What's the difference between lists and tuples?

Apart from tuples being immutable there is also a semantic distinction that should guide their usage. Tuples are heterogeneous data structures (i.e., their entries have different meanings), while lists are homogeneous sequences. Tuples have structure, lists have order.

Using this distinction makes code more explicit and understandable.

One example would be pairs of page and line number to reference locations in a book, e.g.:

my_location = (42, 11)  # page number, line number

You can then use this as a key in a dictionary to store notes on locations. A list on the other hand could be used to store multiple locations. Naturally one might want to add or remove locations from the list, so it makes sense that lists are mutable. On the other hand it doesn't make sense to add or remove items from an existing location - hence tuples are immutable.

There might be situations where you want to change items within an existing location tuple, for example when iterating through the lines of a page. But tuple immutability forces you to create a new location tuple for each new value. This seems inconvenient on the face of it, but using immutable data like this is a cornerstone of value types and functional programming techniques, which can have substantial advantages.

There are some interesting articles on this issue, e.g. "Python Tuples are Not Just Constant Lists" or "Understanding tuples vs. lists in Python". The official Python documentation also mentions this

"Tuples are immutable, and usually contain an heterogeneous sequence ...".

In a statically typed language like Haskell the values in a tuple generally have different types and the length of the tuple must be fixed. In a list the values all have the same type and the length is not fixed. So the difference is very obvious.

Finally there is the namedtuple in Python, which makes sense because a tuple is already supposed to have structure. This underlines the idea that tuples are a light-weight alternative to classes and instances.

If you went for a walk, you could note your coordinates at any instant in an (x,y) tuple.

If you wanted to record your journey, you could append your location every few seconds to a list.

But you couldn't do it the other way around.

The key difference is that tuples are immutable. This means that you cannot change the values in a tuple once you have created it.

So if you're going to need to change the values use a List.

Benefits to tuples:

1. Slight performance improvement.
2. As a tuple is immutable it can be used as a key in a dictionary.
3. If you can't change it neither can anyone else, which is to say you don't need to worry about any API functions etc. changing your tuple without being asked.

Lists are mutable; tuples are not.

From docs.python.org/2/tutorial/datastructures.html

Tuples are immutable, and usually contain an heterogeneous sequence of elements that are accessed via unpacking (see later in this section) or indexing (or even by attribute in the case of namedtuples). Lists are mutable, and their elements are usually homogeneous and are accessed by iterating over the list.

Difference between list and tuple

Tuples and lists are both seemingly similar sequence types in Python.

1. Literal syntax

   We use parenthesis () to construct tuples and square brackets [ ] to get a new list. Also, we can use call of the appropriate type to get required structure — tuple or list.

   someTuple = (4,6)
   someList  = [2,6] 
   

2. Mutability

   Tuples are immutable, while lists are mutable. This point is the base the for the following ones.

3. Memory usage

   Due to mutability, you need more memory for lists and less memory for tuples.

4. Extending

   You can add a new element to both tuples and lists with the only difference that the id of the tuple will be changed (i.e., we’ll have a new object).

5. Hashing

   Tuples are hashable and lists are not. It means that you can use a tuple as a key in a dictionary. The list can't be used as a key in a dictionary, whereas a tuple can be used

   tup      = (1,2)
   list_    = [1,2] 
   
   c = {tup   : 1}     # ok
   c = {list_ : 1}     # error
   

6. Semantics

   This point is more about best practice. You should use tuples as heterogeneous data structures, while lists are homogenous sequences.

Lists are intended to be homogeneous sequences, while tuples are heterogeneous data structures.

As people have already answered here that tuples are immutable while lists are mutable, but there is one important aspect of using tuples which we must remember

If the tuple contains a list or a dictionary inside it, those can be changed even if the tuple itself is immutable.

For example, let's assume we have a tuple which contains a list and a dictionary as

my_tuple = (10,20,30,[40,50],{ 'a' : 10})

we can change the contents of the list as

my_tuple[3][0] = 400
my_tuple[3][1] = 500

which makes new tuple looks like

(10, 20, 30, [400, 500], {'a': 10})

we can also change the dictionary inside tuple as

my_tuple[4]['a'] = 500

which will make the overall tuple looks like

(10, 20, 30, [400, 500], {'a': 500})

This happens because list and dictionary are the objects and these objects are not changing, but the contents its pointing to.

So the tuple remains immutable without any exception

As people have already mentioned the differences I will write about why tuples.

Why tuples are preferred?

Allocation optimization for small tuples

To reduce memory fragmentation and speed up allocations, Python reuses old tuples. If a tuple no longer needed and has less than 20 items instead of deleting it permanently Python moves it to a free list.

A free list is divided into 20 groups, where each group represents a list of tuples of length n between 0 and 20. Each group can store up to 2 000 tuples. The first (zero) group contains only 1 element and represents an empty tuple.

>>> a = (1,2,3)
>>> id(a)
4427578104
>>> del a
>>> b = (1,2,4)
>>> id(b)
4427578104

In the example above we can see that a and b have the same id. That is because we immediately occupied a destroyed tuple which was on the free list.

Allocation optimization for lists

Since lists can be modified, Python does not use the same optimization as in tuples. However, Python lists also have a free list, but it is used only for empty objects. If an empty list is deleted or collected by GC, it can be reused later.

>>> a = []
>>> id(a)
4465566792
>>> del a
>>> b = []
>>> id(b)
4465566792

Source: https://rushter.com/blog/python-lists-and-tuples/

Why tuples are efficient than lists? -> https://stackoverflow.com/a/22140115

The most important difference is time ! When you do not want to change the data inside the list better to use tuple ! Here is the example why use tuple !

import timeit
print(timeit.timeit(stmt='[1,2,3,4,5,6,7,8,9,10]', number=1000000)) #created list
print(timeit.timeit(stmt='(1,2,3,4,5,6,7,8,9,10)', number=1000000)) # created tuple 

In this example we executed both statements 1 million times

Output :

0.136621
0.013722200000000018

Any one can clearly notice the time difference.

A direction quotation from the documentation on 5.3. Tuples and Sequences:

Though tuples may seem similar to lists, they are often used in different situations and for different purposes. Tuples are immutable, and usually contain a heterogeneous sequence of elements that are accessed via unpacking (see later in this section) or indexing (or even by attribute in the case of namedtuples). Lists are mutable, and their elements are usually homogeneous and are accessed by iterating over the list.

In other words, TUPLES are used to store group of elements where the contents/members of the group would not change while LISTS are used to store group of elements where the members of the group can change.

For instance, if i want to store IP of my network in a variable, it's best i used a tuple since the the IP is fixed. Like this my_ip = ('192.168.0.15', 33, 60). However, if I want to store group of IPs of places I would visit in the next 6 month, then I should use a LIST, since I will keep updating and adding new IP to the group. Like this

places_to_visit = [
    ('192.168.0.15', 33, 60), 
    ('192.168.0.22', 34, 60), 
    ('192.168.0.1', 34, 60), 
    ('192.168.0.2', 34, 60), 
    ('192.168.0.8', 34, 60), 
    ('192.168.0.11', 34, 60)
] 

Just a quick extension to list vs tuple responses:

• Due to dynamic nature, list allocates more bit buckets than the actual memory required. This is done to prevent costly reallocation operation in case extra items are appended in the future.

• On the other hand, being static, lightweight tuple object does not reserve extra memory required to store them.

Lists are mutable. whereas tuples are immutable. Accessing an offset element with index makes more sense in tuples than lists, Because the elements and their index cannot be changed.

Along with a lot of the other comments made here the benefit I see in the use of tuples is the flexibility in their being able to have values of different types UNLIKE a list.

Take for example a database table with different values and types designated for each column. A list couldn't replicate this at all (because of its restriction to a singular type of value that it can contain) whereas Tuple can have multiple different types and values with their placements respected for each column (and can even be placed within a list creating your own virtual representation of a database).

That flexibility and restriction (because values cannot be changed)also has its benefits say for the transmission of transaction data (or say something similar to the format of a table). You "seal" the data in the Tuple preventing it from being modifiable before you send protecting it because of what it is designed to do: provide immutability. What difference is this compared to a readonly collection? The fact that you can have different value types.

The applications of it (because of the great use of lists, objects and dictionaries in general) are limited with people generally being of the mindset that an object model would serve as a better option (and in certain cases does), but say you don't want an object model because you prefer to keep that separate to what you have defined as your business entities. Then a tuple might serve you well in what you are trying to achieve.