A sequence is a container of objects which are kept in a specific order. We can identify objects in a sequence by their position or index. Positions are numbered from zero in Python; the element at index zero is the first element.

We call these containers because they are a single object which contains (or collects) any number of other objects. The “any number” clause means that they can collect zero other objects, meaning that an empty container is just as valid as a container with one or thousands of objects.

In some programming languages, they use words like "vector" or "array" to refer to sequential containers. Further in other languages there are very specific implementations of sequential containers. For example, in C or Java, the primitive array has a statically allocated number of positions. In Java, a reference outside that specific number of positions raises an exception. In C, however a reference outside the defined positions of an array is an error that may never be detected. Really.

There are four commonly-used subspecies of sequence containers. The string, the Unicode string, the tuple and the list. A string is a container of single-byte ASCII characters. A Unicode string is a container of multi-byte Unicode (or Universal Character Set) characters. A tuple and a list are more general containers.

When we create a tuple or string, we've created an immutable, or static object. We can examine the object, looking at specific characters or objects. We can't change the object. This means that we can't put additional data on the end of a string. What we can do, however, is create a new string that is the concatenation of the two original strings.

When we create a list, on the other hand, we've created a mutable object. A list can have additional objects appended to it or inserted in it. Objects can be removed from a list, also. A list can grow and shrink; the order of the objects in the list can be changed without creating a new list object.

One other note on string. While string are sequences of characters, there is no separate character data type. A character is simply a string of length one. This relieves programmers from the C or Java burden of remembering which quotes to use for single characters as distinct from multi-character strings. It also eliminates any problems when dealing with Unicode multi-byte characters.