Default value for optional parameters unexpected behaviour?

[Terry Reedy]

On 6/26/2011 2:28 PM, Marc Aymerich wrote:
> Hi,
> I'm trying to define a function that has an optional parameter which
> should be an empty list whenever it isn't given. However, it takes as
> value the same value as the last time the function was executed. What
> is the reason of this behaviour? How does python deal with default
> values (i.e. when are they assigned/created)?

Our fine Language Reference. Compound Statements chapter, Function 
definitions section, says in bold type: "Default parameter values are 
evaluated when the function definition is executed. ". I presume the 
tutorial says this somewhere too. Read both, along with the first 5 
chanpter of the Library reference.

If you want code executed when you call the function, put it in the body 
that is executed when you call the function

def f(lst = None):
   if lst is None:
     lst = []
     ...

-- 
Terry Jan Reedy