python:python_misc:oo_fundamentals_with_python
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python:about_python:about_python_oo_fundamentals [2018/03/23 18:26] – [Class-based object-orientation] mithat | python:python_misc:oo_fundamentals_with_python [2018/08/23 17:06] – [Class-based object-orientation] mithat | ||
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- | ====== | + | ====== Object-Orientation Fundamentals |
===== What you'll need to know ===== | ===== What you'll need to know ===== | ||
- | In what follows, we assume know basic Python programming concepts including: | + | In what follows, we assume |
* How to create and use variables. | * How to create and use variables. | ||
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All these attributes, which can be defined in terms of data, collectively define the oven's state. | All these attributes, which can be defined in terms of data, collectively define the oven's state. | ||
+ | |||
==== Behavior ==== | ==== Behavior ==== | ||
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* And so on. | * And so on. | ||
- | These operations are one that happen to be public-facing (i.e., operations a user can engage). There might very well also be operations that go on inside the oven to support the oven's functioning that the user will never be aware of. All these operations collectively define the oven's behavior. We call the public-facing behavior (i.e., the operations a user can engage) the oven's **interface**. | + | These operations are ones that happen to be public-facing (i.e., operations a user can engage). There might very well also be operations that go on inside the oven to support the oven's functioning that the user will never be aware of. All these operations collectively define the oven's behavior. We call the public-facing behavior (i.e., the operations a user can engage) the oven's **interface**. |
==== Encapsulation and protection ==== | ==== Encapsulation and protection ==== | ||
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Along the same lines, the way I change the clock' | Along the same lines, the way I change the clock' | ||
- | Ξ In fact, even if I wanted to get at the magnetron or directly change the clock' | + | In fact, even if I wanted to get at the magnetron or directly change the clock' |
- | Ξ The "not caring about how it works---I only need to know what it does" is part of **encapsulation** (literally, "to place in a capsule" | + | The "not caring about how it works---I only need to know what it does" is part of **encapsulation** (literally, "to place in a capsule" |
In many languages, properly hiding things the user has no business getting to is considered part of encapsulation. | In many languages, properly hiding things the user has no business getting to is considered part of encapsulation. | ||
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{{: | {{: | ||
- | I have a Farberware 4241 microwave oven in my office. It's small, but it gets the job done. There is a factory somewhere making these by the thousand million. They make all the Farberware 4241s from a master plan. That master plan defines what the Farberware 4241 //is//. In computing terminology, | + | I have a Farberware 4241 microwave oven in my office. It's small, but it gets the job done. There is a factory somewhere making these by the thousand million, and they make all the Farberware 4241s from a master plan. That master plan defines what the Farberware 4241 //is//. In computing terminology, |
Every Farberware 4241 begins its life identical to every other freshly-made Farberware 4241 because they were made using the same master plan. So, the one in my office that I pulled out of the box last month was identical to the one that I bought my best friend last week later because they were built from the same master plan, or, if you will, the same class. But as my best friend and I started using our ovens, the state of each changed. Note that while the behavior designed into the oven---the operations it's capable of---doesn' | Every Farberware 4241 begins its life identical to every other freshly-made Farberware 4241 because they were made using the same master plan. So, the one in my office that I pulled out of the box last month was identical to the one that I bought my best friend last week later because they were built from the same master plan, or, if you will, the same class. But as my best friend and I started using our ovens, the state of each changed. Note that while the behavior designed into the oven---the operations it's capable of---doesn' | ||
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We are going to write a class for implementing one of these: | We are going to write a class for implementing one of these: | ||
- | )) \\ {{tottycounter.jpg? | + | )) \\ {{..: |
In case you've never seen this bit of advanced technology before, it's a clicker-counter or tally-counter. It has two controls: a button you click to advance the counter by one and another you press to reset the count to zero. Our goal is to build one of these in software using object-orientation. | In case you've never seen this bit of advanced technology before, it's a clicker-counter or tally-counter. It has two controls: a button you click to advance the counter by one and another you press to reset the count to zero. Our goal is to build one of these in software using object-orientation. | ||
- | Ξ One way to start building a model for a class is to start listing the public-facing behavior (or the // | + | One way to start building a model for a class is to start listing the public-facing behavior (or the // |
* //click//: makes the count increase by one. | * //click//: makes the count increase by one. | ||
* //reset//: sets the count to zero. | * //reset//: sets the count to zero. | ||
- | Next we can think about what data we'll need to keep track of the state of a clicker-counter. In this case, it's pretty simple: all we really need is one integer to store the count value. | + | Next we can think about what data we'll need to keep track of the state of a clicker-counter. In this case, it's pretty simple: all we really need is one integer to store the //count// value. |
- | Ξ So, a summary of what we need so far is: | + | So, a summary of what we need so far is: |
* a //click// operation | * a //click// operation | ||
* a //reset// operation | * a //reset// operation | ||
- | * an integer to store the count | + | * an integer to store the //count// |
- | Ξ In Python, object **attributes**, | + | In Python, object **attributes**, |
Class names in Python traditionally use [[http:// | Class names in Python traditionally use [[http:// | ||
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</ | </ | ||
- | Class definitions follow the same header/ | + | Class definitions follow the same header/ |
- | The suite of the class definition nests additional compound statements, in this case a set of function definitions---the instance methods. | + | The suite of the class definition nests additional compound statements: function definitions |
In this class definition, we define three instance methods: '' | In this class definition, we define three instance methods: '' | ||
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<WRAP center round tip 80%> | <WRAP center round tip 80%> | ||
- | A common error is to forget to use '' | + | A common error is to forget to use '' |
</ | </ | ||
- | === ✘ The __init__ method and instance variables === | + | === The __init__ method and instance variables === |
In our class definition, the roles of the '' | In our class definition, the roles of the '' | ||
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</ | </ | ||
- | ------------------------------------------ | + | ==== Instantiation and use ==== |
- | ------------------------------------------ | + | |
- | + | ||
- | ==== Ξ Instantiation and use ==== | + | |
Once you have a definition for a class, invoking the name of the class with a pair of parenthesis will instantiate an object from that class. | Once you have a definition for a class, invoking the name of the class with a pair of parenthesis will instantiate an object from that class. | ||
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you can also see that it's an object and the memory location where it's stored. | you can also see that it's an object and the memory location where it's stored. | ||
- | Ξ Stuff like the above is useful for debugging. Most of the time, once you instantiate objects, you just start to use them. So let's instantiate a clicker, click it three times, and print the resulting '' | + | Stuff like the above is useful for debugging. Most of the time, once you instantiate objects, you just start to use them. So let's instantiate a clicker, click it three times, and print the resulting '' |
<code python> | <code python> | ||
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Python' | Python' | ||
- | ==== ♠ Parameterized constructors and methods ==== | + | ==== Parameterized constructors and methods ==== |
The '' | The '' | ||
- | Ξ First we need to redefine the constructor to take a limit parameter and set an instance variable to that value: | + | First we need to redefine the constructor to take a limit parameter and set an instance variable to that value: |
<code python> | <code python> | ||
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</ | </ | ||
- | Ξ Next we need to modify the '' | + | Next we need to modify the '' |
<code python> | <code python> | ||
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</ | </ | ||
- | ⇒ This yields: | + | This yields: |
<code python> | <code python> | ||
class ClickerCounter(): | class ClickerCounter(): | ||
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</ | </ | ||
- | ♠ General methods can also be parameterized. The process is identical to using parameters with constructors. | + | General methods can also be parameterized. The process is identical to using parameters with constructors. |
- | + | ||
- | -------------------------------------------------------- | + | |
- | -------------------------------------------------------- | + | |
==== Next steps ==== | ==== Next steps ==== |
python/python_misc/oo_fundamentals_with_python.txt · Last modified: 2018/11/30 18:45 by mithat