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')
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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.
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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].
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Negative indices count from the end of the tuple, just as with a list. |
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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.
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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
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You can't add elements to a tuple. Tuples have no append or extend method.
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You can't remove elements from a tuple. Tuples have no remove or pop method.
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You can't find elements in a tuple. Tuples have no index method.
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You can, however, use in to see if an element exists in the tuple.
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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.
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| 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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Further Reading on Tuples