Sequence

1. Definition

In Math: $S=x_{1},x_{2},x_{3},\cdots $

Note The sequence of indices: 1, 2, 3, ...

We can refer to any item in the sequence by using its index number S[2]

So we have a concept of the first element, the second element, and so on

Python lists have a concept of positional order, but set do not

• A list is a sequence type
• A set is not a sequence type

2. Built-In Sequence Types

2.1 Mutable

• lists
• bytearrays

2.2 Immutable

• strings
• tuples: in reality, a tuple is more than just a sequence type
• range: more limited than lists, strings, and tuples
• bytes

2.3 Additional Standard Type

2.3.1 collections package

• namedtuple
• deque

2.3.2 array module

• array

3. Homogeneous vs Heterogeneous Sequences

3.1 homogeneous

• each elements is of the same type(a character) 
  • 'python' 
  • Strings

3.2 heterogeneous

• each element may be a different type
  • [1,10.5,'python'] 
  • lists

Homogeneous sequence types usually more efficient in storage wise at least. e.g. prefer using a string of characters, rather than a list or tuple of characters.

4. Features

Sequences are a very common type of iterable. So any sequence type is iterable. But an iterable is not necessarily a sequence type. iterables are more general. Some examples of built-in sequence types are lists, strings, and tuples. 

numbers = [10, 12, 15, 18, 20]
fruits = ("apple", "pineapple", "blueberry")
message = "I love Python ❤️"

They support efficient element access using integer indices via the __getitem()__ special method (indexing) and define a __length()__ method that returns the length of the sequence.

Also, we can use the slicing technique on them. 

# Slicing the sequences
print(numbers[:2])
print(fruits[1:])
print(message[2:])

5. Methods

6. References

https://mathbyteacademy.com/