[Tutor] Python function seem to have a memory ???

Lloyd Kvam lkvam@venix.com
Sat, 11 Aug 2001 10:05:23 -0400

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 collected so long as the function exists since the function's default is a reference to the container.  Thus changes to the container will persist.  

The final wrinkle is to have the function modify the container - it changes its default value 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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Lloyd Kvam
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