An Awful Pickle


Specialist     Come in.
The door opens and Raymond Luxury Yacht enters. He cannot walk straight to the desk as his passage is barred by the strip of wood carrying the degrees, but he discovers the special hinged part of it that opens like a door. Mr Luxury Yacht has his enormous polystyrene nose. It is a foot long.
Specialist     Ah! Mr Luxury Yacht. Do sit down, please.
Mr Luxury Yacht     Ah, no, no. My name is spelled ‘Luxury Yacht’ but it’s pronounced ‘Throatwobbler Mangrove’.
Specialist     Well, do sit down then Mr Throatwobbler Mangrove.
Mr Luxury Yacht     Thank you.

So, we know how to save trivia questions to a file, and how to read them back from a file in the future.  Moreover, we have decided on a particular way of structuring the data which makes a question.  That is, the question is followed by the correct answer and then a number of incorrect answers.   Now we have to translate between a list (which has a concept of elements), and a file (which doesn’t).  Files are “flat” – which is to say that they have no sense of structure, they are simply a stream of data.  A file may record all of the characters which are the questions and answers, but it wouldn’t record the fact that they are a list or, indeed, that they are any kind of Python object.  I was originally just going to run with this to let you find out about files, but I have instead decided to introduce a further concept – the Python pickle!

pickle is a module which allows you to store Python objects including their structure.  That means after you have pickled an object to a file, you can later load that object back up from the file and all the structure associated with that object will be preserved.  While, at the moment, we are only dealing with a list, any object can be pickled – even if it has methods and attributes (ie functions and data which are packaged with the object) – they are saved with the object in the file.  What pickle does is “serialises” the object first before “persisting” it.
To use pickle you must first import it:

import pickle

pickle has two main methods – dump, which dumps an object to a file object and load, which loads an object from a file object.  Note here that the file object referred to here is what is returned by the open() function.  It is not the name of the file.  So to use pickle you must first open() the file (either as ‘w’ if you are dumping an object or as ‘r’ if you are loading one) and store the object that the open() function returns.  I will demonstrate by making a demo list object and pickling it to a file called ‘testfile’:

a = ['A dummy question','The correct answer','A wrong answer']
a
['A dummy question', 'The correct answer', 'A wrong answer']
fileName = "testfile"
fileObject = open(fileName,'w') # open the file for writing
import pickle
pickle.dump(a,fileObject)   # this writes the object a to the file named 'testfile'
fileObject.close()
fileObject = open(fileName,'r')  #open the file for reading
b = pickle.load(fileObject)  #load the object from the file into b
b
['A dummy question', 'The correct answer', 'A wrong answer']
a==b
True

You can see that what is now in b is the same as what is in a (because a==b is True, Python thinks they are the same).  Moreover, this dump/load procedure allows you to preserve the object even when you quit of of python and come back to it later (which is the whole point of this exercise):

fileObject.close()
exit()  # leave python and restart
/home/user> python
Python 2.5 (release25-maint, Dec  9 2006, 14:35:53)
[GCC 4.1.2 20061115 (prerelease) (Debian 4.1.1-20)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> fileName = 'testfile'
>>> fileObject = open(fileName,'r')
>>> c = pickle.load(fileObject)  #load the old object
>>> c
['A dummy question', 'The correct answer', 'A wrong answer']

However, now we try to compare c to the original we see that Python has forgotten a when we exited:

c==a
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined

Which is to say that the only place that python got the object c from was the file when it pickle.load()ed.

Homework:
Make some other objects, dump them to a file and load them again.  Make sure that you name the file first then open() it before you pickle and .close() it afterwards.  Use the attribute ‘w’ when you open a file to dump an object and ‘r’ when you are going to load an object.

Pickle vs cPickle:

Python actually has two pickle modules – pickle, which we used above and cPickle. There are some technical differences between them but for most purposes they can be treated as being exactly the same.  The main difference is that cPickle has been written in the C programming language and, as a result, runs much faster.  While I am using pickle here, in future tutorials I will (try to remember to) use cPickle instead.  When you write your own programs you should use the cPickle module by default as it will run faster (ie. wherever you see pickle, use cPickle instead).  Otherwise the usage is exactly the same.

Spelling Note: It is pickle not pickel.


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9 Responses to An Awful Pickle

  1. Pingback: Tweets that mention An Awful Pickle « Python Tutorials for Kids 8+ -- Topsy.com

  2. Pingback: Linux News » Python4Kids: New Tutorial – An Awful Pickle

  3. wumpus says:

    One thing that should be stressed about pickle is how big a security hole it is. It allows whoever packaged the data to run anything they want (mainly due to duck typing, you can replace any variable with a nasty object with all methods changed to hostile actions).

    I suspect that while 8+ year olds might not be terribly interested in proper software engineering, they may be quite interested in breaking each others programming and preventing same. It might be a good introduction to computer security.

  4. ph says:

    you can put multiple objects in a pickle file

  5. brendanscott says:

    @ph:
    quite right, changed

  6. Pingback: Zasoby o Pythonie | Wiadomości o technologiach IT

  7. I really like this tutorial. I just don’t see how you can do anything without a text editor! So how do I put python in a text editor?

    • brendanscott says:

      Hi Nathan
      I’m not entirely sure what you mean. You use a text editor to type the text of a Python program and save it to a file. However, a text editor can’t run a Python program. You need to “drop to a shell” in order to do that. On Linux, look for a menu entry called “terminal” (or run ‘bash’). On Windows, use the ‘run’ option to run ‘command.exe’ (no quotes). The thing you get when you do either of these is called a ‘shell’. From there you need to make sure you’re in the same directory as the program you saved. If you are, then you run the program with python

      All the earlier tutorials assume you *don’t* have a text editor(!!) . Once you “drop to a shell” then type python and hit the return key and you’ll be in a place where you can run Python without a text editor (redo any of the earlier tutorials). To exit type ctrl-d or type exit() +return key.

      • Spoon says:

        I’m not an expert, but I think what you want is IDLE, which is a text editor made specifically for python, which does useful things like highlighting text and indenting automatically, but also has the function of running the code just by pressing F5, making it unnecessary to save your code and then open shell to run it. IDLE is available for Windows and Linux (probably Mac too).

        By the way, thanks for the tutorial. I’ve been wondering if this was possible for a while, and had heard of pickle, but the official python docs seem useless unless you already know what they’re talking about.

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