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.

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

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.