tuple index when invoke with function with dynamic arguments

Question

>>> class Test(object):
>>>    def test(self,*arg):
>>>       print(arg[0],arg[1])
>>> p = Test()
>>> t = 2,3
>>> p.test(t)

gives me IndexError: tuple index out of range

why is that? and how do i get the value for that tuple?

Answer 1

You passed in just one argument (the whole tuple (2, 3) ), so only arg[0] exists; if you meant the tuple values to be separate arguments, apply them with the *args call syntax:

p.test(*t)

The alternative is to not use the *arg catchall argument in your function definition:

def test(self, arg):

Now your function has two normal positional arguments, self and arg . You can only pass in one argument, and if that is your tuple, arg[0] and arg[1] will work as expected.

Answer 2

Using your demo class:

>>> class Test(object):
>>>    def test(self,*arg):
>>>       print(arg[0],arg[1])

When doing this:

>>> p = Test()
>>> t = 2,3
>>> p.test(t)

arg will have a value of [(1,2),]

When doing this:

>>> p = Test()
>>> t = 2,3
>>> p.test(*t)

arg will have a value of [1,2]

The * in the function means that all remaining arguments (non-keyword) are put into a list for you.

In the first case you send (1,2) has a single argument. In the second case the tuple is made into individual arguments using the * thus you send in 1 and 2 .

For complete documentation on this refer to this Python article: http://docs.python.org/2/reference/expressions.html#calls