Is it possible to set a default value for a variable argument list in Python 3?
Something like:
def do_it(*args=(2, 5, 21)):
pass
I wonder that a variable argument list is of type tuple but no tuple is accepted here.
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2 Answers
If not syntactically, then depending on what behavior you want:
def do_it(*args):
if not args: args = (2, 5, 21)
or
def do_it(a=2, b=5, c=21, *args):
args = (a,b,c)+args
should do it.
Comments
Initializing a list like that usually isn't a good idea.
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)
rint f(3)
This will print
[1]
[1, 2]
[1, 2, 3]
I generally check it something is passed in and if not then init it in the function body.
2 Comments
Mark Rushakoff
If the objects inside the tuple are immutable, then the OP's example is safe since the tuple itself is immutable as well.
Paul Hildebrandt
You are so right. I was thinking of mutable objects which a tuple isn't. Thanks for the correction.