usr/bin/python import os # Changing a directory to "/home/newdir" os.chdir("/home/newdir") The getcwd() Method: T he getcwd() method displays the current working directory. Syntax: os.getcwd() Example: Following is the example to g ive current directory: #!/usr/bin/python import os # This would give location of the current directory os.getcwd() The rmdir() Method: T he rmdir() method deletes the directory, which is passed as an arg ument in the method. Before removing a directory, all the contents in it should be removed. Syntax: os.rmdir('dirname') Example: Following is the example to remove "/tmp/test" directory. It is required to g ive fully qualified name of the directory, otherwise it would search for that directory in the current directory. #!/usr/bin/python import os # This would remove "/tmp/test" os.rmdir( "/tmp/test" ) directory. File & Directory Related Methods: T here are three important sources, which provide a wide rang e of utility methods to handle and manipulate files & directories on Windows and Unix operating systems. T hey are as follows: File Object Methods: T he file object provides functions to manipulate files. OS Object Methods: T his provides methods to process files as well as directories.