Quoting from the python Language Reference[link]:
Default parameter values are evaluated when the function definition is executed.
This means that the expression is evaluated once, when the function is defined, and that that same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use None as the default, and explicitly test for it in the body of the function, e.g. …
I was not aware of this. I have an excuse though, in previous versions of Python, language reference was described as “for language lawyers” [link]. Since I never liked lawyers, I never bothered reading it. I am happy though it was changed in current release and now it is described as “describes syntax and language elements”. I guess I have to read it now.
Below is an example for a bug I had caused by this feature and which made me aware of this issue.
>>> import time >>> def now(t=time.time()): ... print t ... >>> now() 1228988224.36 >>> now() 1228988224.36
UPDATE (Dec, 7):
apt-get install python3 works now. this might be the prefered way for most of us.
Based on sofen’s blog’s post about python 2.6
This was the easiest thing I did recently. It went so smooth that this post is totally redundant.
Seven commands and you are all unicode.
Step 1: Make sure you got all the prerequisites
$ sudo apt-get install build-essential libncursesw5-dev libreadline5-dev libssl-dev libgdbm-dev libbz2-dev libc6-dev libsqlite3-dev tk-dev g++ gcc
Step 2: Grab the bits, extract them and build them
$ wget http://www.python.org/ftp/python/3.0/Python-3.0.tgz $ tar xvzf Python-3.0.tgz $ cd Python-3.0/ $ ./configure $ make $ sudo make install
That’s it my friend. You now have the best software development platform in the entire universe!
$ python3.0 Python 3.0 (r30:67503, Dec 4 2008, 21:27:40) [GCC 4.3.2] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>>
Good luck and enjoy the sweetness of open source software!