Interaction with Lists in Python - Part 3
Modifying
Lists
You can modify the
content of a list as needed. Modifying a list means to change a particular
entry, add a new entry, or remove an existing entry. To perform these tasks,
you must sometimes read an entry. The concept of mod-ification is found within
the acronym CRUD, which stands for Create, Read, Update, and Delete. Here are
the list functions associated with CRUD:
append(): Adds a new entry to the end of the
list.
clear(): Removes all entries from the list.
copy(): Creates a copy of the current list and
places it in a new list.
extend(): Adds items from an existing list and
into the current list.
insert(): Adds a new entry to the position
specified in the list.
pop(): Removes an entry from the end of the
list.
remove(): Removes an entry from the specified
position in the list.
The following
steps show how to perform modification tasks with lists. This is a hands-on
exercise. As the book progresses, you see these same functions used within
application code. The purpose of this exercise is to help you gain a feel for
how lists work.
Open a Python Shell window.
You see the
familiar Python prompt.
Type List1 = [] and press Enter.
Python creates a
list named List1 for you.
Notice that the
square brackets are empty. List1 doesn’t contain any entries. You can create
empty lists that you fill with information later. In fact, this is precisely
how many lists start because you usually don’t know what information they will
contain until the user interacts with the list.
Type len(List1) and press Enter.
The len() function
outputs 0. When creating an application, you can check for an empty list using
the len() func-tion. If a list is empty, you can’t perform tasks such as
removing ele-ments from it because there is nothing to remove.
Type List1.append(1) and press Enter.
Type len(List1) and press Enter.
The len() function
now reports a length of 1.
Type List1[0] and press Enter.
You see the value
stored in element 0 of List1.
Type List1.insert(0, 2) and press Enter.
The insert()
function requires two arguments. The first argument is the index of the
insertion, which is element 0 in this case. The second argument is the object
you want inserted at that point, which is 2 in this case.
Type List1 and press Enter.
Python has added
another element to List1. However, using the insert() function lets you add the
new element before the first element.
Type List2 = List1.copy() and press Enter.
The new list,
List2, is a precise copy of List1. Copying is often used to create a temporary
version of an existing list so that a user can make temporary modifications to
it rather than to the original list. When the user is done, the application can
either delete the temporary list or copy it to the original list.
Type List1.extend(List2) and press Enter.
Python copies all
the elements in List2 to the end of List1. Extending is commonly used to
consolidate two lists.
Type List1 and press Enter.
You see that the
copy and extend processes have worked. List1 now contains the values 2, 1, 2,
and 1.
Type List1.pop() and press Enter.
Python displays a
value of 1. The 1 was stored at the end of the list, and pop() always removes
values from the end.
Type List1.remove(1) and press Enter.
This time, Python
removes the item at element 1. Unlike the pop() function, the remove() function
doesn’t display the value of the item it removed.
Type List1.clear() and press Enter.
Using clear()
means that the list shouldn’t contain any elements now.
Using
operators with lists
Lists can also
rely on operators to perform cer tain tasks. For example, if you want to
create a list that contains four copies of the word Hello, you could use MyList
= ["Hello"] * 4 to fill it. A list allows repetition as needed. The
multiplication operator (*) tells Python how many times to repeat a given item.
It’s essen tial to remember that every repeated ele ment is separate, so what
MyList contains
is ['Hello',
'Hello', 'Hello', 'Hello'].
You can also use
concatenation to fill a list. For example, using MyList = ["Hello"] +
["World"] + ["!"] * 4 creates six
elements in
MyList. The first element is Hello, followed by World and ending with four ele
ments with one exclamation mark (!) in each element.
The membership
operator (in) also works with lists. This Post uses a straightforward and
easy-to-understand method of searching lists (the recommended approach).
However, you can use the membership operator to make things shorter and simpler
by using "Hello" in MyList. Assuming that you have your list filled
with ['Hello', 'World', '!', '!', '!', '!'], the output of this state ment is
True.
Type len(List1) and press Enter.
You see that the
output is 0. List1 is definitely empty. At this point, you’ve tried all the
modification methods that Python provides for lists. Work with List1 some more
using these various functions until you feel comfortable making changes to the
list.
Close the Python Shell window.
0 comments:
Post a Comment