Python is a mature language, but it hasn't stopped evolving, and there are some issues to consider when coding Python, if you want your code to work with the latest version of Python in five five years from now... See http://www.python.org/peps/pep-3000.html for more information.

Unit Tests

Automated testing is generally better than debugging, and is your safety-net for any kind of refactoring, not only when migrating to a newer Python. Use PyUnit (unittest), DocTest or any alternative (see UnitTests) to help you.

• Write unit tests for all new or refactored code

True Division

Since the beginning, Python has yielded an integer result when two integers are divided, e.g. 3/2 => 1. While correct if we assume that dividing integers means integer division (the remainder is accessible through the modulo operator %) it's not always obvious to beginners. This behaviour will change in a future Python version, so that a/b will yield a float as a result even if both a and b are integers, and a new floor division operator // will perform integer division. See See http://www.python.org/peps/pep-0238.html

• Use true and floor division in new code

from __future__ import division # Enable the new behaviour

f = 3/2 # 1.5

i = 3//2 # 1

New style classes

Currently, there are two kinds of classes in Python. The 'classic' or old style classes, and the new style classes. Old style classes will go away in some future version, and while most code will still work when the default switches from old style to new style, there are some differences in semantics, and the new style classes have some extra features. See http://www.python.org/doc/newstyle.html

• Use new style classes in new code

Don't write

class X:
    pass

Write

class X(object):
    pass

Exception class inheritance

Starting from Python 3.0, all exceptions must be derived from BaseException, which will be the base class for KeyboardInterrupt, SystemExit and Exception from Python 2.5. See http://www.python.org/peps/pep-0352.html

• When defining new exception classes, always inherit (directly or indirectly) from Exception

class MyException(Exception): pass

Arguments for raise statement

The raise statement currently accepts different syntaxes. This will be consolidated so that only one argument is allowed for raise: the exception object, created like any other object. Other syntaxes will be deprecated.

• Write the raise statement with the exception object as the only argument

Don't write:

raise MyException "A nasty error"

Instead, write:

raise MyException("A nasty error")

Comparison of incompatible types

In the future, x < y and friends (>, <=, >=) will raise an exception instead of an arbitrary result, if type(x) != type(y) unless the types explicitly define the behaviour for these comparisions.

• Don't compare objects of different types with undefined comparison behaviour

Deprecation of little-used alternative operators

One obvious way to test for inequality already exists: the '!=' operator. The rarely-used '<>' will go away.

• Use only != for inequality comparison

The syntax `foo` for repr() will not exist in Python 3.0. Almost all code currently uses repr(foo) for this purpose, and the backtick syntax is little-known and less readable.

• Use only repr(foo) for the representation of an object

New keywords

All of the names with, as, nonlocal, True and False are becoming keywords. Code that attempts to assign to those names will generate an exception in Python 3.0.

• Don't assign to names that will become keywords

See more

Here are some more tips on writing Python code to be future proof with Python 3.0.

• Collin Winter have some tips in: [http://oakwinter.com/code/porting-setuptools-to-py3k/]

More changes in the future?

The following little code might be useful to run when you upgrade to a new Python version. It shows how some new features in Python is getting gradually enabled. (This example is from Python 2.3.)

>>> import __future__
>>> for x in __future__.all_feature_names:
...     print x, eval('__future__.'+x)
... 
nested_scopes _Feature((2, 1, 0, 'beta', 1), (2, 2, 0, 'alpha', 0), 16)
generators _Feature((2, 2, 0, 'alpha', 1), (2, 3, 0, 'final', 0), 4096)
division _Feature((2, 2, 0, 'alpha', 2), (3, 0, 0, 'alpha', 0), 8192)

The second tuple (e.g. (3, 0, 0, 'alpha', 0) for division) shows when a new feature will become the default.