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Python provides a way to set a default value for function parameters. An example is:

 def f(x=3):
 print(x)

This is for a primitive type, lets try with objects:

 def f(x=list()):
 print(id(x)) 
 f()

> 44289920

 f()

> 44289920

Same object! I was surprised of this being used to the C/C++ way. Done with that, I now understand the default value is not build at invoking time but at definition time. 

So I came to a solution:

 def f(x=list()):
 if len(x) == 0:
 x = list()
 print(id(x))

Solved! But at what price: In my opinion this doesn't seem to be a very clean solution.
This solution rely in the use of `len(x) == 0` as a way to identify the default value which is Ok for my function but not for others so the solution can be generalized as:

 def f(x=None):
 if x is None:
 x = list()

This can be shortened to:

 def f(x=None):
 x = x or list() # a bit shorter version

My question is, is there any shorter or better way to solve this problem? Will it ever be?

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