IT 117: Intermediate Scripting

IT 117: Intermediate Scripting
Class 21

Microphone

Graded Quiz

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Solution to Homework 8

I have posted a solution to homework 8 here.

Let's take a look.

Homework 10

Homework 10 is posted here.

It is due this coming Sunday at 11:59 PM.

Questions

Are there any questions before I begin?

Course Work

Comments in Classes

Comments are important
But only when they are informative
When they tell you something that might be hard to see otherwise
For this reason certain methods do not need to be commented
You do not need to write comments for the following
- Constructors
- Accessors
- Mutators
- Magic Methods
Unless they do something unusual

Tips and Examples

Don't Use "(Object)" When Defining a Class

Sometimes students use the following format for the class header
```
class CLASS_NAME(Object);
```
You don't need to write "(Object)" in the class header
This is unnecessary ...
and should be avoided

Review

A Practical Example of A Class

Let's create a Student class
I need at least three values for each student
- Student ID
- Last Name
- First Name
I will need other attributes for this class
- Preferred first name
- Phonetic spelling of the name
- Email address
- Unix username
But they will be added later
So the constructor sets them to the empty string

Here is the code for the constructor

def __init__(self, id, last_name, first_name):
    self.__id             = id
    self.__last_name      = last_name
    self.__first_name     = first_name
    self.__preferred_name = ""
    self.__phonetic_name  = ""
    self.__email          = ""
    self.__unix_username  = ""

I need accessors for each of these attributes

def get_id(self):
    return self.__id

def get_last_name(self):
    return self.__last_name

def get_first_name(self):
    return self.__first_name

def get_preferred_name(self):
        return self.__preferred_name

def get_phonetic_name(self):
    return self.__phonetic_name

def get_email(self):
    return self.__email

def get_unix_username(self):
    return self.__unix_username

The values for the last four attributes are set later ...
after the object has been created

So I have to create mutators for these attributes

def set_preferred_name(self, preferred_name):
    self.__preferred_name = preferred_name

def set_phonetic_name(self, phonetic_name):
    self.__phonetic_name = phonetic_name

def set_email(self, email):
    self.__email = email

def set_unix_username(self, unix_username):
    self.__unix_username = unix_username

Other Methods for the Student Class

I need to create a __str__ method for this class

def __str__(self):
    return self.__first_name + " " + self.__last_name + ", " + self.__id

Student also needs a method to return the full name of the student
We'll call this method get_full_name
Normally this method will return a string consisting of the first name ...
followed by a space ...
followed by the last name
But some students have a prefered first name
In that case the prefered name will be used in place of the first name

Here is the code

def get_full_name(self):
    if self.__preferred_name:
        return self.__preferred_name + " " + self.__last_name
    else:
        return self.__first_name + " " + self.__last_name

I also need a method get_sort_name

def get_sort_name(self):
    if self.__preferred_name:
        return self.__last_name + ", "  + self.__preferred_name 
    else:
        return self.__last_name + ", "  + self.__first_name

Storing Objects for Future Use

In a previous class we wrote a script to create a dictionary of Student objects
But that dictionary disappeared after the script finished
We need a way to store the students dictionary ...
so we can use it at another time
Most computer languages allow you to store an object on disk
This process is called serializing
Python calls this process pickling
Every Python installation comes with a standard module named pickle
This module contains functions used to serialize an object
First you must import the pickle module
```
import pickle
```
To open a file for writing binary data we must use the wb access mode
```
pickle_file = open(pickle_filename, "wb")
```
To write an object to disk use the dump function
```
pickle.dump(students, pickle_file)
```
Loading an object from a data file is a similar process

First we open the file for binary reading

pickle_file = open(pickle_filename, "rb")

Then we read the previously stored object using the load function
```
students = pickle.load(pickle_file)
```

Import Statements and Global Variables

add_students.py uses code imported from two modules
- student
- pickle
We need import statements at the top of the file ...

but they need to be outside the class definition

from student import Student
import pickle

class Section:
...

Global variables also need to be declared at the top of the file
Again they must be outside the class definition

Attendance

New Material

Magic Methods

If the truth be told, every data type in Python is actually a class
And all variables are instances of these classes
When I write
```
x = 5
```
The Python interpreter creates an integer object ...
that holds the value 5 ...
and the variable x that points to it
But we can do things with numbers that do not involve using a method
We can write expressions using arithmetic operators
```
x += 7
y = x + 8
x > y
```
When I created the Time class I added the difference method
This method gives the difference between two Time objects
I can get the difference between two Time objects like this
```
diff = t1.difference(t2)
```
But wouldn't it be better to be able to write
```
diff = t1 - t2
```
We can if we create a magic method for the - operator

For every arithmetic operator there is an equivalent magic method

Operator	Magic Method
+	__add__(self, other)
-	__sub__(self, other)
*	__mul__(self, other)
//	__floordiv__(self, other)
/	__truediv__(self, other)
%	__mod__(self, other)
**	__pow__(self, other)

These magic methods allow us to use the ordinary operators of arithmetic ...
with objects of a class

I can rename the difference method to __sub__

# returns the difference in seconds between two times
def __sub__(self, other_time):
    return self.__seconds - other_time.__seconds

Now we can use the subtraction operator, - , with Time objects

>>> t1 = Time("10:45:10")
>>> t2 = Time("10:50:00")
>>> t2 - t1
290

When we write
```
>>> t2 - t1
```
The Python interpreter calls __sub__
So we can use any arithmetic operator on class objects ...
as long as the class contains the appropriate magic method

Magic Methods and Boolean Operators

Boolean operators are used to compare two things
All these operators have corresponding magic methods

Here they are

Operator	Magic Method
==	__eq__(self, other)
!=	__ne__(self, other)
<	__lt__(self, other)
>	__gt__(self, other)
<=	__le__(self, other)
>=	__ge__(self, other)

Implementing these magic methods is straightforward

def __eq__(self, other):
    return self.__seconds == other.__seconds

def __ne__(self, other):
    return self.__seconds != other.__seconds

def __lt__(self, other):
    return self.__seconds < other.__seconds

def __gt__(self, other):
    return self.__seconds > other.__seconds

def __le__(self, other):
    return self.__seconds <= other.__seconds

def __ge__(self,other):
    return self.__seconds >= other.__seconds

And they work as you would expect

>>> t1 = Time("10:45:10")
>>> t2 = Time("10:50:00")
>>> t3 = Time("10:50:00")
>>> t1 == t2
False
t2 == t3
True
>>> t1 < t2
True
>>> t1 > t2
False
>>> t1 <= t2
True
>>> t2 <= t3
True
>>> t1 >= t2
False
>>> t2 >= t3
True

Type Conversion Magic Methods

Sometimes you want to convert a value of one type into another
This is done using one of Python's conversion functions
- int
- float
- bool
- str

All of these have corresponding magic methods

Operator	Magic Method
`int`	__int__(self)
`float`	__float__(self)
`bool`	__bool__(self)
`str`	__str__(self)

We have already implemented __str__
How we implement each of the other magic methods is up to us
The __seconds attribute holds an integer
So the simplest way to implement __int__ is just to return that value
```
def __int__(self):
    return self.__seconds
```
To turn an integer into a float all we have to do is multiply by 1.0
```
def __float__(self):
    return self.__seconds * 1.0
```
But what about __bool__?
We are under no obligation to provide this method
But it's a good idea to do so for completeness
In Python, any integer that is not zero is considered to be True
```
>>> bool(0)
False
>>> bool(5)
True
>>> bool(-5)
True
```

So we can define the __bool__ method like this

def __bool__(self):
    return self.__seconds != 0

Here is how they work

>>> t1 = Time("10:15:00")
>>> t2 = Time("00:00:00")
>>> str(t1)
'10:15:0 AM'
>>> bool(t1)
True
>>> bool(t2)
False
>>> int(t1)
36900
>>> int(t2)
0
>>> float(t1)
36900.0

You will find the code for the latest version of the Time class here

More Relational Magic Methods

The relational magic methods for the Time class were straightforward
We simply applied the normal relational operators to two integer second values
```
def __eq__(self, other):
    return self.__seconds == other.__seconds
```
But what if we have two objects that are not so easily compared?
For example two Car objects?
Let's say the Car class has the following attributes
- __manufacturer
- __model
- __year
- __color
What makes two Car objects equal?
Do all of the attributes have to be equal?
The decision is up to the programmer
And depends on what the class will be used for

Two cars could be equal equal if they had the same manufacturer and model

    def __eq__(self, other):
        return self.__manufacturer == other.get_manufacturer() and self.__model == other.get_model()

    def __ne__(self, other):
        return self.__manufacturer != other.get_manufacturer() or self.__model != other.get_model()

Running the following test code

c1 = Car("Honda", "CRV",   "1997", "Blue")
c2 = Car("Honda", "CRV",   "2010", "Green")
c3 = Car("Honda", "Civic", "2015", "Red")
print(c1)
print(c2)
print(c3)
print("c1 == c2 :" , c1 == c2 )
print("c1 == c3 :" , c1 == c3 )
print("c1 != c3 :" , c1 != c3 )

We get

Honda CRV 1997 Blue
Honda CRV 2010 Green
Honda Civic 2015 Red
c1 == c2 : True
c1 == c3 : False
c1 != c3 : True

Putting Test Code into Class Modules

It is always a good idea to write test code for each class
You could write this code as a separate script
The script would have to import the class for testing
But this means we need two separate files
The class definition and the test script
It would be simpler to put the test code in the class file
But we have to be careful how we do this
If we just copy the test code to the bottom of the file
Importing the class will have side effects
Anytime another script imports the class ...
the test code would be run

Here is an example

>>> from car4 import Car
Honda CRV 1997 Blue
Honda CRV 2010 Green
Honda Civic 2015 Red
c1 == c2 : True
c1 == c3 : False
c1 != c3 : True

The test code runs as soon as we import the class
We can prevent this from happening ...

but we need to put this code inside a special if statement

if __name__ == "__main__":
    c1 = Car("Honda", "CRV",   "1997", "Blue")
    c2 = Car("Honda", "CRV",   "2010", "Green")
    c3 = Car("Honda", "Civic", "2015", "Red")
    print(c1)
    print(c2)
    print(c3)
    print("c1 == c2 :" , c1 == c2 )
    print("c1 == c3 :" , c1 == c3 )
    print("c1 != c3 :" , c1 != c3 )

When we run the module at the command line the test code executes

$ ./car3.py 
Honda CRV 1997 Blue
Honda CRV 2010 Green
Honda Civic 2015 Red
c1 == c2 : True
c1 == c3 : False
c1 != c3 : True

But not when we import it
```
>>> from car3 import Car
>>> 
```

Do No Harm