IT 117: Intermediate Scripting

IT 117: Intermediate Scripting
Class 19

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Solution to Homework 7

I have posted a solution to homework 7 here.

Let's take a look.

Homework 9

I have posted homework 9 here.

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

Class Quiz

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Tips and Examples

TypeError When Creating Objects

Sometimes students get the following error when creating a class object

section = Section("it117", "s17", 1)
TypeError: object() takes no parameters

This happens when you misspell the name of the constructor
The name of the constructor should be __init__
Not _init_
Or __init_
Or _init__

Review

The Constructor Is Special

A constructor is a method that creates an object
And returns a pointer to the object
It has no return statement
Every time you create a class you should have a constructor

Defining a Class

The general format for a class is

class CLASS_NAME:
    def __init__(self[, PARAMETER, ...]):
        ...

    def METHOD_NAME(self[, PARAMETER, ...]):
        ...

By convention, all class names in Python are Capitalized
A class definition consists of a number of methods
One of which has the special name __init__
This method is a constructor, which is used to create the object
The class is the template used to create the object
The object is section of RAM
That can hold many values
Objects do not have a name
They only have a location in memory
This location is stored in a variable
We use this variable to work with the object
A class is the code used to create the object

Creating An Object from A Class

The __init__ cannot be called directly
Instead you use the name of the class

The general format is

OBJECT_VARIABLE = CLASS_NAME([PARAMETER, ...])

In a previous class, we created an object like this
```
date = Date(date_string)
```
The variable date points to the object
Which was created by the call to the constructor
We can then use dot notation to access the object's methods
Like this
```
print(date.format_date())
```

The self Variable

Every method defined in a class has self as a parameter
We only use self inside the class
When we need to call a method
Or refer to variables of the class
We cannot use self outside the class
self represents the individual object created by the constructor
If I call the Date constructor to create a date object
```
>>> date= Date("2015-10-03")
```
This object is an instance of the Date class
The object pointed to by date has its own local variables
These variables are called attributes
The year attribute in date_1 has the value 2015
We need to use a variable pointing to an object
To work with that object
We do this using dot notation
We can use this notation to access a method
```
>>> date.format_date()
'October 3, 2015'
```
Or an attribute
```
>>> date.year
2015
```
Inside a class, the variable that points to the object is self
We use dot notation with self to call a method
```
self.get_month_name()
```
Or use an attribute
```
self.year
```

The Time Class

Let's create another class, Time
The constructor takes a time string using the format
```
HH:MM:SS
```
HH is the hours on a 24 hour clock with values from 0 to 23
MM is a two digit minute and SS is a two digit second

Here is the constructor

# expects a string of the form HH:MM:SS
# where hours are expressed in numbers from 0 to 23
def __init__(self, time_string):
    self.hours, self.minutes, self.seconds = time_string.split(":")
    self.hours   = int(self.hours) 
    self.minutes = int(self.minutes)
    self.seconds = int(self.seconds)

Let's also create the format_time method

This method will give us a nicely formated string representing the time

# returns a string in the form HH:MM:SS AM/PM       
def format_time(self):
    hours = self.hours
    am_pm = "AM"
    if hours > 12:
        hours -= 12
        am_pm = "PM"
    return str(hours) + ":" + str(self.minutes) + ":" + str(self.seconds) + " " + am_pm

We can use this as follows

>>> from time_1 import Time
>>> t1 = Time("10:45:30")
>>> t1.format_time()
'10:45:30 AM'

Let's now add the method difference.

It will return the difference between two times in seconds

# returns the difference in seconds between two times           
def difference(self, other_time):
    seconds  = (self.hours - other_time.hours) * 60 * 60
    seconds += (self.minutes - other_time.minutes) * 60
    seconds += self.seconds - other_time.seconds
    return seconds

Which we can use as follows

>>> midnight = Time("00:00:00")
>>> t2 = Time("03:04:13")
>>> t2.difference(midnight)
11053

Changing the Attributes of a Class

Classes have many advantages
One is that we can change the internals of an object
As long as all the methods give the same results
The client code does not care how a method works
In the Time class we represented time with three attributes
- hours
- minutes
- seconds
Instead of using three attributes we can use one
It will store the number of seconds since midnight
This will make the difference calculation easier
We will have to change the other methods so they give the same results
The first thing we need to do is to change the constructor
We will still take a time string
And turn it into integer values for hours, minutes and seconds
But these now will be variables that are local to the constructor

Here is the new code for the constructor

# expects a string of the form HH:MM:SS
# where hours are expressed in numbers from 0 to 23
def __init__(self, time_string):
    hours, minutes, seconds = time_string.split(":")
    hours   = int(hours) 
    minutes = int(minutes)
    seconds = int(seconds)
    self.seconds = hours * 60 * 60 + minutes * 60 + seconds

We need to modify format_time to use seconds

Here is the new code for format_time

# returns a string in the form HH:MM:SS AM/PM               
def format_time(self):
    hours = self.seconds  // (60 * 60)
    remainder = self.seconds  % (60 * 60)
    minutes = remainder // 60 
    seconds  = remainder % 60
    am_pm = "AM"
    if hours > 12:
        hours -= 12
        am_pm = "PM"
    return str(hours) + ":" + str(minutes) + ":" + str(seconds ) + " " + am_pm

The modified code is shown in red
We also have to modify difference

But here the code is much simpler

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

New Material

Accessing Attributes Directly

A constructor creates an object of its class
And returns a pointer to the object
Which we store in a variable
We use this variable to call methods
But we can also use it with attributes
Not only can we get the value of the attribute
```
>>> t1.second
63924
```

But we can change it as well

>>> t1.seconds  = 7000
>>> t1.format_time()
'1:56:40 AM'

Is this a good idea?

Data Hiding

By default, client code can use a variable that points to an object
To change the value of any attribute
This might seem harmless
But it can lead to trouble
The first version of our Time class had three attributes
- hours
- minutes
- seconds

Each of these attributes can be accessed using dot notation

>>> from time_1 import Time
>>> t1 = Time("14:35:12")
>>> t1.hours
14
>>> t1.minutes
35
>>> t1.seconds 
12

Since attributes are variables we can change them directly

>>> t1.hours = 15
>>> t1.minutes = 14
>>> t1.seconds  = 13
>>> t1.format_time()
'3:14:13 PM'

But the latest version of the Time class has one attribute

>>> from time_2 import Time
>>> t1 = Time("14:35:12")
>>> t1.seconds 
52512
t1.format_time()
'2:35:12 PM'

A user can certainly change that value
But it would take some calculating to get the right value for seconds
And it would be easy to make a mistake

Worse than that, they might enter a totally ridiculous value

>>> t1.seconds  = 1000000000
>>> t1.format_time()
'277765:46:40 PM'

We should prevent things like this from happening
To do so we have to change things
So the client code cannot change the attributes directly
This is called data hiding
We hide attributes from the client code by changing their names
Any attribute whose name begins with to underscores, __
Will be invisible to the client code
Let's create a third version of our Time class
In this version the name of the attribute will be __seconds

The constructor will now look like this

# expects a string of the form HH:MM:SS
# where hours are expressed in numbers from 0 to 23
def __init__(self, time_string):
    hours, minutes, seconds  = time_string.split(":")
    hours   = int(hours) 
    minutes = int(minutes)
    seconds  = int(seconds )
    self.__seconds  = hours * 60 * 60 + minutes * 60 + seconds

It is no longer possible to access attributes directly

>>> from time_3 import Time
>>> t1 = Time("14:35:12")
>>> t1.__seconds 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Time' object has no attribute '__seconds '

But the methods still work the way they are supposed to
```
>>> t1.format_time()
'2:35:12 PM'
```

The str Method

Python classes can have certain methods that are not only hidden
They are not called directly
These methods have __ at the beginning and end of their names
They are sometime called magic methods
These methods are never called by name
Instead they are called whenever some action is taken
They are called implicitly
Not explicitly
__init__ is an example of a magic method
The most commonly used magic method after __init__
Is __str__
When the a Time object is created in the client code
```
t1 = Time("11:54:12")
```
The __init__ method of Time is called
Similarly, whenever you try to print an object
Its __str__ method is called
__str__ creates a string representation of the object
At the moment, the Time class has no __str__

If we create a Time object and print it here is what we get

>>> from time_3 import Time
>>> t = Time("09:30:00")
>>> print(t)
<time_3.Time object at 0x1013e8358>

The __str__ method must return a string
It is secretly called every time you print an object of the class
Or use the object where a string is required
We can create an __str__ method for Time

By changing the name of the format_time method to __str__

    # returns a string formated as follows
    # H[H] hours M[M] minutes S[S] seconds 
    def format_seconds (seconds ):
        hours = seconds  // (60 * 60)
        remainder = seconds  % (60 * 60)
        minutes = remainder // 60
        seconds  = remainder % 60
        return str(hours) + ' hours ' + str(minutes)+ ' minutes ' + str(seconds ) + ' seconds s

This __str__ method will be called

Whenever we print an object

>>> print("Here is the time object we just created: ", t)
Here is the time object we just created:  9:30:0 AM

You see the latest version of the Time class here

Accessor and Mutator Methods

A well designed class hides its attributes
But the client code might need the values they contain
To make this happen we need a method for each attribute
That the client code will be allowed to see
These methods are called accessors
It is common practice for the name of an accessor to begin with "get""
Followed by the attribute name
Without the initial __
So another name for an accessor is a getter
Let's say you need to keep track of each computer in an office
You would need to record the following information about each machine
- Manufacturer
- Model
- Serial number
- Processor
- RAM
- Hostname
- Disk size

The first thing we need to do is create a constructor

def __init__(self, manufacturer, model, serial_number, processor, ram, hostname, disk_size):
    self.__manufacturer  = manufacturer
    self.__model         = model
    self.__serial_number = serial_number
    self.__processor     = processor
    self.__ram           = ram
    self.__hostname      = hostname
    self.__disk_size     = disk_size

Since all the attributes are hidden we need to create an accessor for each attribute

def get_manufacturer(self):
    return self.__manufacturer

def get_model(self):
    return self.__model

def get_serial_number(self):
    return self.__serial_number

def get_processor(self):
    return self.__processor

def get_processor(self):
    return self.__processor

def get_ram(self):
    return self.__ram

def get_hostname(self):
    return self.__hostname

def get_disk_size(self):
    return self.__disk_size

The values of the following attributes should never change
- __manufacturer
- __model
- __serial_number
- __processor
That is one of the reasons we hide attributes
You wouldn't want someone to write code that would change the serial number
But some attributes could change with time
- __ram
- __hostname
- __disk_size
These attributes are hidden
So they cannot be changes directly outside the class
We need special methods to change each of these values
Such methods are called mutators
It common practice is to name mutator method with "set"
Followed by the attribute name without the leading __
So mutators are sometimes called setters

Here are the mutators for the Computer class

def set_ram(self, ram):
    self.__ram = ram

def set_hostname(self, hostname):
    self.__hostname = hostname

def set_disk_size(self, disk_size):
    self.__disk_size = disk_siz

Data Validation in The Constructor

Our constructor works
But it is not very smart
I can give it any values I want for the time
Even ones that are ridiculous
```
>>> t1 = Time("500:3000:120000")
```
When I do this, some functions don't work properly
```
>>> t1.format_time()
'571:20:0 PM'
```
The constructor needs to check the input it is given
It should do this before it sets the attribute values
Checking that values make sense before you use them
Is called data validation
We have to be careful when changing the constructor
So we will proceed in a step by step fashion
First we need to be sure that time_string is indeed a string
We can do this with the built-in function isinstance
isinstance takes two arguments
- An object
- The name of a class
It returns True if the object is an instance of the class
```
>>> isinstance("14:35:12", str)
True
```
What should we do if time_string is not a string?
We could print an error message and quit
But that is not the best way to go
Instead we should raise an exception
Exceptions allow a program to respond gracefully
When unexpected things happen
There are specific Python exceptions for different kinds of problems

Here we need to raise a TypeError like this

if not isinstance(time_string, str):
    raise TypeError("Time constructor expects a string argument")

Now if we call the constructor and time_string is not a string

We get the following

>>> t1 = Time(55)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/glenn/workspace-mars/it117/resources_it117/code_it117/example_code_it117/10_chapter_example_code/time_3.py", line 13, in __init__
    raise TypeError('Time constructor expects a string argument')
TypeError: Time constructor expects a string argument

Next, we need to be sure that time_string has the right format
It must have two digits followed by a colon, :
Followed by two more digits and another colon
Then two more digits
I don't want the constructor to get too large
Because then it would be hard to read
So I am going to place the code to check the format
In a separate method called has_valid_format

Here is the code

# returns true if the string has the form HH:MM:SS
def has_valid_format(self,s):
    pattern = re.compile(r"\d\d:\d\d:\d\d")
    match = pattern.match(s)
    return match

Here we use regular expressions to try to match the string
And return a pointer to the match object
If the format is good, this variable will point to a real object
And the calling statement will interpret this as True
If the string does not have right format
The value of match will be None
None is interpreted as False in an if statement
We also need to make sure that the hours value is between 0 and 23
And the minutes and seconds are between 0 and 59

To do this I am going to create a new method time_numbers_in_range

# returns true if the numbers for hours, minutes and seconds 
# are within the proper range
def time_numbers_in_range(self, hours, minutes, seconds ):
    if not 0 <= hours <= 23:
        return False
    if not 0 <= minutes <= 59:
        return False
    if not 0 <= seconds  <= 59:
        return False
    return True

Of course, we have to change the constructor to use these new methods

# expects a string of the form HH:MM:SS
# where hours are expressed in numbers from 0 to 23    
def __init__(self, time_string):
    if not isinstance(time_string, str):
        raise TypeError("Time constructor expects a string argument")
    if not self.has_valid_format(time_string):
        raise ValueError("String must have the format HH:MM:SS")
    hours, minutes, seconds  = time_string.split(":")
    hours   = int(hours) 
    minutes = int(minutes)
    seconds  = int(seconds )
    if not self.time_numbers_in_range(hours, minutes, seconds ):
        raise ValueError("One of the time values is not in the proper range")
    self.__seconds  = hours * 60 * 60 + minutes * 60 + seconds

Hiding Methods

An attribute invisible outside the class
If it's name begins with two underscores, __
This also works for methods
In the example above we created two methods
- has_valid_format
- time_numbers_in_range
These methods are only used by the constructor
They are not intended for use outside the class
So we should hide them by adding __
To the beginning of their names
Most methods in a class will be used by the client code
But any method not needed by the client code
Should be hidden
A class should be a black box
It should have specified inputs and outputs
Users of the class do not need to know how it works
Only what goes in
And what comes out