# Beginner Question

Steven D'Aprano steve+comp.lang.python at pearwood.info
Thu Jun 2 01:20:18 EDT 2016

```On Thursday 02 June 2016 14:21, Igor Korot wrote:

> Hi, guys,
>
> On Wed, Jun 1, 2016 at 9:42 PM, boB Stepp <robertvstepp at gmail.com> wrote:
>> 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???
>
> I think the OP question here is:
>
> Why it is printing the array?

Because he calls the function, then prints the return result.

print(f(1))

calls f(1), which returns [1], then prints [1].

Then he calls:

print(f(2))

which returns [1, 2] (but he expects [2]), then prints it. And so on.

> There is no line like:
>
> t = f(1)
> print t

Correct. But there are lines:

print(f(1))
print(f(2))
print(f(3))

--
Steve

```