When do default parameters get their values set?

[Terry Reedy]

 >>> def f(a="default "+"arg value"): return a

 >>> f.__defaults__
('default arg value',)

Before any calls, the expression has be evaluated.  When f() is called 
with no arg, the local name 'a' is associated with
'default arg value'. Then code execution begins. Use dir(f), for 
instance, to invesigate internals.

-- 
Terry Jan Reedy