When you ask a user to enter something you need to
validate the input so you're sure they've entered a valid value. This is what I'm gonna teach you in
today's video, make sure you stick until the end because I'm gonna give you lots of examples and
tips to make your life easier. So, here we are, I've just created validate_user_inputs.py and here
we're gonna have a look at different validations. So, we're gonna start out with validating if
the user enters a number. So, let's start with user_input is equal to input() "Enter a number" and of course if we want to print So, this wouldn't work because user_input,
the input is actually a string so even if I wrote three, the three would be a string
not an integer or float so, we need to convert it. We can do something like float and then user_input or we can do something like
int, integer and we're gonna stick to integer. So now this will work but what if the user
actually enters a letter? This wouldn't work because int() only converts strings that can
be converted to a number so for example "3", a string "3" could be converted to 3, number 3.
But if we entered "hello", "hello" wouldn't be converted to an integer because how can you
convert a word to an integer? You can't. So, we're going to take advantage of this impossibility
to convert a string, a word to number and we're going to use it to validate, so, for
example if you do something like try user_input, int, user_input, except, ValueError print "invalid" "You need to enter a number", something
like that. So here we are trying to convert what the user entered to a number,
if this error occurs, so ValueError that means that that string couldn't be
converted to a number because it's actually not a number. So that would be invalid
and the user would be shown this This, so let's actually try if we get invalid.
So, "Enter number", "Invalid: you need to enter a number". If you enter a number you don't
see invalid you don't see anything. So, this could be used to check if the user actually
entered the number but we want python to actually keep asking the user for a number
until they enter a number, so to do that we're going to use the while True loop. So, we're
gonna get this, actually I'm gonna write that right here, so, while True, I'm gonna do
this, we're gonna get this and put it in here. So we're gonna try to convert the number if
the number can't be converted you need to tell the user that they entered something invalid and
then down here we can actually print the result, let's say 2 + user_input, because here we are sure
that the user_input is actually a number because otherwise the loop would keep looping and looping
and looping. By the way, of course you need to break out the loop because otherwise that would
keep looping anyway so here if the user_input is converted to an integer correctly then you break.
If the user_input cannot be converted to a number, then this runs, the except runs and the user
is shown this. So let's try it out real quick so "Enter a number", "r", "Invalid", "e",
"Invalid", 5, 5 + 2, 7. So this is the first way you can check, you can validate an input
and here you're checking if the user entered a number, a valid number. You can check
the other way around so if the user entered anything but a number and that's really easy
because you can just do something like this, so let's copy that paste it here. So
here you copy this and you place it here and down here you break. So what happens here?
Here, you try to convert it so if the string can be converted to a number that means that
the user entered a number, we don't want them to enter a number, so "Enter anything but a
number", so we want anything like a special character whatever but not a number. So here as I
said we try to convert it, if the user entered 1, which is not valid, this works because of
course you can convert the number to an integer, so we need to print "Invalid" and the while True
will start again. But if this throws an error, that means that the user actually entered
something different from a number and we take advantage of the ValueError, so here in this
case we want the error to occur so that we know that the user entered something else not a number
and we break out of it. If that makes sense. So, here we can't do something like 2 + because that
would throw an error. Let's actually try that out so "Enter anything but a number", "3", "Invalid, you need to enter..." ah, of
course here you need to enter "Anything...." so that would work "4", "Anything but a number", "2", "Anything but
a number", "hello", "hello", perfect! That works! Another thing we can do is we can check
the presence of an invalid character in the user_input, so for example when setting up
a nickname sometimes you don't want the user to enter certain characters and so you can do
something like this, so, let's comment this out. So first of all you start out with a list of
characters that you don't want to allow so dot slash semicolon, I'm just making this up so it doesn't really make a lot of sense. So
here we print a little space and then nickname something like, input, "Enter a nickname" this, this, this not allowed, something like that. So the user knows that
they cannot enter these characters then, invalid counter. And we are going to use this later on to
check if there is at least one invalid character inside of the user_input if there is we're
going to tell the user "wait you entered something wrong, enter a valid nickname".
So, then, for char in not_allowed_characters if char in nickname, invalid_chars_counter plus
equal 1, we add one. So basically here we loop through these and for each one of these we
check if this is included in the nickname, if this is included in the nickname, if this
is included in nickname. If so we add one to the invalid_chars_counter. Then, after the
loop we do something like if invalid_chars_counter is greater than 0, which means that
we've encountered an invalid character, we print something to the user
and we do something like "Invalid the nickname cannot contain dot slash semicolon" else, we break. And down here we can do something
like, print and print "welcome plus nickname". So basically here we've got the characters that we
don't want the user to have in the nickname, here we get the user_input, we've got the counter of
the invalid characters, we go through this list, if one of these is inside, so we check one by
one, if this is inside of nickname let's say fabio dot something.
That would increase the counter and if the counter is greater than zero
then we've got an invalid character, if not we break and we can use the nickname
down below. So, let's try that out, let's clear. "Enter a nickname", let's
say "fabio.", "Invalid", let's say "programmer/" or something like
that and "fabioprogrammer", something like that, "Welcome fabioprogrammer",
because we didn't write any of these characters in the nickname, so this is working perfectly. Then, last but not least, maybe you want the
user to enter only specific set of letters or values for example, five letters of the alphabet
or maybe all the letters of the alphabet but not like special characters and stuff like that.
Or for example you want the user to enter just one letter and not just three or four letters
or a name or something like that, like for the hangman game which I've made a video about and
here I'm gonna show you how you can do that. So I'm going to comment this out again, let's
go down here, like that. So first of all you need a list of valid characters you want to set
I'm going to copy and paste the whole alphabet. Something like that. And then I'm
going to start with while True print, then ,same user_input, input,
"Enter one letter", just one letter. Then if user_input is not in list_valid_characters,
actually let's write chars which makes more sense . So here I'm checking
a lot of things. I'm checking the length because I've got just one letter per element
which means that if the user enters "ab" in the same input, that wouldn't work because
we don't have an "ab" element in here just an "a" and a "b". So we're checking the
length, we're checking if the user enters the right letters and stuff like that,
so we're checking a lot of things just by doing this. So, if user_input not in
list_valid_chars we're gonna print "Invalid: enter a letter of the alphabet" Something like that. So then, if
the user_input is in the list, in this list, which is actually
a tuple, let's call it tuple. A list would work as well so, no worries.
We're going to break, of course, we are going to break and then down here we can use the
the letter to do whatever we want to do, so, "You entered user_input", I'm using the f string. So here we are giving the user a list of options
and only if the user enters one of these options does the program actually go ahead, otherwise
it keeps looping and looping until they enter the right letter, the right character. So let's
try that out, "Enter one letter", "3", "Invalid". As you can see it doesn't work, doesn't work until
I enter just one letter "You entered r". Perfect!
