The default values are evaluated at the point of function definition in the defining scope, so that
i = 5
def f(arg=i):
print(arg)
i = 6
f() # will print 5
Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:
def f(a, L=[]):
L.append(a)
return L
print(f(1))
print(f(2))
print(f(3))
# This will print
[1]
[1,2]
[1,2,3]
If you don't want the default to be shared between subsequent calls, you can write the function like this instead:
def f(a, L=None):
if L is None:
L = []
L.append(a)
return L
