Learn Python

Python is one of the simplest language which requires less lines of code to accomplish particular task.This tutorial is presently for begineers of Python 3.0 and I am following mit open courseware and free python version of How to think like a computer scientist html version.

I am making the necessary changes in the book for Python 3.0. Right now I am doing it alone so please keep correcting me and any additions to the content a highly welcome.

Python 3.0 Final was recently released on 3rd December 2008.

Differences in coding in Python 3.0 and earlier releases. Python 2

1. print is now a function so always use print()

examples:

Python 3.0

>>>print ("india")

Earlier versions

>>>print "india"

Print can NEVER be used (use the write() method from objects or create your own print function -- with a different name and use it everywhere)

2. Catching exceptions putting the exception in a given var can NEVER be used (there's no compatible way of doing it in a way that's acceptable in both versions, so, just deal with it as you can using the traceback module)

3. socket.send needs bytearray: socket.send(bytearray(str, 'utf-8'))

4. socket.receive gets bytes (so, they need to be decoded: socket.recv(size).decode('utf-8')

5. Some imports:

try:
    import StringIO
except:
    import io as StringIO #Python 3.0

try:
    from urllib import quote_plus, unquote_plus
except ImportError:
    from urllib.parse import quote_plus, unquote_plus #Python 3.0

try:
    import __builtin__
except ImportError:
    import builtins as __builtin__ #Python 3.0

There are way too many others, so, the approach for getting it right is basically running the 2to3 with that import to see the new version of it and then making it work as it used to.

6. True, False assign: in some scripts, to support older python/jython versions, the following construct was used:
__builtin__.True = 1
__builtin__.False = 0

As True and False are keywords now, this assignment will give a syntax error, so, to keep it working, one must do:
setattr(__builtin__, 'True', 1) -- as this will only be executed if True is not defined, that should be ok.

7. The long representation for values is not accepted anymore, so, 2L would not be accepted in python 3.

The solution for something as long_2 = 2L may be something as:
try:
    long
except NameError:
    long = int
long_2 = long(2)

Note that if you want to define a number that's already higher than the int limit, that won't actually help you (in my particular case, that was used on some arithmetic, just to make sure that the number would be coerced to a long, so, that solution is ok -- note: if you were already on python 2.5, that would not be needed as the conversion int -> long is already automatic)

8. raw_input is now input and input should be written explicitly as eval(raw_input('enter value'))
So, to keep backwards compatibility, I think the best approach would be keeping on with the raw_input (and writing the "old input" explicitly, while removing the "new input" reference from the builtins)

try:
    raw_input
except NameError:
    import builtins

    original_input = builtins.input
    del builtins.input
    def raw_input(*args, **kwargs):
        return original_input(*args, **kwargs)
    builtins.raw_input = raw_input

9. The compiler module is gone. So, to parse something, the solution seems to be using ast.parse and to compile, there's the builtins.compile (NOTE: right now, the 2to3 script doesn't seem to get this correctly)