Default Initialization of Starred Variables within the Definition of a Function

Question

It is well-known that in order to set a default value to a variable within a function in Python, the following syntax is used:

def func(x = 0):
    if x == 0:
        print("x is equal to 0")
    else:
        print("x is not equal to 0")

So if the function is called as such:

>>> func()

It results in

'x is equal to 0'

But when a similar technique is used for starred variables, for example,

def func(*x = (0, 0)):

it results in a syntax error. I've tried switching up the syntax by also doing (*x = 0, 0) but the same error is encountered. Is it possible to initialize a starred variable to a default value?

Answer 1

star variables are non standard variables that are meant to allow functions with arbitrary length

*variables is a tuple with all positional arguments (This is usually named args)

**variables is a dictionary with all named arguments (This is usually named kwargs )

They would always be there, just empty if none is provided. You could test if a value is in the dictionary or tuple depending on what type of argument and initialize it.

def arg_test(*args,**kwargs):
   if not args:
      print "* not args provided set default here"
      print args
   else:
      print "* Positional Args provided"
      print args


   if not kwargs:
      print "* not kwargs provided set default here"
      print kwargs
   else:
      print "* Named arguments provided"
      print kwargs

#no args, no kwargs
print "____ calling with no arguments ___"
arg_test()

#args, no kwargs
print "____ calling with positional arguments ___"
arg_test("a", "b", "c")

#no args, but kwargs
print "____ calling with named arguments ___"
arg_test(a = 1, b = 2, c = 3)

Answer 2

The starred variable has a value of an empty tuple () by default. While it's not possible to change that default value due to how starred parameters work (tl;dr: Python assigns un-starred parameters, if any are available and collects the rest inside a tuple; you can read more about them for example in the relevant PEP 3132: https://www.python.org/dev/peps/pep-3132/) you could implement a check at the beginning of the function to find out if x is an empty tuple and then change it accordingly. Your code would look something like this:

def func(*x):
    if x == ():  # Check if x is an empty tuple
        x = (0, 0)
    if x == 0:
        print("x is equal to 0")
    else:
        print("x is not equal to 0")