Python 2.5 to 2.4

Last night we ran into a slight problem.  We are running Python 2.5 on our Macbook’s, but our server is running a very stable 2.4.  No problem right? Well kind of.

The first problem we encountered is related to PEP 308.  Someone who had written a RSS parsing plugin for Django, failed to mention that it minimally needed Python 2.5 to run correctly.  But this is one of the greatest advantages of open source.  I rolled up my sleeves and removed what the PEP calls conditional expressions.  I’ve always referred to these as the Ternary operator [or operation].

I must admit, the style used in Python is nothing like I’ve ever used in C/C++, etc.  An example of what I had to replace was:

author = if entry.has_key('author') else ''

changed to:

if entry.has_key('author'): author =
else: author = ''

You can see how much simpler it is to write the first one.  It may be harder to read at first, but if you think of it as:

x = true_value if condition else false_value

Here is the original code I fixed to make it work with Python 2.4.  There are some other shortcuts going on.  One for example may be a bit more confusing at first, but it is nothing more than a “short cut” to make things simpler to write and read, right?!?  🙂

Take a list:

>>> a = range(0,10)
>>> print a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Now suppose I want a list where I multiply every value in this list by 2.  I could write:

>>> x = []
>>> for number in a:
>>>     x.append(number*2)
>>> print x
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

To make the iteration more simple, Python allows us to comprehend the list into:

>>> x = [number * 2 for number in a]
>>> print x
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

This is best expressed as:

L = [expression for variable in sequence]

You can find out more about Python’s list comprehension here.

When rewriting the code, because I was unable to use PEP 308, I decided to write a classic iteration loop, to make the code a bit simpler to read and understand.  There is no reason to wrap a comprehensive list over multiple lines just because you can.  😉


One response to “Python 2.5 to 2.4

  1. How about

    author = entry.get(author, ”)

    ? I think even Py2.4 had that. 🙂

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