in this video we're going to be talking about f
strings and some of the cool things that you can do with them. so most of you are probably
already aware of what f strings are. it's this kind of notation where you can put in f...
little f or capital f, it actually doesn't matter, and then you have some literal stuff and then
within the string you can put more python code. so this is really useful and it was introduced
in, i think, python, let's see, python 3.6. but what you may not know is that there's a
lot of extra stuff that you can put in between these curly braces to modify what actually
gets printed here. so in this first example we're just printing something out and you
would just see the value is "other dog face" but one of the really cool things that you
can do is just put an equals sign afterwards, and then that will actually print out basically
what's before the equal sign, then the equal sign, and then the value of what's before the equals
sign, so if we go ahead and run this code... let me go ahead and make this bigger for you...
you can see the first one we just have the value is "other dog face" and then in the second one
we see string value equals and then the repr of that variable. so this is extremely useful
especially for debugging purposes if there's some variable in your code like num_value and i
just want to add a quick debug print statement, i can just say, oh let me add in a num_value
equals kind of print statement in here and then immediately you can see it comes
out as num_value equals something, and that's really useful. i wouldn't really use it
in production code, but especially for debugging purposes ... you know i end up using it a lot more
than i probably should. another cool thing about this is that you can actually add spaces basically
anywhere in there and it will preserve the spaces. so if you really like having, you know, everything
nicely printed out, then you can do that, put a space before and after in here and
you see it down here when it's printed out. and this last example you can see that you can
actually have arbitrary expressions there, and so it will print out, you see, num_value % 2.
that part just gets printed out and then the value of that expression gets substituted after
the equals sign. so that's a really cool hidden trick of f strings that you might not have known
about. even though f strings have been in python since python 3.6, this equals sign ability has
only been in since python 3.8, so if you're not using a pretty recent version of python then you
might not have access to this yet. but if you do, that's great, it's super useful. okay next
example, conversions. so if you're not aware, inside the curly braces of an f string after the
expression you can put a exclamation point a, exclamation point r, or exclamation point s, and
what these do is instead of printing the value of this thing, it will additionally do some extra
thing on top of that. so r will call the repr, so this one right here let's print it out you can see
that it get the quotes in the string printed out with it and that's because the repr of a string
actually has the quotes in it. so this second one here is actually equivalent to if i said print the
repr out. the reason that you would want to use this bang r instead of writing repr out is because
you're probably doing this for debugging purposes. let's say that thing that you
actually wanted to print out was, you know, a nice formatted thing "string value"
but something wasn't working quite right so you say let me just print out the repr so
i can see exactly what's being printed, and that'll help clear things up. so what--
what's this bang a here? this exclamation point a? that actually stands for "ascii" so
if you are not familiar with what that does, it's similar to repr. it's very
similar to repr, except that all of the basically non-ascii characters get replaced
by an ascii safe escaped version of it, so this dog face that i have here, you know really
cute but um, if we're limited to just ascii strings i need to escape that with this backslash
u001f kind of stuff. so in python 3 i don't really see a big need for doing this kind of thing but
this is more similar to how python 2 reprs worked, but maybe you just don't want to print out dog
faces because... you know... well i don't know why anyone wouldn't want to print out a
dog face instead of this nasty thing, but i guess you can do that. and then of
course your next question is probably: well you said there was a, r, and s, so what's
the purpose of s? well if you do that it calls the string conversion operator on the type. and
you might think: well why would i ever want to do that when the default is to do that? you know,
if i did nothing what it's going to do is call the string conversion operator and print it out
that way, it's not going to print out the repr, and the reason that this bang s exists is
because this actually gets applied before another thing that you can do which is
formatting, and that will be our next example. so if you're wondering about the
bang s it's for formatting. so if you do one of these things and then do something that i'm
going to show you in the formatting operator, it will apply this either string conversion
or repr conversion or ascii conversion and then apply the formatting after that, so it can
be useful. i usually don't ever do that though. okay so let's get to the formatting example. here,
so i've got a whole bunch of little examples and we'll just, you know, see how they all work. so
basically every type can define what it means to define its own formatting. so i have here, you
know, a float value, and here i have a datetime, and in my f string i can put a colon and then
format string. so... the format strings are actually specific to the type, so the things that
i can put here for a datetime are different than the things that i put here for a numeric value,
so you can see here if i have a datetime i can print it out year month day like that, and that'll
work. so i'll go ahead and run it and you can see now equals 2021 06 14. so that format
string works for datetimes and then for, you know, numeric values. this .2f is
actually a format string telling me i want two decimal places. so you don't have to
manually round things, you can just say i want two decimal places and then the formatting
library, or the formatting function within the number class that you're using, will handle
the rounding and displaying part for you. so what's actually going on under the hood here is
a little bit easier to see if you define your own class. so here i've defined this MyClass and let's
see what it does. it's just an empty class and all it does is define a format method which takes a
format spec and is supposed to return a string. so let's see what happens when we use our
own class in one of these format, you know, things and see what actually happens when it
gets called. so when i go ahead and run it, you can see that i see my class dunder format
called with format_spec equals blah blah blah so you can see this blah blah my format stuff is
exactly what was after the colon inside of the f string in the curly braces. so that is getting
passed as an argument the format_spec argument to the dunder format method of the class, and you
can do whatever you want with that information. so it's totally up to you. in this case i just, you
know, totally ignored the information and just always return MyClass(). this is a way to
allow your own class to decide what are some special ways that i can be printed out. so
that's not something that i do super often, but it's kind of more important that you just
know that that's what the syntax is doing. basically whatever you pass here is just
getting passed to a special function of the class. and so you can look up each class
in the documentation is going to tell you what its format strings are. and you know how to make use
of them so especially for numbers this, you know, automatically rounding to two decimal places, i
do that all the time. so just be aware that that's another thing. all right well that's all i've
got on f strings and little f string tricks. i hope you enjoyed this little more informal video
and i just wanted to give a quick shout out to my patrons on patreon, thank you guys so much i
really appreciate your support see you next time.
