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Intermediate Python

Class 1

What you'll need

A text editor
Terminal
Python 3
- Installation guide
- If you don't have this installed, you can use repl.it for today

Welcome

Girl Develop It is here to provide affordable and accessible programs to learn software through mentorship and hands-on instruction.

Some "rules":

We are here for you!
Every question is important
Help each other
Have fun

Welcome

Who are you?
What do you hope to get out of the class?
What music did you listen to today?

Where we're headed

Object-oriented programming (OOP)
Handling errors
Modules and packages
Using 3rd party packages
How the web works
Automated testing
Building a web app with Python
A dash of NLP

Object

Oriented

Programming

What is OOP?

A modeling technique

When programs get large, we need a way to organize our code...

A modeling technique

Group properties and behavior into logical units

classes: The blueprints that create...
instances: The real objects.

Photo credit: Wikimedia Commons

Classes implement...

methods: The behavior that instances will have (refuel, setSpeed)
attributes: The properties that instances will have (fuel, maxspeed)

Let's develop it (conceptually)

Photo credit: David Guo

Note: What are the properties of this marker? ...color... What are its behaviors? ...writes... We could have a Marker class, and each of these individual marker is a marker instance.

In code...

class DryEraseMarker:
    def __init__(self, color):
        self.color = color

    def write(self, text):
        return self.color + ': ' + text


blue_marker = DryEraseMarker(color='blue')
print(blue_marker.write('hi'))  # => 'blue: hi'

red_marker = DryEraseMarker(color='red')
print(red_marker.write('bye'))  # => 'red: bye'

Let's develop it (in code)

Create a file called marker.py and add the following code:

class DryEraseMarker:
    def __init__(self, color):
        self.color = color

__init__ is a special method for initializing instances
self is the instance we're initializing
This is where we set attributes

Let's develop it (in code)

Now write the following code:

class DryEraseMarker:
    def __init__(self, color):
        self.color = color

blue_marker = DryEraseMarker(color='blue')
print(blue_marker.color)

Now we're instantiating a Marker
Try creating multiple instances
Try changing an attribute of a marker

Let's develop it (in code)

class DryEraseMarker:
    def __init__(self, color):
        self.color = color

blue_marker = DryEraseMarker(color='blue')
print(blue_marker.color)  # => 'blue'

my_multi_color_marker = DryEraseMarker(color='red')
my_multi_color_marker.color = 'green'
print(my_multi_color_marker.color)  # => 'green'

Let's develop it (in code)

Now change your file to have the following:

class DryEraseMarker:
    def __init__(self, color):
        self.color = color

    def write(self, text):
        return self.color + ': ' + text
        

blue_marker = DryEraseMarker(color='blue')
print(blue_marker.color)
print(blue_marker.write('hi'))

write is an instance method
self is the instance calling the method

Let's develop it (in code)

class DryEraseMarker:
    def __init__(self, color):
        self.color = color

    def write(self, text):
        return self.color + ': ' + text
        

blue_marker = DryEraseMarker(color='blue')
print(blue_marker.color)
print(blue_marker.write('hi'))  # => 'blue: hi'

Instantiate another DryEraseMarker and call write with some text

Let's develop it (in code)

class DryEraseMarker:
    def __init__(self, color):
        self.color = color

    def write(self, text):
        return self.color + ': ' + text
        

blue_marker = DryEraseMarker(color='blue')
print(blue_marker.color)
print(blue_marker.write('hi'))  # => 'blue: hi'

red_marker = DryEraseMarker(color='red')
red_marker.write('bye')  # => 'red: bye'

Inheritance

Process by which a "child" class derives data and behavior of a parent class

Photo credit: baseread.com

Note: The Vehicle class will have methods like move and steer. Because WheeledVehicle inherits from Vehicle, it doesn't need to implement those.

Let's develop it (conceptually)

How can we represent other types of writing implements?

NOTE: Diagram class's suggestions on whiteboard

Let's develop it (in code)

Create a file called writing.py and add the following code:

class WritingImplement:
    def __init__(self, color):
        self.color = color

    def write(self, text):
        return self.color + ': ' + text


class DryEraseMarker(WritingImplement):
    def erase(self):
        return ''

DryEraseMarker inherits from WritingImplement
What attributes does a DryEraseMarker have? What behaviors?
Try instantiating a DryEraseMarker again and calling write and erase on it

Let's develop it (in code)

class WritingImplement:
    def __init__(self, color):
        self.color = color

    def write(self, text):
        return self.color + ': ' + text


class DryEraseMarker(WritingImplement):
    def erase(self):
        return ''

blue_marker = DryEraseMarker(color='blue')
print(blue_marker.write('hi'))  # => 'blue: hi'
print(blue_marker.erase())  # => ''

In general, inheritance represents "is-a" relationships

A DryEraseMarker is a WritingImplement

Representing more objects

class WritingImplement:
    def __init__(self, color):
        self.color = color

    def write(self, text):
        return self.color + ': ' + text


class EraseableWritingImplement(WritingImplement):
    def erase(self):
        return ''

class DryEraseMarker(EraseableWritingImplement):
    def uncap(self):
        return 'popped the cap off..'

class Pen(WritingImplement):
    pass

Let's develop it

Copy the following code:

class WritingImplement:
    def __init__(self, color):
        self.color = color

    def write(self, text):
        return self.color + ': ' + text


class EraseableWritingImplement(WritingImplement):
    def erase(self):
        return ''


class DryEraseMarker(EraseableWritingImplement):
    def uncap(self):
        return 'popped the cap off..'

How would you implement a pencil?

Let's develop it

class Pencil(EraseableWritingImplement):
    def sharpen(self):
        return 'super sharp!'

Overriding behavior

A subclass can override the behavior of a superclass

Overriding behavior

A subclass can override the behavior of a superclass

class BoldMarker(WritingImplement):
    # Override WritingImplement's write() method
    def write(self, text):
        # We call super() to get the parent
        # class's method
        ret = super().write(text)  # => 'black: hi'
        # Then we modify the return value of
        # the parent class
        return ret.upper()

bold_marker = BoldMarker('black')
print(bold_marker.write('hi'))  # => BLACK: HI

Types of attributes

instance variable (you've seen this): specific to an instance
- color of a dry erase marker
class variable: specific to a class, shared by all instances
- brand of dry erase markers

class DryEraseMarker(WritingImplement):
    brand = 'Expo'


blue_marker = DryEraseMarker('red')
green_marker = DryEraseMarker('green')

# instance variables differ
blue_marker.color != green_marker.color  # => True
# class variables are shared
blue_marker.brand == green_marker.brand  # => True

Let's develop it (together)

Add a class variable to one or more of the classes you've written.

This is open-ended. Think: what properties should be shared among all instances of a class?

POINTED = 0
CHISEL = 1

class DryEraseMarker(WritingImplement):
    label = 'low odor; dry erase'
    tip = CHISEL

POINTED = 0
CHISELED = 1
class DryEraseMarker(WritingImplement):
    label = 'low odor; dry erase'
    tip = CHISELED

if blue_marker.tip == 0:
    print('pointy')
else:
    print('chisely')

# More readable
if blue_marker.tip == POINTED:
    print('pointy')
else:
    print('chisely')

Error handling

Errors will happen

Sometimes errors are expected
Need a way to do something when an error occurs

Example: Trying to open a file

open('doesnotexist.txt')
# FileNotFoundError: [Errno 2] No such file or directory: 'doesnotexist.txt'

Example: Trying to open a file

Handling the error:

try:
    open('doesnotexist.txt')
except FileNotFoundError:
    print('File does not exist.')
    print('But the show must go on...')

Raising errors

Sometimes we want our programs to raise an error
Enforces contraints
Use the raise keyword

raise Exception('some useful error message')

Enforcing constraints with errors

class WritingImplement:
    def __init__(self, color):
        if not isinstance(color, str):
            raise TypeError('color must be a string')
        self.color = color


# this will crash! (and that's a good thing)
bad_pen = WritingImplement(color=42)
# TypeError: color must be a string

Let's develop it

Copy the following into your program:

class WritingImplement:
    def __init__(self, color):
        if not isinstance(color, str):
            raise TypeError('color must be a string')
        self.color = color

Write code to create a WritingImplement called pen
Pass a bool as the color
Handle the TypeError and print "No pen, no problem"

Extra credit:

Instead of printing when a TypeError occurs, instantiate the pen again, passing "black" as the color. then write "back in black" with the pen

try:
    pen = WritingImplement(color=True)
except TypeError:
    print('No pen, no problem')

try:
    pen = WritingImplement(color=True)
except TypeError:
    pen = WritingImplement(color='black')

print(pen.write('back in black'))

Summary

OOP helps us organize programs
Groups data (attributes) and behavior (methods) into classes
Classes are blueprints that create instances
Inheritance allows child classes to derive data and behavior of a parent class
try/except for handling errors, raise for raising errors

Files

class1.md

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class1.md

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Intermediate Python

Class 1

What you'll need

Welcome

Welcome

Where we're headed

Object

Oriented

Programming

What is OOP?

A modeling technique

A modeling technique

A modeling technique

Let's develop it (conceptually)

In code...

Let's develop it (in code)

Let's develop it (in code)

Let's develop it (in code)

Let's develop it (in code)

Let's develop it (in code)

Let's develop it (in code)

Inheritance

Let's develop it (conceptually)

Let's develop it (in code)

Let's develop it (in code)

In general, inheritance represents "is-a" relationships

Representing more objects

Let's develop it

Let's develop it

Overriding behavior

Overriding behavior

Overriding behavior

Types of attributes

Types of attributes

Let's develop it (together)

Error handling

Errors will happen

Example: Trying to open a file

Example: Trying to open a file

Raising errors

Raising errors

Enforcing constraints with errors

Let's develop it

Summary

Further reading

Questions?