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While Loops
9:32Repeating things in code is a frequent need. "while" repeats code until an expression is no longer True. Let's explore how to use it.
In programming, a while loop is
code that continuously repeats
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until a certain condition
is no longer met.
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If I think about my day today,
I've already used a few of these loops.
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In the shower this morning,
while there was soap on my body, I rinsed.
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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,
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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.
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While the user hasn't entered the correct
password, prompt for a retry.
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That's a great idea.
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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
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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.
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This is like one of those do not
try this at home warnings, okay?
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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.
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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.
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Do not try this at home.
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Okay.
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So if the user doesn't get the password
right, we should let them try again.
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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.
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And that word is while and that's
followed by an expression that will be
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checked each iteration through the loop.
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This is very much like an if statement,
so if the expression is true,
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continue the loop.
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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.
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So we've got a colon, so we're into
a block of code that will repeat.
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And that's what we'll do
is we'll just ask again.
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And we'll say password = input
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("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,
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execution returns to
the while expression again.
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And if this is true,
it will run the block again.
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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.
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Okay, let's give this a run, so
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we'll say python password_checker.
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Enter the super secret password.
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So I'm gonna enter in food,
invalid password, please try again.
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So because password, which was food,
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did not equal open sesame,
this line of code and so
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we're here now, invalid password,
please try again.
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Again, let's print it out to the screen.
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Let's do one more,
the password to my luggage.
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And you'll see that, again,
12345 is not equal to opensesame.
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And it will continue until
this condition is false.
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So let's make that false,
opensesame is actually equal
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to opensesame, welcome to secret town.
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Again, this is totally not secure at all,
anyone could actually just read this file
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here and see our password in plain
text right there, not a good idea.
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Another thing to take note of is this,
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we'll never see that retry
message if this is ever false,
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the loop would never run because
the condition is false from the get go.
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You know what?
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Let's add an additional check, we don't
want hackers to keep on trying and
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using brute force to
figure out our password.
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Let's only allow them three
attempts before we break out of it.
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So we'll keep track of our attempt count.
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So this one here,
we'll say attempt_count = 1.
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And then we'll increment by one
after each password attempt.
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So we'll say, here,
we'll say attempt_count.
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And much like the in-place
addition that we did on strings,
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you can do that on numbers too.
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So attempt_count += 1.
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So that's attempt_count =
attempt_count +1, more or less, right?
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Just some nice short hand.
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So at the start of this loop, we can check
and see if the count is more than three,
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all right?
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So we can say if attempt_count > 3.
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What should we do?
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So you can actually stop a program running
but first we need to import a module and
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that module name is sys, which is not for
sister, it's short for system.
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Import sys.
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And the name of the function
that we're gonna call is exit.
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So we'll do sys.exit.
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Now the way that sys.exit works,
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if you pass any value to sys.exit,
it's considered an error.
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So whoever ran the program will get back
the fact that an error happened in their
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code, which is kind of what we
want to have happened here.
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So let's give it a message here.
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We'll say too many invalid
password attempts.
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So let's clear this, see what happens.
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So we will run python password_checker.
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Okay, so let's try spam.
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Well, that didn't work.
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Let's try lumberjack.
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No?
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Cheeseshop?
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How about hovercraft?
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So this should be our last attempt, right?
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Too many invalid password attempts.
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And see, it printed it out there,
too, really nice.
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Were those password choices confusing?
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One thing that I always like to make sure
that people know when they're just getting
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started learning Python is that
the language is not based on the snake.
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It is quite possibly,
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surprisingly, named after the British
comedy group Monty Python.
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Now, I like to mention this because you're
gonna see Python code in documentation and
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blog posts that use some
very strange examples.
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You'll see lots of spam and
eggs, and lumberjacks, and
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hovercrafts full of eels
which is super weird.
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Especially if you don't know that all of
these are based on Monty Python skits.
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So actually,
I do recommend watching some Monty Python.
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Now not only because they are some of
the most amazing absurdist comedy sketches
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ever created but also because it will
help with some of the more inside jokes.
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It'll help make sense.
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I always feel for those of you who
might be perplexed by the references.
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But also I can not even imagine
what that must be like for
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learners who have English as a second or
third language.
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A parrot is a speaking bird, no?
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Why is the code talking about
a parrot being electrocuted?
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Is it because it speaks?
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No, I'm sorry, it's because of a joke
that was made in the late 1970s.
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How did you miss that one?
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Check the teacher's notes for more.
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One more thing that I'd like
to tackle stylistically here
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is that I don't like how our password
is in the middle of this code,
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it's kind of down here in this code,
right?
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It's a value that we
might want to change but
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it will remain constant during
the running of this program.
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So one thing that we can do is to create
a variable near the top of the file and
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I'm gonna call it, underneath our import
here, I'm gonna call it MASTER_PASSWORD.
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Now note that I used all capital letters.
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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.
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And then I'm gonna use
MASTER_PASSWORD here.
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This MASTER_PASSWORD is not something
that I ever plan on changing while
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the program is running.
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It's a constant value.
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And I'm conveying that to readers of this
code by using all capital letters and
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placing it at the top of the file so
it's the first thing that they see.
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And then I'm gonna use
that in our while loop.
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Now if we ever wanted
to change the password,
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you just need to tweak
the constant variable.
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Check the teacher's notes for
more on constants.
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While loops are great for
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when you want code to run until
a condition is no longer true.
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Now, in this case,
you aren't sure when the loop will end.
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There's another type of
loop that is great for
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when you have a certain amount
of items to loop through.
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It's called for and I'll show you what
it's good for right after this break.
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