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[MUSIC PLAYING]

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DAVID MALAN: All right,
this is CS50's introduction

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to programming with Python.

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My name is David Malan, and
this is our look at style.

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Now, up until now, we've been writing
code that's hopefully at least correct.

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That is, the code does
what you intended to do.

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And hopefully, it's
also well designed, that

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is, you're using relatively
few lines of code

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to achieve some goal while still
ensuring that it's readable.

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You're perhaps using functions that save
you the trouble of reinventing wheels,

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so to speak.

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But it's possible that your code isn't
necessarily manifesting the best style

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as well, which is
another form of quality

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that you can ascribe to some code.

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Now style, or the right type of
style to use, is rather subjective,

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and it depends typically on
the programmer, on the company,

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on the course, or on the language
that you're actually using.

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But within the Python
community, it turns out

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that there's some pretty
regimented standards

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that most all python
Programmers adhere to,

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or rather, are expected to adhere
to, because this particular language

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and community has tried to
codify some of their preferences

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for how your code should look in
the form of something called PEP 8.

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So a PEP or P-E-P--

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Python enhancement proposal--
is a set of proposals

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that the community within
the world of Python

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typically generate to not only
propose new ideas, but also to codify,

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ultimately, certain standards.

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And PEP 8 happens to be such a proposal
that standardized, or rather tried

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to standardize, what our code
should look like-- that is to say,

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it's quite possible to write
code that's not only correct,

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it might even be well designed,
but it's just really a mess.

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And it just doesn't look very good.

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It's not very pretty.

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It's therefore harder to read.

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It's harder for others to read.

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And therefore, it's just
not as maintainable.

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And any time you make something
harder to read or less maintainable,

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you're just increasing the
probability, dare say down

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the line, of introducing bugs.

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So it's a good thing for your
code to be properly formatted,

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just like in the world of writing
emails or essays or books or documents

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or beyond.

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It tends to be good
practice to capitalize

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certain words, at least in English,
use good punctuation, use paragraph

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breaks and the like.

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So even if you're relatively
new to programming, at least

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in English or your own
human language, odds

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are you've had quite a bit of
practice with writing language

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in the human world that also
just looks good as well.

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Well, what does it mean to look good
in the world of programming code?

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Well, let me propose that
if you look at PEP 8 itself,

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which is available on the
internet via this address here,

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it turns out that it
tries to standardize

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a number of details that would
be manifest in your own code

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once you've written a number of lines.

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And the overarching premise
of PEP 8, and really

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the notion of style in the
Python community especially,

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is that "readability counts."

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And typically, languages--
Python among them--

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come with what's generally
known as a style guide.

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A style guide, not unlike
PEP 8, is some kind

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of guide-- a document, either
printed or perhaps internet

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based-- that just tries to standardize
what everyone's code should look like.

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So a course that you're taking
might have its own style guide.

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A company you're working for
might have its own style guide.

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You, down the road as a
professional programmer,

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might have your own style
guide for your own code.

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But within the Python community,
they've tried generally

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to standardize for the most
part a lot of these details.

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And ultimately-- quote, unquote too--

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a style guide is about consistency,
consistency with this style guide,

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in the context of Python is important.

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Consistency within a
project is more important.

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And consistency within one module
or function is the most important.

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So that is to say, these
aren't necessarily hard

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and fast rules, but
rather guidelines that

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should guide the design
of your own code.

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Now, how do you go about
designing, or how do you

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go about styling your code well?

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Well, it boils down to things
like this and many more--

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indentation, using
consistent indentation.

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Now, in some languages,
it doesn't strictly

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have to be there, when you indent
one line of code under another,

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or it could be one space, or two
spaces, three spaces, or four-- maybe

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even eight, or an actual tab.

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In the world of Python, they
tried to put an end to this debate

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and they prescribed that we all just
agree to use four spaces consistently.

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Tabs versus spaces-- no
tabs, spaces instead.

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And indeed, typically in
something like VS Code,

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when you hit Tab on
your keyboard, depending

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on how the program is
configured, it should generally

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convert even things like tabs
to individual spaces, like four.

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What about maximum line length?

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Your code tends to get
less and less readable

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the longer and longer
your lines of code get,

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especially if they start
to scroll over the screen.

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So Python too tries to standardize
what the maximum line length is.

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You just shouldn't go past a
certain number of characters

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to the right of your screen.

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Blank lines-- using some
number of blank lines,

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for instance, between blocks
of code, perhaps among comments

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also just lends itself to
making your code more readable.

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Why?

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Because it's not just
a wall of text, a wall

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of code that you or other programmers
have to see by adding in blank lines.

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And whitespace, more generally,
can make it a little easier

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to wrap your mind
around what's going on.

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And then imports-- even
something like this,

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when you're importing this library
or that, or this module or package,

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Python too prescribes just how
and where you should generally

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put those lines of code
that say import or from.

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And Python, via PEP 8, also prescribes
any number of details as well.

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And

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These aren't details that we
necessarily preach along the way,

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but rather practice as we write
each of these examples in class.

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And it turns out, too, that as
you see more and more code, well,

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you just get accustomed to the
certain rules of proposals like these.

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So how do you go about, though,
checking if your code is

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in conformance with something like PEP
8, or a style guide more generally?

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Well, you can certainly
read the style guide itself

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and then look at your own code,
and compare the two left and right.

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And decide, oh, I need to fix this
and this other thing in my own code.

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But there's also tools,
being programmers,

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that can help us solve
these problems as well.

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And one of the most popular
in the world of Python

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is a program called pylint,
which is an example of what's

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generally known as a linter, which
is a program that rather statically

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analyzes.

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That is-- reads your code
top to bottom, left to right,

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and tries to figure out if there
are potentially mistakes therein,

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or at least inconsistencies with
something like a prescribed style

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guide.

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This is something that you can
install via the usual ways,

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using something like pip.

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And its documentation is here.

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But it turns out there's other tools
out there as well that are perhaps

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a little less noisy than pylint.

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It turns out if you run
pylint on most of the programs

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you've written thus far,
odds are you're going

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to be overwhelmed with just how
many things you apparently did wrong

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stylistically, even though your
code may very well be both correct

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and well designed.

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So a little less noisy,
at least initially,

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might be this program here, that's the
de facto standard within the Python

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community for formatting
your code for you.

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Pycodestyle, formerly known as PEP 8--

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a program as well-- is a
program that you can not only

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run on your computer,
documented at this URL here,

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it will actually take care of
the process of reformatting

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your code for you if it's a bit messy.

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That is, if the style of your code,
your indentation and blank lines

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and other details as well, are not
in accordance with the style guide,

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something like pycodestyle
will just fix it for you.

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But another one, an
alternative nowadays that's

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actually gaining steam, that's
perhaps even more popular nowadays,

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quite simply called black.

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And black is a program that you
too can install with pip here,

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and it's documented at this URL.

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And the etymology of the
name black is actually

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an allusion to Henry Ford, who
invented cars way back when, and only

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sold quite a few black
models of the same.

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And indeed, he's generally known as
saying a quote along these lines,

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any customer can have a car
painted any color that he

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wants, so long that it is black.

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So if you think about that for a
moment, it's not quite a choice.

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And indeed, that's the spirit of this
particular formatter called black,

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which is that it's opinionated,
more so than others.

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With a lot of these formatters
that exist out there, you tend to--

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or your company tends
to, or your course tends

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to configure them with certain rules.

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So there might be different
ways to do indentation.

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There might be different ways
to do imports or blank lines,

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and different companies
and different people

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might reasonably disagree, and therefore
have their own style guide here,

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their own style guide there.

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And it just tends to waste a lot of
time, is the thinking, if all of us

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don't even agree on
some of these basics.

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So this particular formatter, called
black, is opinionated in the sense

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that it just makes a lot
of these decisions for you.

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And if you don't like the way black
is formatting your code, tough--

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tough, that's the way
it's going to do it.

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And this is a trend, perhaps now, at
least within the Python community,

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of quibbling a little less
over these stylistic details

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so that you and I can ultimately focus
all the more on writing good code

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and solving actual problems.

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And let's go ahead and do just this.

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Let me go ahead and open up
vscode here, where in advance I've

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created a program called
students.py, whose purpose in life

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is just to create at
the top of this program

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a dictionary containing key
value pairs, the keys of which

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are the names of some
students, the values of which

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are the houses at Hogwarts in which they
live, and then I've just got a for loop

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here that iterates over
each of those students,

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printing out for now
each of their names.

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You can imagine certainly doing
more with the values as well,

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but for now, this is a simple program.

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But it's already manifesting
poor style, arguably-- certainly

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inconsistent with PEP 8 itself.

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And I know this from just experience,
eyeballing it, and realizing, wow,

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this is not a good thing
that this first line of code

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goes completely off my own screen.

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Feels like it's probably
a little long-- and I'm

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going to have to scroll to
even see what's going on.

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And this one's more subtle,
but if you look at line three

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here, even though I've indented
technically sufficiently-- so

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long as my code is indented
underneath the for loop,

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and any subsequent indentation,
if I had more lines of code,

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were similarly indented,
the code would work.

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And it would be correct.

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But it just does not accord with
PEP 8, which prescribes again,

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four spaces of
indentation at each level.

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So how can I go about fixing this?

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Well, if I've already installed
one of these formatters,

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for instance, something like
black, I could literally just

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do this with my terminal window.

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I'm going to run the command black,
space, students.py, and hit enter.

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And voila, you'll see
that the students.py file

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has been automatically reformatted.

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And at the top of my file,
I have now a dictionary--

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same dictionary as before, but
just so much more readable.

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Not only does it not wrap around
to the edge of the screen,

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you can also see each of the key
values pairs one line at a time.

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And you can see that it lends itself
to even adding more down the road.

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It turns out it's not incorrect to
have a final trailing comma like this,

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even though it's not strictly
necessary here on the new line six.

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Why?

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Well, Padma in Ravenclaw
is the very last student

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in this particular dictionary.

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But just in case, as
is often the case, I

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might go in and start adding
more key value pairs later.

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A common source of mistake is
to accidentally forget that, oh,

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I didn't have a comma there previously.

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And here I am, adding
more key value pairs.

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So that is the kind of detail
that black not only fixes for you,

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but again, has an opinion on as well.

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So moving forward, as you
write code of your own,

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it's good to get ingrained into
you some of the lessons of and some

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of the guidelines in a style guide like
PEP 8, but know that there's tools,

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be pycodestyle, or black, or
something else altogether,

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that you can use as a programmer to
help you focus more on correctness,

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more on design, more on
solving actual problems,

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while still ensuring that your code is
now automatically formatted as well.

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