Re: Beginner Question

From	Igor Korot <ikorot01@gmail.com>
Newsgroups	comp.lang.python
Subject	Re: Beginner Question
Date	2016-06-02 00:21 -0400
Message-ID	<mailman.75.1464841301.1839.python-list@python.org> (permalink)
References	<ea789a85-a90c-4ff8-ab34-4b6dc1fc6c2f@googlegroups.com> <CANDiX9Jb2_eYhz0cUBJKcKz2Z7XEmTCXXedyMiAE7jpdnEd3GA@mail.gmail.com> <CA+FnnTwv80nimQY-FXDY8KrZ1gbMTqydGtSwBQt_6it8ztZCXg@mail.gmail.com>

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Hi, guys,

On Wed, Jun 1, 2016 at 9:42 PM, boB Stepp <robertvstepp@gmail.com> wrote:
> On Wed, Jun 1, 2016 at 7:55 PM, Marcin Rak <mrak@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???

I think the OP question here is:

Why it is printing the array?
There is no line like:

t = f(1)
print t

So, why the first print does print the list? The return value should
be thrown away...

Thank you.

>
> 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
> --
> https://mail.python.org/mailman/listinfo/python-list

Thread

Beginner Question Marcin Rak <mrak@sightlineinnovation.com> - 2016-06-01 17:55 -0700
  Re: Beginner Question boB Stepp <robertvstepp@gmail.com> - 2016-06-01 20:42 -0500
    Re: Beginner Question Marcin Rak <mrak@sightlineinnovation.com> - 2016-06-02 06:10 -0700
  Re: Beginner Question Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-02 13:53 +1000
  Re: Beginner Question Igor Korot <ikorot01@gmail.com> - 2016-06-02 00:21 -0400
    Re: Beginner Question Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-02 15:20 +1000
      Re: Beginner Question Igor Korot <ikorot01@gmail.com> - 2016-06-02 09:38 -0400
        Re: Beginner Question sohcahtoa82@gmail.com - 2016-06-02 11:02 -0700

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