How to pass tuple as argument in Python?

It's because that's not a tuple, it's two arguments to the add method. If you want to give it one argument which is a tuple, the argument itself has to be (3, 'three'):

Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.

>>> li = []

>>> li.append(3, 'three')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: append() takes exactly one argument (2 given)

>>> li.append( (3,'three') )

>>> li
[(3, 'three')]

>>> 

Add more parentheses:

li.append((3, 'three'))

Parentheses with a comma create a tuple, unless it's a list of arguments.

That means:

()    # this is a 0-length tuple
(1,)  # this is a tuple containing "1"
1,    # this is a tuple containing "1"
(1)   # this is number one - it's exactly the same as:
1     # also number one
(1,2) # tuple with 2 elements
1,2   # tuple with 2 elements

A similar effect happens with 0-length tuple:

type() # <- missing argument
type(()) # returns <type 'tuple'>