[Tutor] newbie question about default arguments

[Pujo Aji]

Hello Josh,

you wrote a different problem. The tutorial should be like this:

*Important warning:* The default value is evaluated only once. This makes a
difference when the default is a mutable object such as a list, dictionary,
or instances of most classes. 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]

 If you don't want the default to be shared between subsequent calls, you
can write the function like this instead:

def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L


Look the second example use None instead of []

To explain more detail let's create an example :

def f1(a, L=[]):
    L.append(a)
    return L

def f2(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

def main():
    m = f2(3)
    print m     # produce [3]
    m = f2(4)
    print m     # produce [4]

    # ----------------

    m = f1(3)
    print m # produce[3]
    m = f1(4)
    print m # produce[3,4]

    pass

in f1 : when we don't put the second argument the L=[] is created only once.
The second input in f1 will be accumulated.

in f2: when we don't put the second argument L is given a None value. Inside
this function if L is None then L = [] this will always make L = [] that's
way no accumulation happened.

Hope this help.
pujo




On 3/29/06, Josh Adams <jadams at iastate.edu> wrote:
>
> Hi all,
> I was going through the tutorial at http://docs.python.org/tut/node6.htmlwhen I
> came to the bit about default arguments with this code:
>
> def f(a, L=[]):
>     L.append(a)
>     return L
>
> print f(1)
> print f(2)
> print f(3)
>
> returns:
> [1]
> [1, 2]
> [1, 2, 3]
>
> >From the postings here, I think I understand that this occurs because L
> is only
> initialized when f is first run.  However, this code gives some different
> results:
>
> def f2(a, L=[]):
>      if L == []:
>              L = []
>      L.append(a)
>      return L
>
> print f2(1)
> print f2(2)
> print f2(3)
>
> returns:
> [1]
> [2]
> [3]
>
> I'm not too clear on why this doesn't return the same results as the
> first.  Can
> someone enlighten me?
>
> Thanks,
> Josh
>
>
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor
>
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