Heads up! To view this whole video, sign in with your Courses account or enroll in your free 7-day trial. Sign In Enroll
Well done!
You have completed Python Comprehensions!
You have completed Python Comprehensions!
Preview
Let’s take the loops from the last video and see how we can simplify them with list comprehension!
Resources
The Syntax
# list comprehension to produce a list
[expression for temp_var in iterable]
# list comprehension with .join() to produce a string
"".join([expression for temp_var in iterable])
Related Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign upRelated Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign up
In the last video, we looked at two
basic for loops and their syntax.
0:00
Now, I'll teach you how to use Python
list comprehension to make your code more
0:05
efficient.
0:10
We'll start with the list
comprehension syntax, as promised,
0:12
it's only one line, let's break it down.
0:16
These square brackets are used
to hold the values generated by
0:19
the list comprehension.
0:23
This should look quite familiar,
because Python lists are also held in
0:26
square brackets, for and in our
keywords that are part of the syntax.
0:31
These should also look pretty familiar
since they're part of the for
0:37
loop syntax as well.
0:42
Iterable, is the original list or
string that we want to iterate over.
0:43
Temp_var, is the temporary variable name
given to the value that we've iterated to.
0:49
Expression, is a valid block of code
that gets run and returns a value.
0:56
This is the action that is to be taken,
like are capitalizing of names.
1:03
Let's open up list_comprehension.py.
1:08
In there, you'll find our two
loops from the previous video,
1:12
as well as the list_comprehension
syntax template ready to go.
1:17
Let's turn our name capitalizing loop
into a one-line list comprehension.
1:24
First, let's uncomment the syntax.
1:30
We can either simply
delete this hash here, or
1:33
use a handy shortcut to toggle comments.
1:36
CMD + / on macOS or CTRL + / on Windows.
1:40
Then we'll start replacing parts of
the syntax template with the real deal.
1:46
We'll start with original iterable,
that's simply this list we've got here.
1:51
So I'll copy and paste and replace that.
1:56
Temporary variable, is simply this one,
2:00
that we've got in our loop,
nametag, so I'll copy and paste.
2:03
And finally expression, is simply this
one, is what changes to our nametag.
2:08
Now, you'll notice that this append syntax
isn't needed in the list comprehension
2:17
because the surrounding square
brackets are already in place
2:23
to append each return value from
the expression to the new_nametags list.
2:27
And here is our one line
compared to our four line loop.
2:34
Let's comment out our
original for loop and
2:38
then uncomment our print statement at
the bottom, make sure to save the file.
2:44
You can save the file by
going to File > Save or
2:50
simply using the handy shortcut CMD + S
for macOS or CTRL + S for Windows.
2:54
If your file is unsaved in workspaces,
you'll see this orange dot.
3:01
Once it is saved, the dot goes away.
3:06
Let's run list_comprehension.py now.
3:09
With Python3, 2, you'll see a list with
our names all capitalized as expected.
3:12
Just like we saw with the original for
loop, congrats,
3:21
you've just written your
first list comprehension.
3:24
Now let's try the same with our string,
the welcome banner.
3:29
We're gonna scroll up here to the loop,
uncomment the template and get replacing.
3:33
Original_iterable is our welcome string,
3:40
temporary_variable is the letter
that we used in the loop,
3:45
and expression is letter.upper,
3:50
where we uppercase every letter, great.
3:54
Let's comment out the loop and
uncomment the print statement.
3:58
Let's make sure to save and
then run the file.
4:04
Now another handy command line tip for
you.
4:07
You can press the Up arrow to go
back through your command history.
4:10
So I just pressed up here to get to
the command to run the file again.
4:14
But wait, that's not a string,
that's a list of individual characters.
4:20
List comprehension, denoted by the use
of square brackets, always produces
4:24
a list as its result, regardless of
the type of iterable used as the source.
4:30
As you can see with our banner example,
a string, when manipulated with
4:36
list comprehension, returns a list of
individual characters of that string.
4:40
There are many ways to convert
our list back to a string, but
4:47
the most efficient way of doing so here is
with the join method, let's break it down.
4:51
Iterable, is the iterable object that
contains values that we want to join
4:57
together to make a string.
5:01
For us, this is the return
value of a list comprehension.
5:04
Glue is a string that we want to use
to join our list values together.
5:10
I've called this glue because it's
what holds the string together.
5:15
In our case, we don't need any special
characters between our letters, so
5:19
we'll supply an empty string here.
5:24
So now that we know that,
5:28
let's use the join method to turn
this list back into a string.
5:29
We'll supply the glue, which is an empty
string, and then add the join syntax,
5:35
and surround our entire comprehension
syntax with wrapping brackets.
5:40
Let's make sure to save, and then we'll
run the file again by pressing Up and
5:46
Enter, and there we have it, nice work!
5:51
You've just learnt how to use the basic
list comprehension syntax with
5:55
two types of iterables, lists and strings.
6:00
You need to sign up for Treehouse in order to download course files.
Sign upYou need to sign up for Treehouse in order to set up Workspace
Sign up