Files

You can open and use files for reading or writing by creating an object of the file class and using its
read, readline or write methods appropriately to read from or write to the file. The ability to read
or write to the file depends on the mode you have specified for the file opening. Then finally, when you
are finished with the file, you call the close method to tell Python that we are done using the file.

Using file

Example 12.1. Using files
#!/usr/bin/python
# Filename: using_file.py
poem = '''\
Programming is fun
When the work is done
if you wanna make your work also fun:
use Python!
'''
f = file('poem.txt', 'w') # open for 'w'riting
f.write(poem) # write text to file
f.close() # close the file
f = file('poem.txt') # if no mode is specified, 'r'ead mode is assumed by default
while True:
line = f.readline()
if len(line) == 0: # Zero length indicates EOF
break
print line, # Notice comma to avoid automatic newline added by Python
f.close() # close the file


Output
$ python using_file.py
Programming is fun
When the work is done
if you wanna make your work also fun:
use Python!

How It Works
First, we create an instance of the file class by specifying the name of the file and the mode in which
we want to open the file. The mode can be a read mode ('r'), write mode ('w') or append mode
('a'). There are actually many more modes available and help(file) will give you more details
about them.
We first open the file in write mode and use the write method of the file class to write to the file
and then we finally close the file.
Next, we open the same file again for reading. If we don't specify a mode, then the read mode is the default
one. We read in each line of the file using the readline method, in a loop. This method returns a
complete line including the newline character at the end of the line. So, when an empty string is returned,
it indicates that the end of the file has been reached and we stop the loop.
Notice that we use a comma with the print statement to suppress the automatic newline that the
print statement adds because the line that is read from the file already ends with a newline character.
Then, we finally close the file.
Now, see the contents of the poem.txt file to confirm that the program has indeed worked properly.