On 24/11/13 13:05, Rafael Knuth wrote:
"a" and "b" on the left side are unchangable tuples and they simply get unpacked on the right side.
Be careful about terminology here. a,b is a single tuple with two values. But a and b are variables not tuples.
Tuples are collections of (one or more) values. In python they are separated by commas. Thus a single valued tuple looks like
(5,) A double valued tuple like (3,4) And so on. Note the parentheses are optional and only needed for disambiguating the tuple. 3,4 is also a double valued tuple. Variables are names that refer to values. A value can be any Python object (including a tuple!). So a = None a = 5 a = 's' a = (1,2) are all valid values for the variable 'a' And t = ( (1,2), (2,3) ) is a single tuple composed of two other tuples. So a,b = 3,4 is assigning the tuple on the right side to the tuple on the left. It *simultaneously* assigns 3 to a and 4 to b. It doesn't matter what a and b were storing previously it creates a new tuple on the right and assigns it to another newly created one on the left. Note that the right side could be an existing tuple but the one on the left is always a new tuple. Thus a,b = t # see above would only create one new tuple (on the left) and a would have the value t[0], or (1,2) and b would be t[1] or (2,3). The final thing that makes your case complicated is that a,b appear on both sides of the assignment. But if you remember that the assignments are effectively happening simultaneously it all follows the same rules. Tuple assignment/unpacking is a powerful technique in Python. Without it we would need to introduce extra variables so that a,b = b, a+b would become old_a = a a = b b = old_a + b HTH -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.flickr.com/photos/alangauldphotos _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
