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Whether we like it or not, someone, somewhere, is going to turn an instance of our class into a string. Or an int. Let's make sure it happens like we want.
You can read more about the magic methods. There are a lot of them, though, so don't try and memorize them all. You'll find that, over time, you memorize the most useful ones. Here are the ones I recommend you start with:
-
__str__- Control how your instances turn into strings -
__int__- Controlint()conversion -
__init__- Customize the initialization of your instances
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[MUSIC]
0:00
Now that we have a handle on creating
classes, let's talk about one of the most
0:04
powerful features of Python's classes,
magic methods.
0:08
You've already seen one magic method,
the __init__ one,
0:11
remember we call it magic because it's
a method that Python will call for you.
0:14
As you can probably imagine, there
are tons of Magic Methods that Python uses
0:18
behind the scenes to
make things run smoothly.
0:21
You'll be learning about some of them, but
0:24
I've linked to the full documentation
about them in the teacher's notes.
0:25
I really like Magic Methods,
because despite their name,
0:29
they let us peel back some of the magic
in Python and see how things are working.
0:32
One of the things that happens behind
the scenes, is conversion between types.
0:35
Think about when you use
the str function on an integer.
0:39
What happens?
0:42
It's actually controlled
by a Magic Method.
0:43
Let's go to WorkSpaces and check it out.
0:45
Let's start with our
illustrious Thief class.
0:47
Right now if we turn
a Thief into a string,
0:50
we get something that's well,
less than useful.
0:54
Not really that great.
0:58
So remember that the 0x blah, blah,
1:00
blah bit refers to where
the object is stored in memory.
1:03
Handy but not really handy.
1:07
Wouldn't it be easier to have it
tell us the name of the character?
1:11
Well, let's make it do that then.
1:14
I think this is probably something we'd
want all characters to have though,
1:16
so we'll do it on the Character class and
not on the Thief class.
1:20
You'll often find the implement
Magic Methods on your base classes, but
1:25
not on your specific
implementation classes.
1:29
Those that you use directly,
1:32
usually only have very specific code
that they need to do their jobs.
1:34
The more generic code goes into
the parent classes further up the chain.
1:37
So let's add a new method here.
1:42
And we'll call it __str__.
1:47
As you probably could have guessed.
1:51
So we actually don't need that line.
1:55
So this method should return
a string that you want to use to
1:57
identify your object,
whenever it's turned into a string.
2:03
So in this case, let's return.
2:07
Let's return two things.
2:12
Let's return the self.__class__.__name__,
so we can see if it's a thief,
2:13
or an archer, or a barbarian, or
whatever and let's return self.name.
2:19
So now, if we recheck what
happens when we run play.py.
2:24
We see Thief: Kenneth, so that's awesome,
that's very cool, that's much,
2:31
much friendlier, and it's better output.
2:35
A related method that's often on
a written is __repr__, __repr__
2:38
which is used to give the official
string representation of an instance.
2:43
Now this was mostly used for debugging and
2:48
should present as much useful
information as possible.
2:50
Right now, our rpg gives back what
our old string looked like, and
2:53
I think that's good output for it,
so I'm gonna leave that alone.
2:57
So that's converting to a string,
but what about other conversions?
3:00
More complex conversions, like turning
an instance into a dictionary or
3:04
other object while possible,
are beyond the scope of this course.
3:07
We can though easily set up
converting our objects into floats.
3:11
Now I don't think any of our existing
classes would work well for this, so
3:16
let's make a new one.
3:19
And we'll do this out here in the OO
Python directory not in the rpg directory.
3:20
Now I'm going to call this numstring.py.
3:25
Let's make a class that holds a number,
holds an int or a float, or
3:29
whatever as a string.
3:33
But, it'll let us turn it into
an int whenever we want to.
3:36
Let's just do this, so we'll say class
NumString, cuz that's a catchy name.
3:40
And then def __init__ (self and value).
3:47
And self.value = str (value).
3:51
And then we'll define the __str__,
and we'll return self.value.
3:57
And we'll define __int__, and
we'll return the int(self.value).
4:04
And let's also define __float__,
which returns the float(self.value).
4:13
Now, I'm sure you could guess
what __int__ and __float__ did.
4:20
Even before I told you.
4:25
Since you've seen __str__.
4:26
But it's still good to see them.
4:27
Hopefully, you can see why you probably
wouldn't want to use this as real code.
4:31
It's good for experimentation though.
4:36
You should always play around, and
4:37
try just whatever silly
ideas you can come up with.
4:39
So, in the interest of being silly,
let’s try it out.
4:43
Let’s come down here,
and then go into python.
4:47
And I’m going to say from
numstring import NumString.
4:50
And I’m going to say five = NumString (5).
4:56
And if I look at five,
I get back to this NumString object.
5:01
So if I do a str(five) and
there's '5' as a string.
5:07
And if I print(five),
5:14
I get the string version printed
out which drops the quote marks.
5:16
What if we do int(five), then we get
back a 5, and what if we do float(five)?
5:21
Then we get back 5.0.
5:28
So cool.
5:29
We have an object that represents
itself as a string, but
5:30
that we can turn into a float or init.
5:33
Now, how is this different from
any other number in a string?
5:35
It's not.
5:39
But hopefully you've gotten
a peek behind the curtain here.
5:39
Now what happens if we try and
do some math with our NumString.
5:43
Any guesses before I type this out?
5:47
So I'm going to clear
the screen real quick.
5:50
So let's try five + 4.
5:53
Type error.
5:55
We can't add NumStrings at int.
5:56
Fair enough, so what if we do five
+ a NumString that represents four?
5:59
Can't add NumStrings, another type error.
6:08
What about five + the string of '4'?
6:09
Type error.
6:16
Seems like no matter what I do,
I can't do any math with it.
6:18
Let's see if we can fix
that in the next video.
6:22
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