When can I pass * and ** in the argument of a Python function? i.e.:
def fun_name(arg1, *arg2 , ** arg3):
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1Did you read this? docs.python.org/reference/…S.Lott– S.Lott2010-09-17 14:12:46 +00:00Commented Sep 17, 2010 at 14:12
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3 Answers
As you've stated your question, you aren't using them in the arguments (which occur when you are calling a function), you are using them in the parameters which occur when you are creating a function. The * and ** operators serve different purposes in each of those situations.
When you are defining a function, they specify that positional arguments will be placed in a tuple and that keyword arguments will be placed in a dict. Yes I did just say arguments, but they are applied to paramaters in this case.
def example(*args, **kwargs):
print "args: {0}".format(args)
print "kwargs: {0}".format(kwargs)
example(1, 2, 'a', foo='bar', bar='foo')
when run, this outputs:
args: (1, 2, 'a')
kwargs: {'foo': 'bar', 'bar': 'foo'}
Do you see what I mean when I say that we applied it to the paramaters in the function definition? the arguments are 1, 2, 'a', foo='bar', bar='foo'. the paramaters are *args, **kwargs.
Now here's an example applying them to arguments.
def example2(a, b, foo=None, bar=None):
print "a: {0}, b:{1}, foo:{2}, bar: {3}".format(a, b, foo, bar)
args = (1, 2)
kwargs = {'foo': 'bar', 'bar': 'foo'}
example2(*args, **kwargs)
This outputs:
a: 1, b:2, foo:bar, bar: foo
You can see that when we apply them to arguments (that is when we are calling the function), * has the effect of expanding a list or tuple to fill the positional arguments of a function and ** has the effect of expanding a dictionary to fill the keyword arguments of a function. You just need to make sure that there are enough and not too much arguments in total after the expansions have taken place.
in the last example, the arguments are *args, **kwargs and the parameters are a, b, foo=None, bar=None
Comments
When can I pass * and ** in the argument of a Python function? i.e.:
Short answer: when you require variable number of argument to be passed to your function.
That said, I honestly think that this a very broad question. You will be much better off reading more about these concepts, trying them off and then asking an specific questions here.
@Wooble's answer will help you; but what will help you even more is to understand what *args and **kwargs do. Then you can use them as befits the situations you encounter.
You can learn more about the variable and keyword arguments concepts for example at:
Comments
Any time you want your function to accept non-named positional arguments (*), or additional named arguments (**).
It's certainly possible to include *args, **kwargs in every function definition, although this might not be a great idea if passing more arguments won't actually have any effect.
