Changing Internal Representation
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Introduction to Classes and the 'self' Parameter
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Good morning, class! Today, we're going to dive into classes and objects in Python. Can anyone tell me what a class is?
Isn't it a blueprint for creating objects?
Exactly! And when we create an instance from a class, we call that instance an object. Now, in Python, every method in a class must include 'self' as its first argument. Can anyone explain why?
I think it's because 'self' refers to the instance of the object?
That's correct! 'Self' allows methods to access attributes of the class instance. Think of it as a way for the methods to say, 'I'm working with this specific object.'
Internal Representation of a Point
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Let's illustrate with a common example: a point in 2D space, represented by x and y coordinates. When we created a point, how do we initialize it?
We use the '__init__' method, right?
Yes! The '__init__' function sets the initial values of x and y for our point. What about accessing these values?
We use 'self.x' and 'self.y' to refer to the x and y attributes of the current object.
Perfect! So 'self' keeps track of which object's attributes we are accessing. Remember this when we manipulate data inside our classes!
Changing Representations - Cartesian to Polar Coordinates
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Now, let’s consider how we could represent points using polar coordinates instead of Cartesian coordinates. Why would we want to do that?
Maybe for performance reasons when calculating distances?
Exactly! If we often need the distance from the origin, using r directly saves us from computing it every time. What’s the formula to get r?
It's the square root of x squared plus y squared!
Right! Remember, regardless of the internal changes we make, the functions we expose to users should remain consistent.
Using Default Arguments
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Let’s broaden our understanding by discussing default arguments. Why would we use them in our point class?
To set a default position for the point, like (0, 0) if no values are provided!
Correct! This makes our class more flexible. If a user creates a point without arguments, it defaults to the origin. How would we define that?
We could set a and b to zero in the init method!
Exactly! This is a simple way to avoid errors while initializing.
Key Takeaway - Functionality vs. Implementation
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Before we conclude, what’s the main takeaway when we talk about internal representation versus the external interface?
That we can change how data is represented internally without changing how users interact with the object!
Precisely! This separation allows for optimization without compromising user experience. Great job, everyone!
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
In this section, we delve into how Python uses classes and the 'self' parameter to manage object attributes. Using examples like points and their translation, we cover how internal representations can be changed without altering the public interface of functions. The section emphasizes understanding how data encapsulation works in object-oriented programming.
Detailed
Changing Internal Representation
This section focuses on the principles of object-oriented programming (OOP) in Python, particularly how the internal representation of data can be manipulated without changing the way functions behave externally.
Key Concepts:
- Classes and Objects: Python allows programmers to define a data type through classes, creating instances known as objects. For instance, a class defining a point could have attributes like x and y for its coordinates.
- 'Self' Parameter: Every method within a class requires a first argument
self, which refers to the instance of the object itself. This allows methods to access and modify the object's attributes. - Internal Representation: The section illustrates two ways to internally represent a point—using Cartesian coordinates (x, y) and polar coordinates (r, θ). The transition between these representations is possible while keeping the interface (i.e., how users interact with the functions) consistent.
- Functionality vs. Implementation: Modifications in internal representation shouldn't affect how users interact with the object. For example, changing to polar coordinates improves efficiency for certain functions but requires conversion for others.
- Default Arguments: Demonstrating the use of default values in function arguments, the section cites examples where points can default to the origin if no coordinates are provided.
This understanding of data encapsulation, method attributes, and the use of constructors in class definitions is essential for mastering Python's object-oriented features.
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Significance of Self in Python Classes
Chapter 1 of 5
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Chapter Content
We saw a skeleton implementation of a heap. This had a special function called init, which was used to create the heap, and then we had functions insert and delete. Now one thing which we did not explain is this argument self that run through this. This is a convention which we have in python that every function defined inside a class should have as its first parameter the name self, now it need not be called self, but it is less confusing to always call it, self.
Detailed Explanation
In Python, when you create a class, like a heap, you define functions to manage its data. The first parameter of these functions is conventionally named 'self'. This refers to the instance (or object) of the class. It's like saying, 'this function applies to this specific object of the class'. For example, if you have a heap object named 'h' and you call 'h.insert(17)', inside the insert function, 'self' refers to 'h', allowing you to manipulate its contents.
Examples & Analogies
Think of 'self' as your personal identification in a group. If you were in a classroom and the teacher asked a question, you might respond, 'In my opinion, ...'. Here, 'my' refers to you, just like 'self' refers to the specific object or instance in the Python class.
Defining Point with Coordinates
Chapter 2 of 5
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Chapter Content
Our first example is just a representation of a point x y. So, we are just thinking about a normal coordinate system, where we have the x-axis, the y-axis. Therefore, a given point is given some coordinate like a comma b. This is a point with x-coordinate a and y-coordinate b, this is a familiar concept that all of you must have seen in mathematics somehow.
Detailed Explanation
In this example, we define a Point class that represents a point in a 2D space using x and y coordinates. When initializing a Point, you pass the x and y values, and these values are assigned to 'self.x' and 'self.y', respectively. This ensures that every point object accurately holds its own unique coordinates.
Examples & Analogies
Imagine plotting your home on a map using coordinates. Your home might be located at (3, 4) where '3' represents the right position from the left edge of the map (x-axis) and '4' is the upward position from the bottom (y-axis). Each home on the map corresponds to a unique pair of coordinates.
Shifting the Point's Position
Chapter 3 of 5
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Chapter Content
So, you want to say shift this to 3 plus delta x and 4 plus delta y. This function we called translate. So, it takes the amount of shift as the argument, delta x and delta y.
Detailed Explanation
The translate function modifies the position of a point by shifting it according to delta x and delta y values. When you call 'p.translate(2,1)', the current point's coordinates are updated: x becomes 'self.x + delta x' and y becomes 'self.y + delta y'. This example highlights how you can change an object's internal state using methods in a class.
Examples & Analogies
Think of the point as a character in a video game. If you want to move your character to the right and slightly up by specific amounts, you would tell the game to shift your character’s position. The 'shift' here is similar to how we apply 'delta' movements to our point.
Calculating the Distance from Origin
Chapter 4 of 5
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Chapter Content
Supposing we want to compute the distance of a point from the origin. So, we want to know what is this distance by Pythagoras' theorem is nothing but the square root of x square plus y square.
Detailed Explanation
To calculate the distance from the origin (0, 0) to a point (x, y), we use the Pythagorean theorem. This involves squaring the x and y coordinates, adding them together, and taking the square root. The method does not require any parameters other than 'self' because it inherently works with the point's own coordinates.
Examples & Analogies
Imagine you are standing on the ground (the origin) and need to find out how far away a tree is located at point (3, 4) in the park. By using the Pythagorean theorem (like measuring the sides of a right triangle), you can find out the straight-line distance to the tree.
Changing Internal Representation
Chapter 5 of 5
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Chapter Content
We could change our implementation, so that we do not keep the internal representation in terms of x and y, we actually keep it in terms of r and theta.
Detailed Explanation
In programming, you can change how data is internally stored without affecting how it behaves from the user's perspective. For example, instead of using Cartesian coordinates (x, y) to represent a point, you can use polar coordinates (r, theta). Despite this change, the methods still work the same way, showing the flexibility of object-oriented design.
Examples & Analogies
Imagine organizing your closet by seasons instead of colors. You still access clothes the same way (like putting on a shirt) regardless of how they are organized. Changing the internal organization makes finding things more efficient, just as switching from (x, y) to (r, theta) can be more efficient for certain tasks.
Key Concepts
-
Classes and Objects: Python allows programmers to define a data type through classes, creating instances known as objects. For instance, a class defining a point could have attributes like x and y for its coordinates.
-
'Self' Parameter: Every method within a class requires a first argument
self, which refers to the instance of the object itself. This allows methods to access and modify the object's attributes. -
Internal Representation: The section illustrates two ways to internally represent a point—using Cartesian coordinates (x, y) and polar coordinates (r, θ). The transition between these representations is possible while keeping the interface (i.e., how users interact with the functions) consistent.
-
Functionality vs. Implementation: Modifications in internal representation shouldn't affect how users interact with the object. For example, changing to polar coordinates improves efficiency for certain functions but requires conversion for others.
-
Default Arguments: Demonstrating the use of default values in function arguments, the section cites examples where points can default to the origin if no coordinates are provided.
-
This understanding of data encapsulation, method attributes, and the use of constructors in class definitions is essential for mastering Python's object-oriented features.
Examples & Applications
Defining a simple class Point with x and y attributes initialized in the init method: 'class Point: def init(self, x, y): self.x = x; self.y = y'.
To create a point at (3, 4): 'p = Point(3, 4)' where p is an object of the Point class.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Classes create, objects relate; 'self' is key, to navigate.
Stories
Imagine a magician (the class) creating magic (objects) with a special wand (self), making each spell (method) unique!
Memory Tools
To remember the key parts: C for Class, O for Object, S for self, I for attributes, and D for Default arguments—C, O, S, I, D.
Acronyms
COSA
Class
Object
Self
Attributes—the four pillars of Python OOP!
Flash Cards
Glossary
- Class
A blueprint for creating objects that encapsulates data and functions to manipulate that data.
- Object
An instance of a class that contains the attributes and behaviors defined by its class.
- self
A conventional name for the first parameter of instance methods in Python, which refers to the object itself.
- Attribute
A variable bound to an instance of a class, which holds data specific to that object.
- __init__
A special method in Python classes, known as a constructor, which is called when an instance of the class is created.
- Default Arguments
Predefined values for function parameters that are used when no argument is provided during the function call.
- Internal Representation
The way data is stored and manipulated within a class, which may be different from how it is presented to the user.
Reference links
Supplementary resources to enhance your learning experience.