3.3. Introducing Tuples

3.3. Introducing Tuples

A tuple is an immutable list. A tuple can not be changed in any way once it is created.

Example 3.15. Defining a tuple

>>> t = ("a", "b", "mpilgrim", "z", "example") 1
>>> t
('a', 'b', 'mpilgrim', 'z', 'example')
>>> t[0]                                       2
>>> t[-1]                                      3
>>> t[1:3]                                     4
('b', 'mpilgrim')
1 A tuple is defined in the same way as a list, except that the whole set of elements is enclosed in parentheses instead of square brackets.
2 The elements of a tuple have a defined order, just like a list. Tuples indices are zero-based, just like a list, so the first element of a non-empty tuple is always t[0].
3 Negative indices count from the end of the tuple, just as with a list.
4 Slicing works too, just like a list. Note that when you slice a list, you get a new list; when you slice a tuple, you get a new tuple.

Example 3.16. Tuples Have No Methods

>>> t
('a', 'b', 'mpilgrim', 'z', 'example')
>>> t.append("new")    1
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'append'
>>> t.remove("z")      2
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'remove'
>>> t.index("example") 3
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'index'
>>> "z" in t           4
1 You can't add elements to a tuple. Tuples have no append or extend method.
2 You can't remove elements from a tuple. Tuples have no remove or pop method.
3 You can't find elements in a tuple. Tuples have no index method.
4 You can, however, use in to see if an element exists in the tuple.

So what are tuples good for?

  • Tuples are faster than lists. If you're defining a constant set of values and all you're ever going to do with it is iterate through it, use a tuple instead of a list.
  • It makes your code safer if you “write-protect” data that does not need to be changed. Using a tuple instead of a list is like having an implied assert statement that shows this data is constant, and that special thought (and a specific function) is required to override that.
  • Remember that I said that dictionary keys can be integers, strings, and “a few other types”? Tuples are one of those types. Tuples can be used as keys in a dictionary, but lists can't be used this way.Actually, it's more complicated than that. Dictionary keys must be immutable. Tuples themselves are immutable, but if you have a tuple of lists, that counts as mutable and isn't safe to use as a dictionary key. Only tuples of strings, numbers, or other dictionary-safe tuples can be used as dictionary keys.
  • Tuples are used in string formatting, as you'll see shortly.
Tuples can be converted into lists, and vice-versa. The built-in tuple function takes a list and returns a tuple with the same elements, and the list function takes a tuple and returns a list. In effect, tuple freezes a list, and list thaws a tuple.

Further Reading on Tuples