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Why Lists and Tuples?
This seems to be the first question that always comes up when teaching beginners about tuples: why do we need tuples if we have lists? Some of the reasoning may be historic; Python’s creator is a mathematician by training, and he has been quoted as seeing a tuple as a simple association of objects and a list as a data structure that changes over time. In fact, this use of the word “tuple” derives from mathematics, as does its frequent use for a row in a relational database table.
The best answer, however, seems to be that the immutability of tuples provides some integrity—you can be sure a tuple won’t be changed through another reference elsewhere in a program, but there’s no such guarantee for lists. Tuples, therefore, serve a similar role to “constant” declarations in other languages, though the notion of constantness is associated with objects in Python, not variables.
Tuples can also be used in places that lists cannot—for example, as dictionary keys (see the sparse matrix example in Chapter 8). Some built-in operations may also require or imply tuples, not lists, though such operations have often been generalized in recent years. As a rule of thumb, lists are the tool of choice for ordered collections that might need to change; tuples can handle the other cases of fixed associations.
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