python - Default parameter value for objects -

python provides way set default value function parameters. example is:

def f(x=3):     print(x) 

this primitive type, lets try objects:

def f(x=list()):     print(id(x))      f() 

44289920

f() 

44289920

same object! surprised of being used c/c++ way. done that, understand default value not build @ invoking time @ definition time.

so came solution:

def f(x=list()):     if len(x) == 0:         x = list()     print(id(x)) 

solved! @ price: in opinion doesn't seem clean solution. solution rely in use of len(x) == 0 way identify default value ok function not others solution can generalized as:

def f(x=none):     if x none:         x = list() 

this can shortened to:

def f(x=none):     x = x or list()  # bit shorter version 

my question is, there shorter or better way solve problem? ever be?

i still prefer none approach, here new option think about: if defined type, create new instance of it.

def f(x=list):     if isinstance(x, type): x = x()     print(id(x))