- Controlling Conversion 6:24
- Dream Car 1 objective
- Math 6:22
- Multiplication 2 objectives
- Emulating Built-ins 8:01
- Iterable Album 1 objective
- Subclassing Built-ins 10:28
- Double 3 objectives
- Frustration 1 objective
- Constructicons 5:26
- Dream Vacation 1 objective
- Special Methods 7:30
- Proper Properties 2 objectives
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00:00
Controlling Conversion
6:24Whether 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.
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Now that we have a handle on creating
classes, let's talk about one of the most
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powerful features of Python's classes,
magic methods.
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You've already seen one magic method,
the __init__ one,
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remember we call it magic because it's
a method that Python will call for you.
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As you can probably imagine, there
are tons of Magic Methods that Python uses
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behind the scenes to
make things run smoothly.
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You'll be learning about some of them, but
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I've linked to the full documentation
about them in the teacher's notes.
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I really like Magic Methods,
because despite their name,
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they let us peel back some of the magic
in Python and see how things are working.
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One of the things that happens behind
the scenes, is conversion between types.
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Think about when you use
the str function on an integer.
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What happens?
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It's actually controlled
by a Magic Method.
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Let's go to WorkSpaces and check it out.
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Let's start with our
illustrious Thief class.
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Right now if we turn
a Thief into a string,
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we get something that's well,
less than useful.
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Not really that great.
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So remember that the 0x blah, blah,
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blah bit refers to where
the object is stored in memory.
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Handy but not really handy.
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Wouldn't it be easier to have it
tell us the name of the character?
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Well, let's make it do that then.
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I think this is probably something we'd
want all characters to have though,
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so we'll do it on the Character class and
not on the Thief class.
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You'll often find the implement
Magic Methods on your base classes, but
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not on your specific
implementation classes.
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Those that you use directly,
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usually only have very specific code
that they need to do their jobs.
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The more generic code goes into
the parent classes further up the chain.
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So let's add a new method here.
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And we'll call it __str__.
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As you probably could have guessed.
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So we actually don't need that line.
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So this method should return
a string that you want to use to
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identify your object,
whenever it's turned into a string.
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So in this case, let's return.
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Let's return two things.
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Let's return the self.__class__.__name__,
so we can see if it's a thief,
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or an archer, or a barbarian, or
whatever and let's return self.name.
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So now, if we recheck what
happens when we run play.py.
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We see Thief: Kenneth, so that's awesome,
that's very cool, that's much,
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much friendlier, and it's better output.
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A related method that's often on
a written is __repr__, __repr__
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which is used to give the official
string representation of an instance.
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Now this was mostly used for debugging and
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should present as much useful
information as possible.
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Right now, our rpg gives back what
our old string looked like, and
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I think that's good output for it,
so I'm gonna leave that alone.
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So that's converting to a string,
but what about other conversions?
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More complex conversions, like turning
an instance into a dictionary or
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other object while possible,
are beyond the scope of this course.
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We can though easily set up
converting our objects into floats.
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Now I don't think any of our existing
classes would work well for this, so
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let's make a new one.
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And we'll do this out here in the OO
Python directory not in the rpg directory.
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Now I'm going to call this numstring.py.
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Let's make a class that holds a number,
holds an int or a float, or
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whatever as a string.
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But, it'll let us turn it into
an int whenever we want to.
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Let's just do this, so we'll say class
NumString, cuz that's a catchy name.
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And then def __init__ (self and value).
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And self.value = str (value).
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And then we'll define the __str__,
and we'll return self.value.
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And we'll define __int__, and
we'll return the int(self.value).
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And let's also define __float__,
which returns the float(self.value).
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Now, I'm sure you could guess
what __int__ and __float__ did.
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Even before I told you.
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Since you've seen __str__.
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But it's still good to see them.
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Hopefully, you can see why you probably
wouldn't want to use this as real code.
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It's good for experimentation though.
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You should always play around, and
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try just whatever silly
ideas you can come up with.
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So, in the interest of being silly,
let’s try it out.
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Let’s come down here,
and then go into python.
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And I’m going to say from
numstring import NumString.
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And I’m going to say five = NumString (5).
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And if I look at five,
I get back to this NumString object.
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So if I do a str(five) and
there's '5' as a string.
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And if I print(five),
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I get the string version printed
out which drops the quote marks.
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What if we do int(five), then we get
back a 5, and what if we do float(five)?
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Then we get back 5.0.
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So cool.
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We have an object that represents
itself as a string, but
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that we can turn into a float or init.
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Now, how is this different from
any other number in a string?
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It's not.
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But hopefully you've gotten
a peek behind the curtain here.
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Now what happens if we try and
do some math with our NumString.
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Any guesses before I type this out?
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So I'm going to clear
the screen real quick.
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So let's try five + 4.
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Type error.
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We can't add NumStrings at int.
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Fair enough, so what if we do five
+ a NumString that represents four?
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Can't add NumStrings, another type error.
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What about five + the string of '4'?
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Type error.
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Seems like no matter what I do,
I can't do any math with it.
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Let's see if we can fix
that in the next video.
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