How to list Keys In Dictionary Python
Python Tutorial for Beginners 5: Dictionaries - Working with Key-Value Pairs
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hey there how's it going everybody in
this video we'll be learning about
dictionaries and how we can work with
them and Python
so dictionaries allow us to work with
key value pairs and if you're coming
from another programming language then
you may have heard these called hash
maps or associative arrays so when I say
that we'll be working with key value
pairs these are two linked values where
the key is a unique identifier where we
can find our data and the value is that
data so we can actually think of that
almost like a real physical dictionary
where we look up for definitions so in
that example each word that we look up
would be the key and the definition of
that word would be the value so let's go
ahead and take a look at some examples
so let's say that we wanted to represent
a student using a dictionary so to do
this we could just create a student
variable and set this equal to these
curly braces and that's how we start our
dictionary now within our curly braces
here we will first put in our key so
let's say that we want a key of name now
we're going to put in a colon to
separate our key from our value and for
the name here we'll just start off with
John and now let's add some more keys
and values and to separate these keys
and values we're going to put in a comma
so next let's put in a key of age and a
value of 25 and lastly we'll put in a
key of courses and for this value we'll
pass in a list of courses here so we'll
say math and Comp Sci so now let's print
out our student and see how that looks
so if we save that and run it and we can
see that it prints out all of our keys
and values now let's just get a value of
one key so to do this we can add square
brackets after our dictionary and then
specify the key that we want to access
so I can put in square brackets here and
let's say that we want to access the
name of that student dictionary so I'll
just pass in the name key and run that
and we can see that that gave us that
value of that name key now if I wanted
the courses instead then I could just
pass in that courses for the key if I
run that then you can see that we got
this list of math and Comp sign so we
can see that these values and our
dictionary can be just about anything
our name is a string our age is an
integer and the courses are
list now all of our keys are currently
strings but they can actually be any
immutable data type so usually these
would there be strings or injures but
there are a few more data types that
they can be as well so for example
instead of name as our key here if for
some reason we wanted this to be an
integer so I'll just pass in one so a 1
is a valid key and now if I access that
key of 1 and run that and you can see
that that gave us John but I'm gonna set
that back to be a string for now and
keep that as name so what happens if we
try to access a key that doesn't exist
so for example I'll try to access the
key of phone for a phone number so if we
run this then we can see that we get a
key error because that phone key doesn't
exist now throwing an error when a key
doesn't exist might not always be what
we want so sometimes we might just want
to return none or a default value so to
do this we can use the dictionaries get
method so instead of accessing this key
this way if we were instead to say not
get and use the get method so let's just
go ahead and get the name since we know
that that key already exists so if I run
that then we could see that that works
just like before and gave us the value
of John but if I try to access a key
that doesn't exist so we'll try to
access that phone key again if I save
that and run it then by default this get
method returns none instead of an error
and we can also specify a default value
for keys that don't exist so to do this
we can just pass the default value that
we want as a second argument to this get
method so I'll just put in a comma here
and we'll put in a string that just says
not found so if I save that and run it
now we can see that four keys that don't
exist it returns not found okay so let's
look at how we can add a new entry to
our dictionary so let's say that we
wanted to add that phone number to our
student dictionary and we'll set this
just above our print statement here so
to do this it's just as easy as saying
student and then we will set the key
that we want to set and set this equal
to and we'll just set this equal to a
string of five five five five five five
five
if I save that and run it then we can
see that it found that value of the
phone key when we ran our print
statement now if a key already exists if
we set its value like this then it will
update the value of that key so for
example if right below this I was to say
student and name is equal to and we'll
just pass in Jane if I save that and
then a print out I'll comment out that
for now if I print out our entire
student variable then we can see down
here that the value for name was updated
when we assigned it to Jane now we can
also update values using the update
method now this is especially useful
when we want to update multiple values
at a time so for example let's say that
we wanted to add this phone number
update this name and also update the age
as well so to do this all in one shot we
could say student not update and this
takes in a dictionary as an argument and
the dictionary is just everything that
we either want to add or update so we
can say that we want to update that name
to Jane and we will update the age to
let's say 26 and we also want to add
this phone key and that phone key will
just set as what we had before five five
five five five five five now if I save
that and run it and we can see that just
by running this one statement we updated
the name to Jane the age is now 26 and
it has this new key of phone number okay
so now let's say that we wanted to
delete a specific key and its value now
one way that we can do this is by using
the del keyword which stands for delete
so let me just remove these updates here
and then we can say so let's say that we
wanted to delete the students age so
it's as easy as just saying it del
student age and now if we run this then
we can see that now the only keys that
exist are named and courses so that age
key was deleted now another way that we
can remove a key and value is with the
pop method so if we remember from our
video on lists the pop method will
remove but also return and that value so
that allows us to grab the removed value
with a variable
so we could say age is equal to and do a
student not pop and what we want to pop
is that age so if I save that then we'll
also print the age here below student if
I run that and we can see that the age
and value were removed from our
dictionary but we also created that age
variable that contained the value that
we popped off so that popped off 25 okay
now let's look at how we can loop
through all the keys and values of our
dictionary so first if we want to see
how many keys we have in our dictionary
that we can just print out its length
with the Elian function so if I was to
say print le n of student and run that
then we can see that it returns 3
because we have 3 keys and our student
dictionary now if we wanted to see all
of these keys then we could just print
out student dot keys if I run that then
we can see that that gave us all of the
keys of our dictionary if we wanted all
of our values then we could print out
student not values if I run that you can
see that that gives us only two values
now if we wanted to see the keys and
values then we could use this items
method if I run this then we can see
that now we have these pairs of key and
value pairs so we have name John age 25
courses with the list and we'll be
coming back to these pairs in just one
second so if we wanted to loop through
all of the keys and values in our
dictionary we might be tempted to loop
through the same way we loop through our
list but if we just loop through our
list without using any method then it'll
just loop through the keys so for
example if you were to say four key in
student and then print out that key if
we run this then we can see that it just
looped through and printed out all of
those keys now in order to loop through
the keys and values we'll need to use
that items method that we just saw a
second ago and so we'll just plug that
in there we'll say student items and now
these come and a pair of two values so
instead of just key we're also going to
have to access the value so we can say
four key value and Stu
not items and now we'll print out the
key and that value so if I run that so
we can see that each loop through this
key variable was equal to each key and
this value variable was equal to each
value okay so I think that's going to do
it for this video I hope that now
everyone feels comfortable working with
dictionaries and the functionality
that's available to us and in the next
video we'll be going over conditionals
and how to write if-else and Elif
statements
we'll also be learning more about
boolean and boolean operators now if
anyone has any questions about what we
covered in this video then feel free to
ask in the comment section below and
I'll do my best to answer those and if
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