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Title: More Special Methods (Video) – Real Python

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Description: In the previous lesson, I showed you even more about descriptors than was covered in part one of the course and gave you an overview of .__slots__. In this lesson, I’ll continue down the dunder rabbit hole, and if you’re lucky, you’ll see a smiling…

Open Graph Description: In the previous lesson, I showed you even more about descriptors than was covered in part one of the course and gave you an overview of .__slots__. In this lesson, I’ll continue down the dunder rabbit hole, and if you’re lucky, you’ll see a smiling…

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Discussion (3)https://realpython.com/lessons/python-class-special-methods/#discussion
00:00https://realpython.com/lessons/python-class-special-methods/#t=0.58
In the previous lesson, I showed you even more about descriptors than was coveredhttps://realpython.com/lessons/python-class-special-methods/#t=0.58
in part one of the course and gave you an overview of .__slots__. In this lesson,https://realpython.com/lessons/python-class-special-methods/#t=4.0
I’ll continue down the dunder rabbit hole, and if you’re lucky,https://realpython.com/lessons/python-class-special-methods/#t=8.79
you’ll see a smiling cat. Is an Alice in Wonderland reference too obscure?https://realpython.com/lessons/python-class-special-methods/#t=12.06
00:18https://realpython.com/lessons/python-class-special-methods/#t=18.32
Continuing on our journey to dunder all the things,https://realpython.com/lessons/python-class-special-methods/#t=18.32
let me remind you that everything in Python is an object.https://realpython.com/lessons/python-class-special-methods/#t=21.69
So when you go to do key things like operations,https://realpython.com/lessons/python-class-special-methods/#t=25.33
they are actually implemented with methods. They’d have to be ‘cause,https://realpython.com/lessons/python-class-special-methods/#t=27.98
well, they’re objects. And using dunder methods, you can performhttps://realpython.com/lessons/python-class-special-methods/#t=32.1
operations like adding, subtracting, in facthttps://realpython.com/lessons/python-class-special-methods/#t=37.04
all the math things, as well as bitwise operations, convert things …https://realpython.com/lessons/python-class-special-methods/#t=40.65
You’ve seen .__str__() and .__repr__() in previous parts of this course,https://realpython.com/lessons/python-class-special-methods/#t=45.73
and this goes for all conversion.https://realpython.com/lessons/python-class-special-methods/#t=50.45
00:52https://realpython.com/lessons/python-class-special-methods/#t=52.3
Calling bool() or int() or float() invokes a corresponding dunder method. That alsohttps://realpython.com/lessons/python-class-special-methods/#t=52.3
goes for comparison.https://realpython.com/lessons/python-class-special-methods/#t=57.41
All the things you might do in an if statement, like comparing for equals, arehttps://realpython.com/lessons/python-class-special-methods/#t=59.32
done with dunder underneath.https://realpython.com/lessons/python-class-special-methods/#t=62.7
01:05https://realpython.com/lessons/python-class-special-methods/#t=65.04
The cool thing about all this is you can override these methods in your ownhttps://realpython.com/lessons/python-class-special-methods/#t=65.04
objects and be able to use these language features on the objects themselves.https://realpython.com/lessons/python-class-special-methods/#t=68.57
01:14https://realpython.com/lessons/python-class-special-methods/#t=74.19
Let’s go visit the Great Down Dunder. No?https://realpython.com/lessons/python-class-special-methods/#t=74.19
Enter the Dunder Dome. Dunder the Boardwalk? Okay,https://realpython.com/lessons/python-class-special-methods/#t=78.23
I’ll stop now. Nah, one more. Dunderworld,https://realpython.com/lessons/python-class-special-methods/#t=82.77
a movie about tight pants and object methods. Entertain me.https://realpython.com/lessons/python-class-special-methods/#t=86.14
01:29https://realpython.com/lessons/python-class-special-methods/#t=89.55
Leave one of your own in the comments.https://realpython.com/lessons/python-class-special-methods/#t=89.55
01:33https://realpython.com/lessons/python-class-special-methods/#t=93.36
A common reason to override operations is to implement classes where thosehttps://realpython.com/lessons/python-class-special-methods/#t=93.36
operations are natural.https://realpython.com/lessons/python-class-special-methods/#t=97.2
If you want to build math concepts like matrices or vectors,https://realpython.com/lessons/python-class-special-methods/#t=99.1
there’s no reason why you shouldn’t be able to take advantage of the mathhttps://realpython.com/lessons/python-class-special-methods/#t=102.2
operators, adding two of them together, for example. Of course,https://realpython.com/lessons/python-class-special-methods/#t=105.68
most of this is already in the standard library or third-party libraries likehttps://realpython.com/lessons/python-class-special-methods/#t=109.12
NumPy, so don’t write it yourself, but if you wanted to,https://realpython.com/lessons/python-class-special-methods/#t=112.48
this is what it’d look like.https://realpython.com/lessons/python-class-special-methods/#t=115.83
01:58https://realpython.com/lessons/python-class-special-methods/#t=118.09
A Vector class takes an .x and .y component, and .__add__() is what you came herehttps://realpython.com/lessons/python-class-special-methods/#t=118.09
to see. Overriding this method makes the Vector class work with the add operator.https://realpython.com/lessons/python-class-special-methods/#t=123.05
02:08https://realpython.com/lessons/python-class-special-methods/#t=128.71
The signature takes on one other object,https://realpython.com/lessons/python-class-special-methods/#t=128.71
the thing being added to this instance.https://realpython.com/lessons/python-class-special-methods/#t=131.35
The result of .__add__() should be a new vector with its attributes being thehttps://realpython.com/lessons/python-class-special-methods/#t=134.67
addition of this vector and the one passed in. To make this visible inhttps://realpython.com/lessons/python-class-special-methods/#t=139.13
the REPL, I’ve also implemented .__repr__().https://realpython.com/lessons/python-class-special-methods/#t=144.08
02:27https://realpython.com/lessons/python-class-special-methods/#t=147.74
Of course, this class is incomplete.https://realpython.com/lessons/python-class-special-methods/#t=147.74
A good Vector class would implement all the math stuff,https://realpython.com/lessons/python-class-special-methods/#t=149.84
but this is enough so that you get the idea. Importing …https://realpython.com/lessons/python-class-special-methods/#t=152.9
02:41https://realpython.com/lessons/python-class-special-methods/#t=161.09
creating a vector … looking at the vector,https://realpython.com/lessons/python-class-special-methods/#t=161.09
which called .__repr__() …https://realpython.com/lessons/python-class-special-methods/#t=165.84
02:50https://realpython.com/lessons/python-class-special-methods/#t=170.73
creating another vector … and adding them together results in a third.https://realpython.com/lessons/python-class-special-methods/#t=170.73
.__add__() for the win.https://realpython.com/lessons/python-class-special-methods/#t=175.47
It’s a Dunderful Life. I told you, leave a comment.https://realpython.com/lessons/python-class-special-methods/#t=177.32
03:00https://realpython.com/lessons/python-class-special-methods/#t=180.98
I’m not going to stop anytime soon.https://realpython.com/lessons/python-class-special-methods/#t=180.98
03:04https://realpython.com/lessons/python-class-special-methods/#t=184.74
There are a ton of these dunders in the base object,https://realpython.com/lessons/python-class-special-methods/#t=184.74
which you can override. Using dunder methods,https://realpython.com/lessons/python-class-special-methods/#t=187.45
you can create context managers, muck around with dynamic class creationhttps://realpython.com/lessons/python-class-special-methods/#t=190.23
and instantiation control,https://realpython.com/lessons/python-class-special-methods/#t=195.11
check whether something is inside something else, is an instance of somethinghttps://realpython.com/lessons/python-class-special-methods/#t=196.94
else, or how long it is, control what happens when the format() function is called.https://realpython.com/lessons/python-class-special-methods/#t=201.45
03:27https://realpython.com/lessons/python-class-special-methods/#t=207.03
In a previous lesson, you saw how to control some of those attribute managementhttps://realpython.com/lessons/python-class-special-methods/#t=207.03
things, but it goes much further. There are multiple mechanisms for getting,https://realpython.com/lessons/python-class-special-methods/#t=210.89
setting,https://realpython.com/lessons/python-class-special-methods/#t=214.89
adding, and removing attributes, and that’s not it.https://realpython.com/lessons/python-class-special-methods/#t=215.39
03:40https://realpython.com/lessons/python-class-special-methods/#t=220.26
There are over 125 dunder methods and attributes on the base class,https://realpython.com/lessons/python-class-special-methods/#t=220.26
which you automatically inherit just by writing a class definition.https://realpython.com/lessons/python-class-special-methods/#t=224.34
There are so many that if you laid them end to end,https://realpython.com/lessons/python-class-special-methods/#t=228.66
you could use a submarine to travel 20,000 leagues—say it with me—https://realpython.com/lessons/python-class-special-methods/#t=230.95
dunder the sea. Before moving on to the next lesson,https://realpython.com/lessons/python-class-special-methods/#t=235.87
a quick little tangent about things that can go wrong.https://realpython.com/lessons/python-class-special-methods/#t=240.56
04:03https://realpython.com/lessons/python-class-special-methods/#t=243.43
Depending on what you’re doing with a class,https://realpython.com/lessons/python-class-special-methods/#t=243.43
you can see different kinds of errors.https://realpython.com/lessons/python-class-special-methods/#t=245.3
An AttributeError is raised if you attempt to access an attribute that’s not onhttps://realpython.com/lessons/python-class-special-methods/#t=247.78
the object or class. A TypeError is raised if you’re trying to do anhttps://realpython.com/lessons/python-class-special-methods/#t=251.88
operation on the object that isn’t implemented. For example, calling len(),https://realpython.com/lessons/python-class-special-methods/#t=256.52
which invokes .__len__() on an integer. And some classes thathttps://realpython.com/lessons/python-class-special-methods/#t=261.17
expect to be extended will define a method and have it raise a NotImplementedError.https://realpython.com/lessons/python-class-special-methods/#t=265.96
04:30https://realpython.com/lessons/python-class-special-methods/#t=270.0
That way, if the extender didn’t override it,https://realpython.com/lessons/python-class-special-methods/#t=270.0
you’d get this exception.https://realpython.com/lessons/python-class-special-methods/#t=273.85
There’s another way of accomplishing this, though, called an abstract base class,https://realpython.com/lessons/python-class-special-methods/#t=275.95
which I’ll cover in a later lesson. If you’re writing classes,https://realpython.com/lessons/python-class-special-methods/#t=279.53
there are a few common mistakes to look out for.https://realpython.com/lessons/python-class-special-methods/#t=284.08
04:46https://realpython.com/lessons/python-class-special-methods/#t=286.8
Forgetting to include the self argument in a method will cause either anhttps://realpython.com/lessons/python-class-special-methods/#t=286.8
exception if you had no arguments at all, or having a reference to the objecthttps://realpython.com/lessons/python-class-special-methods/#t=290.51
stuffed in whatever you wrote as the first argument.https://realpython.com/lessons/python-class-special-methods/#t=294.75
04:58https://realpython.com/lessons/python-class-special-methods/#t=298.43
I tend to make this mistake when I decide a function that I’ve written reallyhttps://realpython.com/lessons/python-class-special-methods/#t=298.43
shouldn’t be a function and turn it into a method, moving it inside my class.https://realpython.com/lessons/python-class-special-methods/#t=301.87
05:06https://realpython.com/lessons/python-class-special-methods/#t=306.74
Another case is confusing when you are supposed to be using the class itselfhttps://realpython.com/lessons/python-class-special-methods/#t=306.74
vs an instance. This isn’t as common as the previous error,https://realpython.com/lessons/python-class-special-methods/#t=310.65
but can happen when you’re supposed to be passing around a class referencehttps://realpython.com/lessons/python-class-special-methods/#t=315.17
rather than instance reference to, say, a utility method or function.https://realpython.com/lessons/python-class-special-methods/#t=318.43
05:23https://realpython.com/lessons/python-class-special-methods/#t=323.51
Somewhat related is confusing what are class attributes and instance attributes.https://realpython.com/lessons/python-class-special-methods/#t=323.51
05:29https://realpython.com/lessons/python-class-special-methods/#t=329.11
I covered a tricky version of this bug in part one of the course, where I showedhttps://realpython.com/lessons/python-class-special-methods/#t=329.11
the accidental creation of an instance attribute instead of modifying anhttps://realpython.com/lessons/python-class-special-methods/#t=333.33
existing class attribute. Not quite mistakes,https://realpython.com/lessons/python-class-special-methods/#t=337.99
but some things that can lead to problematic code are using non-public membershttps://realpython.com/lessons/python-class-special-methods/#t=341.88
outside a class.https://realpython.com/lessons/python-class-special-methods/#t=346.97
05:48https://realpython.com/lessons/python-class-special-methods/#t=348.48
Things with leading underscores are a sign by the coder that they might changehttps://realpython.com/lessons/python-class-special-methods/#t=348.48
or have side effects that you’re unaware of. Generally,https://realpython.com/lessons/python-class-special-methods/#t=352.36
if you aren’t inside the class, you shouldn’t be using non-public members.https://realpython.com/lessons/python-class-special-methods/#t=355.76
06:01https://realpython.com/lessons/python-class-special-methods/#t=361.1
The entire third part of this course could be summed up in don’t misusehttps://realpython.com/lessons/python-class-special-methods/#t=361.1
object-oriented relationships.https://realpython.com/lessons/python-class-special-methods/#t=365.04
Coders new to OO or coming from object-oriented heavy languages will tend tohttps://realpython.com/lessons/python-class-special-methods/#t=367.54
create oodles of objects.https://realpython.com/lessons/python-class-special-methods/#t=371.68
06:13https://realpython.com/lessons/python-class-special-methods/#t=373.35
This is often overkill in Python, and likewise,https://realpython.com/lessons/python-class-special-methods/#t=373.35
you can always tell a programmer who’s recently discovered the beauty ofhttps://realpython.com/lessons/python-class-special-methods/#t=377.26
operator overloading when they tend to overload the operators to save typing.https://realpython.com/lessons/python-class-special-methods/#t=380.76
06:25https://realpython.com/lessons/python-class-special-methods/#t=385.03
Instead of writing a named method that is clear,https://realpython.com/lessons/python-class-special-methods/#t=385.03
they might override .__add__() instead.https://realpython.com/lessons/python-class-special-methods/#t=387.55
06:30https://realpython.com/lessons/python-class-special-methods/#t=390.82
My general advice here is stick with the intent of the operator.https://realpython.com/lessons/python-class-special-methods/#t=390.82
If you’re not actually adding things together, you shouldn’t override .__add__().https://realpython.com/lessons/python-class-special-methods/#t=395.02
06:39https://realpython.com/lessons/python-class-special-methods/#t=399.38
Classes aren’t just for you.https://realpython.com/lessons/python-class-special-methods/#t=399.38
They’re for the core programmers as well. Next up,https://realpython.com/lessons/python-class-special-methods/#t=403.18
I’ll show you some classes in the standard library.https://realpython.com/lessons/python-class-special-methods/#t=405.94
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