Pickle

Python provides a standard module called pickle using which you can store any Python object in a
file and then get it back later intact. This is called storing the object persistently.
There is another module called cPickle which functions exactly same as the pickle module except
that it is written in the C language and is (upto 1000 times) faster. You can use either of these modules,
although we will be using the cPickle module here. Remember though, that we refer to both these
modules as simply the pickle module.
Pickling and Unpickling

Example 12.2. Pickling and Unpickling
#!/usr/bin/python
# Filename: pickling.py
import cPickle as p
#import pickle as p
shoplistfile = 'shoplist.data' # the name of the file where we will store the object
shoplist = ['apple', 'mango', 'carrot']
# Write to the file
f = file(shoplistfile, 'w')
p.dump(shoplist, f) # dump the object to a file
f.close()
del shoplist # remove the shoplist
# Read back from the storage
f = file(shoplistfile)
storedlist = p.load(f)
print storedlist


Output
$ python pickling.py
['apple', 'mango', 'carrot']


How It Works
First, notice that we use the import..as syntax. This is handy since we can use a shorter name for a
module. In this case, it even allows us to switch to a different module (cPickle or pickle) by
simply changing one line! In the rest of the program, we simply refer to this module as p.
To store an object in a file, first we open a file object in write mode and store the object into the open
file by calling the dump function of the pickle module. This process is called pickling.
Next, we retrieve the object using the load function of the pickle module which returns the object.
This process is called unpickling.

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.