Why do we need tuples in Python (or any immutable data type)?

1. immutable objects can allow substantial optimization; this is presumably why strings are also immutable in Java, developed quite separately but about the same time as Python, and just about everything is immutable in truly-functional languages.

2. in Python in particular, only immutables can be hashable (and, therefore, members of sets, or keys in dictionaries). Again, this afford optimization, but far more than just "substantial" (designing decent hash tables storing completely mutable objects is a nightmare -- either you take copies of everything as soon as you hash it, or the nightmare of checking whether the object's hash has changed since you last took a reference to it rears its ugly head).

Example of optimization issue:

$ python -mtimeit '["fee", "fie", "fo", "fum"]'
1000000 loops, best of 3: 0.432 usec per loop
$ python -mtimeit '("fee", "fie", "fo", "fum")'
10000000 loops, best of 3: 0.0563 usec per loop

None of the answers above point out the real issue of tuples vs lists, which many new to Python seem to not fully understand.

Tuples and lists serve different purposes. Lists store homogenous data. You can and should have a list like this:

["Bob", "Joe", "John", "Sam"]

The reason that is a correct use of lists is because those are all homogenous types of data, specifically, people's names. But take a list like this:

["Billy", "Bob", "Joe", 42]

That list is one person's full name, and their age. That isn't one type of data. The correct way to store that information is either in a tuple, or in an object. Lets say we have a few :

[("Billy", "Bob", "Joe", 42), ("Robert", "", "Smith", 31)]

The immutability and mutability of Tuples and Lists is not the main difference. A list is a list of the same kind of items: files, names, objects. Tuples are a grouping of different types of objects. They have different uses, and many Python coders abuse lists for what tuples are meant for.

Please don't.

Edit:

I think this blog post explains why I think this better than I did:

• Understanding tuples vs. lists in Python - E-Scribe