Sequences

Sequences are ordered sets of objects with an index of non-negative integers. In this and the next few sections, we will use the interactive interpreter to illustrate the different types. Please feel free to type along on your own computer.

Sometimes it surprises people that string is actually a sequence type. But if you look closely, strings are series of characters put together. Strings are enclosed by either single, double, or triple quotes. Note in the following examples, the quotes have to match, and triple quotes allow the string to span different lines:

    >>> a = "networking is fun"
    >>> b = 'DevOps is fun too'
    >>> c = """what about coding?
    ... super fun!"""
    >>>

The other two commonly used sequence types are lists and tuples. Lists are the sequence of arbitrary objects. Lists can be created by enclosing the objects in square brackets. Just like string, lists are indexed by non-zero integers that start at zero. The values of lists are retrieved by referencing the index number:

    >>> vendors = ["Cisco", "Arista", "Juniper"]
    >>> vendors[0]
    'Cisco'
    >>> vendors[1]
    'Arista'
    >>> vendors[2]
    'Juniper'

Tuples are similar to lists, created by enclosing the values in parentheses. Like lists, the values in the tuple are retrieved by referencing its index number. Unlike list, the values cannot be modified after creation:

    >>> datacenters = ("SJC1", "LAX1", "SFO1")
    >>> datacenters[0]
    'SJC1'
    >>> datacenters[1]
    'LAX1'
    >>> datacenters[2]
    'SFO1' 

Some operations are common to all sequence types, such as returning an element by index as well as slicing:

    >>> a
    'networking is fun'
    >>> a[1]
    'e'
    >>> vendors
    ['Cisco', 'Arista', 'Juniper']
    >>> vendors[1]
    'Arista'
    >>> datacenters
    ('SJC1', 'LAX1', 'SFO1')
    >>> datacenters[1]
    'LAX1'
    >>>
    >>> a[0:2]
    'ne'
    >>> vendors[0:2]
    ['Cisco', 'Arista']
    >>> datacenters[0:2]
    ('SJC1', 'LAX1')
    >>>

There are also common functions that can be applied to sequence types, such as checking the number of elements and minimum and maximum values:

    >>> len(a)
    17
    >>> len(vendors)
    3
    >>> len(datacenters)
    3
    >>>
    >>> b = [1, 2, 3, 4, 5]
    >>> min(b)
    1
    >>> max(b)
    5

It would come as no surprise that there are various methods that apply only to strings. It is worth noting that these methods do not modify the underlying string data itself and always return a new string. If you want to use the new value, you would need to catch the return value and assign it to a different variable:

    >>> a
    'networking is fun'
    >>> a.capitalize()
    'Networking is fun'
    >>> a.upper()
    'NETWORKING IS FUN'
    >>> a
    'networking is fun'
    >>> b = a.upper()
    >>> b
    'NETWORKING IS FUN'
    >>> a.split()
    ['networking', 'is', 'fun']
    >>> a
    'networking is fun'
    >>> b = a.split()
    >>> b
    ['networking', 'is', 'fun']
    >>>

Here are some of the common methods for a list. This list is a very useful structure in terms of putting multiple items in and iterating through them. For example, we can make a list of datacenter spine switches and apply the same access list to all of them by iterating through them one by one. Since a list's value can be modified after creation (unlike tuple), we can also expand and contrast the existing list as we move along the program:

    >>> routers = ['r1', 'r2', 'r3', 'r4', 'r5']
    >>> routers.append('r6')
    >>> routers
    ['r1', 'r2', 'r3', 'r4', 'r5', 'r6']
    >>> routers.insert(2, 'r100')
    >>> routers
    ['r1', 'r2', 'r100', 'r3', 'r4', 'r5', 'r6']
    >>> routers.pop(1)
    'r2'
    >>> routers
    ['r1', 'r100', 'r3', 'r4', 'r5', 'r6']