wiki:Python3Issues

Python 3 Changes Of Which To Be Aware

Python 3 is the first Python release ever which introduced backwards-incompatible changes. There's a pretty complete list of those changes here: http://docs.python.org/dev/3.0/whatsnew/3.0.html

This document is a summary of those most likely to affect us and for which we can prepare.

  • The division operator changes.

Python 3 concludes a long-standing debate in the Python community by changing the '/' operator to return a float even if its arguments are int. In other words, in Python 2, 3/2 == 1. In Python 3, 3/2 == 1.5.

We can get "true" division now in Python 2 by adding this at the top of a file:

from __future__ import division

Here's an example of the difference under Python 2.5:

    >>> 3/2
    1
    >>> from __future__ import division
    >>> 3/2
    1.5
    >>> 

Our code involves lots of math, so this is a potentially big change for us. However, we're well positioned to handle it if we take action now. It's pretty simple -- we'll have to retest any modules affected by this change, and I think that means most of them. We'll have a lot more modules in the future than we have now, so making this change will only get more expensive as time goes on.

For that reason, I recommend that we immediately begin using "true" division in all our new code, and that we change our existing code to use "true" division as we get the opportunity to do so.

Last but not least, note that in both Python 2 & 3, the operator always gives the truncating behavior. 32 == 1 and 113 == 3. (Note that the latter doesn't round up to 4.)

  • The different between Python ints and longs disappears.

In Python 2, ints are represented with C longs, so their minimum range is +/- 2147483647. Python 2 longs are effectively unlimited. (If you create a number big enough, eventually your computer will run out of memory or Python will die, but I've created a number with > 78,000 digits and Python was fine with it.)

In Python 3, longs go away and ints are effectively unlimited.

  • Absolute imports become the default.

This change will only affect code inside packages, and I debated even mentioning it here. Suffice it to say that if we need to change our import statements for Python 3, the changes will be minor, straightforward, and easy to find (since import statements are almost always confined to the first pages of modules).

  • "Everything you thought you knew about binary data and Unicode has changed."

Fortunately for us we're not manipulating too much text, so the effect on us will be relatively minor. In addition, there's not much we can do now to future-proof our code. (It's hard to write code of much significance that's compatible with both Python 2 & 3.)

  • The builtin file() constructor is gone. The Python 2 documentation says, "When opening a file, it's preferable to use open() instead of invoking this constructor directly." Despite this, I often used the file() constructor because it allowed me to read in a file in one line like so:

s = file("foo.txt", "rb").read()

Last modified 10 years ago Last modified on Aug 7, 2009, 2:28:59 PM