Beginner Question

boB Stepp robertvstepp at gmail.com
Wed Jun 1 21:42:34 EDT 2016


On Wed, Jun 1, 2016 at 7:55 PM, Marcin Rak <mrak at sightlineinnovation.com> wrote:
> Hi to all
>
> I have a beginner question to which I have not found an answer I was able to understand.  Could someone explain why the following program:
>
> def f(a, L=[]):
>     L.append(a)
>     return L
>
> print(f(1))
> print(f(2))
> print(f(3))
>
> gives us the following result:
>
> [1]
> [1,2]
> [1,2,3]
>
> How can this be, if we never catch the returned L when we call it, and we never pass it on back to f???

This comes up rather frequently.  In fact, if you just copy your
function (Which is used in the official Python tutuorial.) and paste
it into Google you will get some relevant hits.  One such is:

https://pythonconquerstheuniverse.wordpress.com/category/python-gotchas/

As the link will explain the behavior you observe is a consequence of
two things:  When Python assigns the default argument for the empty
list and that lists are *mutable*.

Enjoy!


-- 
boB


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