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## 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")
    >>> t
    ('a', 'b', 'mpilgrim', 'z', 'example')
    >>> t[0]
    'a'
    >>> t[-1]
    'example'
    >>> t[1:3]
    ('b', 'mpilgrim')

| 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.
| 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]`.
| Negative indices count from the end of the tuple, just as with a list.
| 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")
    Traceback (innermost last):
      File "<interactive input>", line 1, in ?
    AttributeError: 'tuple' object has no attribute 'append'
    >>> t.remove("z")
    Traceback (innermost last):
      File "<interactive input>", line 1, in ?
    AttributeError: 'tuple' object has no attribute 'remove'
    >>> t.index("example")
    Traceback (innermost last):
      File "<interactive input>", line 1, in ?
    AttributeError: 'tuple' object has no attribute 'index'
    >>> "z" in t
    True

| You can't add elements to a tuple. Tuples have no `append` or `extend` method.
| You can't remove elements from a tuple. Tuples have no `remove` or `pop` method.
| You can't find elements in a tuple. Tuples have no `index` method.
| 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

- How to Think Like a Computer Scientist teaches about tuples and shows how to concatenate tuples.
- Python Knowledge Base shows how to sort a tuple.
- Python Tutorial shows how to define a tuple with one element.