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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 'a' >>> t[-1] 'example' >>> t[1:3] ('b', 'mpilgrim')
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
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.|
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