PYTHON - FILES I/OThis chapter covers all the basic I/O functions available in Python. For more functions, please refer to standard Python documentation. Printing to the Screen The simplest way to produce output is using the print statement where you can pass zero or more expressions separated by commas. This 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?"; This produces 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. These functions are:
The 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 This prompts 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 The 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 This would produce the following result against 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 use 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. This 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 are parameter details:
Once a file is opened and you have one file object, you can get various information related to that file. Here is a list of all attributes related to file object:
#!/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 This produces the following result:
Name of the file: foo.txt
Closed or not : False Opening mode : wb Softspace flag : 0 The close()Method The 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 reassigned to another file. It is a good 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() This produces the following result:
Name of the file: foo.txt
Reading and Writing Files The 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 The write() method writes any string to an open file. It is important to note that Python strings can have binary data and not just text. The 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("foo.txt", "wb") fo.write( "Python is a great language.\nYeah its great!!\n"); # Close opend file fo.close() The above method would create foo.txt file and would write given 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 The read() method reads a string from an open file. It is important to note that Python strings can have binary data, apart from text data. Syntax
fileObject.read([count]);
Here, passed parameter is the number of bytes to be read from the opened file. This method starts reading from the beginning 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 us take a file foo.txt, which we created above.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "r+") str = fo.read(10); print "Read String is : ", str # Close opend file fo.close() This produces the following result:
Read String is : Python is
File Positions The 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 beginning of the file. The seek(offset[, from]) method changes the current file position. The offset argument indicates the number of bytes to be moved. The from argument specifies the reference position from where the bytes are to be moved. If from is set to 0, it means use the beginning 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 us take a file foo.txt, which we created above.
#!/usr/bin/python
# Open a file fo = open("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() This produces 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. To use this module you need to import it first and then you can call any related functions. The rename() Method The rename() method takes two arguments, 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 argument. 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. The os module has several methods that help you create, remove, and change 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 argument 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 change the current directory. The chdir() method takes an argument, 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 go into "/home/newdir" directory:
#!/usr/bin/python
import os # Changing a directory to "/home/newdir" os.chdir("/home/newdir") The getcwd()Method The getcwd() method displays the current working directory. Syntax
os.getcwd()
Example Following is the example to give current directory:
#!/usr/bin/python
import os # This would give location of the current directory os.getcwd() The rmdir()Method The rmdir() method deletes the directory, which is passed as an argument 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 give 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" directory. os.rmdir( "/tmp/test" ) File and Directory Related Methods There are two important sources, which provide a wide range of utility methods to handle and manipulate files and directories on Windows and Unix operating systems. They are as follows: File Object Methods: The file object provides functions to manipulate files. OS Object Methods: This provides methods to process files as well as directories. A file object is created using open function and here is a list of functions which can be called on this object:
1. file.close() Description The method close() closes the opened file. A closed file cannot be read or written any more. Any operation, which requires that the file be opened will raise a ValueError after the file has been closed. Calling close() more than once is allowed. Python automatically closes a file when the reference object of a file is reassigned to another file. It is a good practice to use the close() method to close a file. Syntax Following is the syntax for close() method:
fileObject.close();
Parameters
This method does not return any value. Example The following example shows the usage of close() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "wb") print "Name of the file: ", fo.name # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
2. File.flush() Description The method flush() flushes the internal buffer, like stdio's fflush. This may be a no- op on some file-like objects. Python automatically flushes the files when closing them. But you may want to flush the data before closing any file. Syntax Following is the syntax for flush() method:
fileObject.flush();
Parameters
This method does not return any value. Example The following example shows the usage of flush() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "wb") print "Name of the file: ", fo.name # Here it does nothing, but you can call it with read operation. fo.flush() # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
3. File.fileno() Description The method fileno() returns the integer file descriptor that is used by the underlying implementation to request I/O operations from the operating system. Syntax Following is the syntax for fileno() method:
fileObject.fileno();
Parameters NA Return Value This method returns the integer file descriptor. Example The following example shows the usage of fileno() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "wb") print "Name of the file: ", fo.name fid = fo.fileno() print "File Descriptor: ", fid # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
File Descriptor: 3 4. File.isatty() Description The method isatty() returns True if the file is connected (is associated with a terminal device) to a tty(-like) device, else False. Syntax Following is the syntax for isatty() method:
fileObject.isatty();
Parameters NA Return Value This method returns true if the file is connected (is associated with a terminal device) to a tty(-like) device, else false. Example The following example shows the usage of isatty() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "wb") print "Name of the file: ", fo.name ret = fo.isatty() print "Return value : ", ret # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Return value : False 5. File.next() Description The method next() is used when a file is used as an iterator, typically in a loop, the next() method is called repeatedly. This method returns the next input line, or raises StopIteration when EOF is hit. Combining next() method with other file methods like readline() does not work right. However, usingseek() to reposition the file to an absolute position will flush the read-ahead buffer. Syntax Following is the syntax for next() method:
fileObject.next();
Parameters NA Return Value This method returns the next input line. Example The following example shows the usage of next() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line for index in range(5): line = fo.next() print "Line No %d - %s" % (index, line) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Line No 0 - This is 1st line Line No 1 - This is 2nd line Line No 2 - This is 3rd line Line No 3 - This is 4th line Line No 4 - This is 5th line 6. File.read([size]) Description The method read() reads at most size bytes from the file. If the read hits EOF before obtaining size bytes, then it reads only available bytes. Syntax Following is the syntax for read() method:
fileObject.read( size );
Parameters size -- This is the number of bytes to be read from the file. Return Value This method returns the bytes read in string. Example The following example shows the usage of read() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line line = fo.read(10) print "Read Line: %s" % (line) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Read Line: This is 1s 7. File.readline([size]) Description The method readline() reads one entire line from the file. A trailing newline character is kept in the string. If the size argument is present and non-negative, it is a maximum byte count including the trailing newline and an incomplete line may be returned. An empty string is returned only when EOF is encountered immediately. Syntax Following is the syntax for readline() method:
fileObject.readline( size );
Parameters size -- This is the number of bytes to be read from the file. Return Value This method returns the line read from the file. Example The following example shows the usage of readline() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line line = fo.readline() print "Read Line: %s" % (line) line = fo.readline(5) print "Read Line: %s" % (line) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Read Line: This is 1st line Read Line: This 8. file.readline([sizehint]) Description The method readline() reads one entire line from the file. A trailing newline character is kept in the string. If the size argument is present and non-negative, it is a maximum byte count including the trailing newline and an incomplete line may be returned. An empty string is returned only when EOF is encountered immediately. Syntax Following is the syntax for readline() method:
fileObject.readline( size );
Parameters size -- This is the number of bytes to be read from the file. Return Value This method returns the line read from the file. Example The following example shows the usage of readline() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line line = fo.readline() print "Read Line: %s" % (line) line = fo.readline(5) print "Read Line: %s" % (line) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Read Line: This is 1st line Read Line: This 9. file.seek(offset[,whence]) Description The method seek() sets the file's current position at the offset. The whence argument is optional and defaults to 0, which means absolute file positioning, other values are 1 which means seek relative to the current position and 2 means seek relative to the file's end. There is no return value. Note that if the file is opened for appending using either 'a' or 'a+', any seek() operations will be undone at the next write. If the file is only opened for writing in append mode using 'a', this method is essentially a no-op, but it remains useful for files opened in append mode with reading enabled (mode 'a+'). If the file is opened in text mode using 't', only offsets returned by tell() are legal. Use of other offsets causes undefined behavior. Note that not all file objects are seekable. Syntax Following is the syntax for seek() method:
fileObject.seek(offset[, whence])
Parameters
This method does not return any value. Example The following example shows the usage of seek() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line line = fo.readline() print "Read Line: %s" % (line) # Again set the pointer to the beginning fo.seek(0, 0) line = fo.readline() print "Read Line: %s" % (line) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Read Line: This is 1st line Read Line: This is 1st line 10. file.tell() Description The method tell() returns the current position of the file read/write pointer within the file. Syntax Following is the syntax for tell() method:
fileObject.tell()
Parameters NA Return Value This method returns the current position of the file read/write pointer within the file. Example The following example shows the usage of tell() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line line = fo.readline() print "Read Line: %s" % (line) # Get the current position of the file. pos = fo.tell() print "Current Position: %d" % (pos) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Read Line: This is 1st line Current Position: 18 11. file.truncate([size]) Description The method truncate() truncates the file's size. If the optional size argument is present, the file is truncated to (at most) that size.. The size defaults to the current position. The current file position is not changed. Note that if a specifiedsize exceeds the file's current size, the result is platform-dependent. Note: This method would not work in case file is opened in read-only mode. Syntax Following is the syntax for truncate() method:
fileObject.truncate( [ size ])
Parameters size -- If this optional argument is present, the file is truncated to (at most) that size. Return Value This method does not return any value. Example The following example shows the usage of truncate() method.
#!/usr/bin/python
# Open a file fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line line = fo.readline() print "Read Line: %s" % (line) # Now truncate remaining file. fo.truncate() # Try to read file now line = fo.readline() print "Read Line: %s" % (line) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Read Line: This is 1st line Read Line: 12. file.write(str) Description The method write() writes a string str to the file. There is no return value. Due to buffering, the string may not actually show up in the file until the flush() or close() method is called. Syntax Following is the syntax for write() method:
fileObject.write( str )
Parameters str -- This is the String to be written in the file. Return Value This method does not return any value. Example The following example shows the usage of write() method.
#!/usr/bin/python
# Open a file in write mode fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line str = "This is 6th line" # Write a line at the end of the file. fo.seek(0, 2) line = fo.write( str ) # Now read complete file from beginning. fo.seek(0,0) for index in range(6): line = fo.next() print "Line No %d - %s" % (index, line) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Line No 0 - This is 1st line Line No 1 - This is 2nd line Line No 2 - This is 3rd line Line No 3 - This is 4th line Line No 4 - This is 5th line Line No 5 - This is 6th line 13. file.writelines(sequence) Description The method writelines() writes a sequence of strings to the file. The sequence can be any iterable object producing strings, typically a list of strings. There is no return value. Syntax Following is the syntax for writelines() method: fileObject.writelines( sequence ) Parameters sequence -- This is the Sequence of the strings. Return Value This method does not return any value. Example The following example shows the usage of writelines() method.
#!/usr/bin/python'
# Open a file in witre mode fo = open("foo.txt", "rw+") print "Name of the file: ", fo.name # Assuming file has following 5 lines # This is 1st line # This is 2nd line # This is 3rd line # This is 4th line # This is 5th line seq = ["This is 6th line\n", "This is 7th line"] # Write sequence of lines at the end of the file. fo.seek(0, 2) line = fo.writelines( seq ) # Now read complete file from beginning. fo.seek(0,0) for index in range(7): line = fo.next() print "Line No %d - %s" % (index, line) # Close opend file fo.close() When we run above program, it produces following result:
Name of the file: foo.txt
Line No 0 - This is 1st line Line No 1 - This is 2nd line Line No 2 - This is 3rd line Line No 3 - This is 4th line Line No 4 - This is 5th line Line No 5 - This is 6th line Line No 6 - This is 7th line OS Object Methods This provides methods to process files as well as directories.
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