The function definition needs to store the default value.  In the common case that will be a
 reference to an immutable constant such as None or 2.  

This example sets the default to a container (a list).  That container is not garbage collec
ted so long as the function exists since the function's default is a reference to the contai
ner.  Thus changes to the container will persist.  

The final wrinkle is to have the function modify the container - it changes its default valu
e each time it is run.  This seems very sneaky, yet delightfully useful at times.

Simon Vandemoortele wrote:
>  
>  I am making my first contact with python through the means of the tutorial
>  (http://www.python.org/doc/current/tut/) and I would like some clarification
>  on the example:
>  
>  --- quote ---
>  Important warning: The default value is evaluated only once. This makes a
>  difference when the default is a mutable object such as a list or dictionary.
>  For example, the following function accumulates the arguments passed to it on
>  subsequent calls:
>  
>  def f(a, l = []):
>      l.append(a)
>      return l
>  print f(1)
>  print f(2)
>  print f(3)
>  
>  This will print
>  
>  [1]
>  [1, 2]
>  [1, 2, 3]
>  --- end quote ---
>  
>  One thing I find astonishing about this is the fact that python functions
>  seem to have memory; each call of f() leads to a different result ! Does this
>  mean that the variable 'l' keeps its content even after the function returns
>  ? This seems very strange to me as I have never seen it in other languages.
>  
>  Some explanation/comments/corrections ?
>  Thx, Simon
>  
>  --
>  If you took all the students that felt asleep in class and laid them
>  end to end, they'd be a lot more comfortable.
>                  -- "Graffiti in the Big Ten"
>  
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