[Tutor] How default arg of function works

[Mats Wichmann]

On 06/14/2018 05:01 AM, Deepak Dixit wrote:
> Thanks a lot for this information.
> 
> On Thu, Jun 14, 2018, 4:28 PM Alan Gauld via Tutor <tutor at python.org> wrote:

>> Yes, the default argument object is created when the
>> function is defined (ie before it is even called the
>> first time) and the same reference to that obje3ct is
>> always used for the default argument.

this turns out to be one of the "surprises" in Python that catches quite
a few people.  once you know how things work under the covers, it makes
sense why it is so, but it's a little unusual on the surface.

def (to define a function) is actually an executable statement that is
run when encountered.  The result is a function object, bound to the
name you def'd.

One of the long-time Python luminaries wrote about this ages ago, I just
went and hunted up the link in case you're interested (not that lots of
other people haven't written about it, but Fredrik Lundh's comments are
usually worth a read, and he's got some advice for you if you're seeking
to get less surprising behavior (that is, don't use an empty list as the
default, instead use a placeholder like None that you can check for):

http://www.effbot.org/zone/default-values.htm