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Repeating things in code is a frequent need. "while" repeats code until an expression is no longer True. Let's explore how to use it.
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In programming, a while loop is
code that continuously repeats
0:01
until a certain condition
is no longer met.
0:04
If I think about my day today,
I've already used a few of these loops.
0:07
In the shower this morning,
while there was soap on my body, I rinsed.
0:12
Afterwards, while I was wet,
I dried my body.
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There's two loops and
I was just waking up.
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Then while my coffee cup was empty,
I filled it, once my cup was full,
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I stopped pouring, while there was cereal
left in my bowl, I took some bites.
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Loops occur all the time in all
applications that we build,
0:32
while data is loading,
show that waiting spinner.
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While the deal that will expire eventually
hasn't yet, update the countdown clock.
0:38
While the user hasn't entered the correct
password, prompt for a retry.
0:43
That's a great idea.
0:48
Let's write a simple little
password checker loop.
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So this is not going to be very robust and
you should never actually use this code
0:53
in a final application but it's a pretty
good example of how while loops work.
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So I'm just giving you this warning now.
1:04
This is like one of those do not
try this at home warnings, okay?
1:06
Right, so let's do this,
let's create a new file, we'll say File,
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New File, and
we will call this password_checker .py.
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So what we'll do right from the start
is we'll prompt for a password.
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It'll say password = input.
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Please enter a super secret password.
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This is not a, it's the.
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Please enter the super secret password.
1:44
Gave some space there,
now already that's a bad idea.
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So here this password is
gonna be right on the screen.
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Right as you type it out.
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Someone could look over their shoulder and
capture it.
1:56
Do not try this at home.
1:58
Okay.
2:01
So if the user doesn't get the password
right, we should let them try again.
2:01
Password typos are super common.
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So what we'll do is we'll use a new
keyword that kicks off our loop.
2:09
And that word is while and that's
followed by an expression that will be
2:13
checked each iteration through the loop.
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This is very much like an if statement,
so if the expression is true,
2:21
continue the loop.
2:26
So that expression is if password is
not equal to whatever the password is.
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So we'll say opensesame,
that's my password by the way.
2:34
So we've got a colon, so we're into
a block of code that will repeat.
2:40
And that's what we'll do
is we'll just ask again.
2:44
And we'll say password = input
2:45
("Invalid password, try again: ").
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And what happens is after the last
line of the while block is finished,
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there's only one right now, so
after this line is finished,
2:59
execution returns to
the while expression again.
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And if this is true,
it will run the block again.
3:05
So let's go ahead and write a message so
that we can see if we were successful,
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let's see, let them into this little
secret world we have over here.
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So welcome to secret town.
3:18
Okay, let's give this a run, so
3:21
we'll say python password_checker.
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Enter the super secret password.
3:31
So I'm gonna enter in food,
invalid password, please try again.
3:36
So because password, which was food,
3:42
did not equal open sesame,
this line of code and so
3:45
we're here now, invalid password,
please try again.
3:50
Again, let's print it out to the screen.
3:55
Let's do one more,
the password to my luggage.
3:57
And you'll see that, again,
12345 is not equal to opensesame.
3:59
And it will continue until
this condition is false.
4:04
So let's make that false,
opensesame is actually equal
4:08
to opensesame, welcome to secret town.
4:13
Again, this is totally not secure at all,
anyone could actually just read this file
4:18
here and see our password in plain
text right there, not a good idea.
4:22
Another thing to take note of is this,
4:26
we'll never see that retry
message if this is ever false,
4:29
the loop would never run because
the condition is false from the get go.
4:33
You know what?
4:38
Let's add an additional check, we don't
want hackers to keep on trying and
4:39
using brute force to
figure out our password.
4:43
Let's only allow them three
attempts before we break out of it.
4:45
So we'll keep track of our attempt count.
4:49
So this one here,
we'll say attempt_count = 1.
4:53
And then we'll increment by one
after each password attempt.
4:59
So we'll say, here,
we'll say attempt_count.
5:04
And much like the in-place
addition that we did on strings,
5:09
you can do that on numbers too.
5:11
So attempt_count += 1.
5:13
So that's attempt_count =
attempt_count +1, more or less, right?
5:15
Just some nice short hand.
5:19
So at the start of this loop, we can check
and see if the count is more than three,
5:21
all right?
5:26
So we can say if attempt_count > 3.
5:27
What should we do?
5:33
So you can actually stop a program running
but first we need to import a module and
5:34
that module name is sys, which is not for
sister, it's short for system.
5:40
Import sys.
5:46
And the name of the function
that we're gonna call is exit.
5:48
So we'll do sys.exit.
5:51
Now the way that sys.exit works,
5:56
if you pass any value to sys.exit,
it's considered an error.
5:58
So whoever ran the program will get back
the fact that an error happened in their
6:04
code, which is kind of what we
want to have happened here.
6:07
So let's give it a message here.
6:10
We'll say too many invalid
password attempts.
6:12
So let's clear this, see what happens.
6:18
So we will run python password_checker.
6:23
Okay, so let's try spam.
6:29
Well, that didn't work.
6:31
Let's try lumberjack.
6:32
No?
6:33
Cheeseshop?
6:35
How about hovercraft?
6:37
So this should be our last attempt, right?
6:40
Too many invalid password attempts.
6:43
And see, it printed it out there,
too, really nice.
6:44
Were those password choices confusing?
6:47
One thing that I always like to make sure
that people know when they're just getting
6:50
started learning Python is that
the language is not based on the snake.
6:53
It is quite possibly,
6:58
surprisingly, named after the British
comedy group Monty Python.
7:00
Now, I like to mention this because you're
gonna see Python code in documentation and
7:05
blog posts that use some
very strange examples.
7:09
You'll see lots of spam and
eggs, and lumberjacks, and
7:13
hovercrafts full of eels
which is super weird.
7:15
Especially if you don't know that all of
these are based on Monty Python skits.
7:19
So actually,
I do recommend watching some Monty Python.
7:23
Now not only because they are some of
the most amazing absurdist comedy sketches
7:27
ever created but also because it will
help with some of the more inside jokes.
7:31
It'll help make sense.
7:35
I always feel for those of you who
might be perplexed by the references.
7:37
But also I can not even imagine
what that must be like for
7:41
learners who have English as a second or
third language.
7:44
A parrot is a speaking bird, no?
7:48
Why is the code talking about
a parrot being electrocuted?
7:50
Is it because it speaks?
7:54
No, I'm sorry, it's because of a joke
that was made in the late 1970s.
7:56
How did you miss that one?
8:01
Check the teacher's notes for more.
8:02
One more thing that I'd like
to tackle stylistically here
8:04
is that I don't like how our password
is in the middle of this code,
8:07
it's kind of down here in this code,
right?
8:10
It's a value that we
might want to change but
8:13
it will remain constant during
the running of this program.
8:16
So one thing that we can do is to create
a variable near the top of the file and
8:19
I'm gonna call it, underneath our import
here, I'm gonna call it MASTER_PASSWORD.
8:24
Now note that I used all capital letters.
8:30
This is a naming convention for constants.
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So we'll say MASTER_PASSWORD and I'm gonna
get rid of this here, put that here.
8:35
And then I'm gonna use
MASTER_PASSWORD here.
8:40
This MASTER_PASSWORD is not something
that I ever plan on changing while
8:46
the program is running.
8:51
It's a constant value.
8:52
And I'm conveying that to readers of this
code by using all capital letters and
8:54
placing it at the top of the file so
it's the first thing that they see.
8:58
And then I'm gonna use
that in our while loop.
9:02
Now if we ever wanted
to change the password,
9:05
you just need to tweak
the constant variable.
9:07
Check the teacher's notes for
more on constants.
9:10
While loops are great for
9:12
when you want code to run until
a condition is no longer true.
9:13
Now, in this case,
you aren't sure when the loop will end.
9:18
There's another type of
loop that is great for
9:22
when you have a certain amount
of items to loop through.
9:25
It's called for and I'll show you what
it's good for right after this break.
9:27
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