What's the difference between lists and tuples

Question

What's the difference?

What are the advantages / disadvantages of tuples / lists?

Answer 1

List is mutable and tuples is immutable. The main difference between mutable and immutable is memory usage when you are trying to append an item.

When you create a variable, some fixed memory is assigned to the variable. If it is a list, more memory is assigned than actually used. E.g. if current memory assignment is 100 bytes, when you want to append the 101th byte, maybe another 100 bytes will be assigned (in total 200 bytes in this case).

However, if you know that you are not frequently add new elements, then you should use tuples. Tuples assigns exactly size of the memory needed, and hence saves memory, especially when you use large blocks of memory.

Answer 2

This is an example of Python lists:

my_list = [0,1,2,3,4]
top_rock_list = ["Bohemian Rhapsody","Kashmir","Sweet Emotion", "Fortunate Son"]

This is an example of Python tuple:

my_tuple = (a,b,c,d,e)
celebrity_tuple = ("John", "Wayne", 90210, "Actor", "Male", "Dead")

Python lists and tuples are similar in that they both are ordered collections of values. Besides the shallow difference that lists are created using brackets "[ ... , ... ]" and tuples using parentheses "( ... , ... )", the core technical "hard coded in Python syntax" difference between them is that the elements of a particular tuple are immutable whereas lists are mutable (...so only tuples are hashable and can be used as dictionary/hash keys!). This gives rise to differences in how they can or can't be used (enforced a priori by syntax) and differences in how people choose to use them (encouraged as 'best practices,' a posteriori, this is what smart programers do). The main difference a posteriori in differentiating when tuples are used versus when lists are used lies in what meaning people give to the order of elements.

For tuples, 'order' signifies nothing more than just a specific 'structure' for holding information. What values are found in the first field can easily be switched into the second field as each provides values across two different dimensions or scales. They provide answers to different types of questions and are typically of the form: for a given object/subject, what are its attributes? The object/subject stays constant, the attributes differ.

For lists, 'order' signifies a sequence or a directionality. The second element MUST come after the first element because it's positioned in the 2nd place based on a particular and common scale or dimension. The elements are taken as a whole and mostly provide answers to a single question typically of the form, for a given attribute, how do these objects/subjects compare? The attribute stays constant, the object/subject differs.

There are countless examples of people in popular culture and programmers who don't conform to these differences and there are countless people who might use a salad fork for their main course. At the end of the day, it's fine and both can usually get the job done.

To summarize some of the finer details

Similarities:

1. Duplicates - Both tuples and lists allow for duplicates

2. Indexing, Selecting, & Slicing - Both tuples and lists index using integer values found within brackets. So, if you want the first 3 values of a given list or tuple, the syntax would be the same:

   >>> my_list[0:3]
   [0,1,2]
   >>> my_tuple[0:3]
   [a,b,c]
   

3. Comparing & Sorting - Two tuples or two lists are both compared by their first element, and if there is a tie, then by the second element, and so on. No further attention is paid to subsequent elements after earlier elements show a difference.

   >>> [0,2,0,0,0,0]>[0,0,0,0,0,500]
   True
   >>> (0,2,0,0,0,0)>(0,0,0,0,0,500)
   True
   

Differences: - A priori, by definition

1. Syntax - Lists use [], tuples use ()

2. Mutability - Elements in a given list are mutable, elements in a given tuple are NOT mutable.

   # Lists are mutable:
   >>> top_rock_list
   ['Bohemian Rhapsody', 'Kashmir', 'Sweet Emotion', 'Fortunate Son']
   >>> top_rock_list[1]
   'Kashmir'
   >>> top_rock_list[1] = "Stairway to Heaven"
   >>> top_rock_list
   ['Bohemian Rhapsody', 'Stairway to Heaven', 'Sweet Emotion', 'Fortunate Son']
   
   # Tuples are NOT mutable:       
   >>> celebrity_tuple
   ('John', 'Wayne', 90210, 'Actor', 'Male', 'Dead')
   >>> celebrity_tuple[5]
   'Dead'
   >>> celebrity_tuple[5]="Alive"
   Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   TypeError: 'tuple' object does not support item assignment
   

3. Hashtables (Dictionaries) - As hashtables (dictionaries) require that its keys are hashable and therefore immutable, only tuples can act as dictionary keys, not lists.

   #Lists CAN'T act as keys for hashtables(dictionaries)
   >>> my_dict = {[a,b,c]:"some value"}
   Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   TypeError: unhashable type: 'list'
   
   #Tuples CAN act as keys for hashtables(dictionaries)
   >>> my_dict = {("John","Wayne"): 90210}
   >>> my_dict
   {('John', 'Wayne'): 90210}
   

Differences - A posteriori, in usage

1. Homo vs. Heterogeneity of Elements - Generally list objects are homogenous and tuple objects are heterogeneous. That is, lists are used for objects/subjects of the same type (like all presidential candidates, or all songs, or all runners) whereas although it's not forced by), whereas tuples are more for heterogenous objects.

2. Looping vs. Structures - Although both allow for looping (for x in my_list...), it only really makes sense to do it for a list. Tuples are more appropriate for structuring and presenting information (%s %s residing in %s is an %s and presently %s % ("John","Wayne",90210, "Actor","Dead"))

Answer 3

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.

Answer 4

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

Answer 5

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.

Answer 6

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.