Once-only evaluation of default parameter values function definitions

[Shalabh Chaturvedi]

Fred Ma wrote:
> Michael Geary wrote:
> 
> 
>>>  Example#1
>>>  ---------
>>>  def f(a, L=[]):
>>>      L.append(a)
>>>      return L
>>>
>>>  Example#2
>>>  ---------
>>>  def f(a, L=None):
>>>      if L is None:
>>>          L = []
>>>      L.append(a)
>>>      return L
>>
>>Here's a simpler example:
>>
>>
>>>>>a = []
>>>>>b = a
>>>>>a
>>
>>[]
>>
>>>>>b
>>
>>[]
>>
>>>>>a.append(1)
>>>>>a
>>
>>[1]
>>
>>>>>b
>>
>>[1]
>>
>>See what happened? The statement 'b = a' didn't make a copy of a and
>>store it in b. Instead, it made the name b refer to the same object
>>as a. So, when I said b.append(1), it meant exactly the same thing
>>as if I'd said a.append(1).
> 
> 
> Thanks, Mike.  That's alot clearer.  The default value looks like it's
> a persistent object.  It is similar to a static local variable in C++.
> In Example#1, that object actually got changed.  In contrast, in
> Example#2, the code prevents L from remaining bound to None beyond the
> first line, so the None object never gets changed.  In fact, my 2nd
> explanation was close: that there is an unnamed persistent object that
> holds the default value to be bound to L for invocations that don't
> explicity supply an argument for L. The place I went wrong was to say
> that L never gets bound to None again, after it has been bound to a
> caller supplied object on a previous invocation of the function.
>
> Fred

That is correct, L will be bound to None whevever there is no argument 
supplied. Also note that the None object is immutable, and cannot be 
changed, so this mechanism is safe.

The default value is as 'persistent' as the function itself. Functions 
are objects and spring into existance only after Python encounters and 
executes the 'def' statement. Python also evaluates the default values 
at this point and keeps these objects around, along with the function 
definition.

If at a later point you do 'del f', you will delete the function and the 
default values (since there is no more use for them, the function cannot 
be called). Assuming, of course, that f was the only reference to the 
function.

Since the 'name-binding' concept in Python is not exactly the same as 
references in C++, I suggest you also read 'How to think like a Pythonista':

http://starship.python.net/crew/mwh/hacks/objectthink.html

for an excellent coverage of one of Python's basic concepts.

--
Shalabh