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Python

Image credit: xkcd.com

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Intermediate Python

Class 2

Let's develop it

Spotify

Let's develop it

Song

  • Data: artist, title
  • Behavior: play()

Let's develop it

Create a file called music.py and implement a Song class.

class Song:
    def __init__(self, artist, title):
        # TODO...
  • Finish the constructor.
  • Add a play() method that prints out 'PLAYING: ' along with the artist and title.
song = Song(artist='Hot Chip', title='The Warning')
song.play()  # => 'PLAYING: Hot Chip - The Warning'

Let's develop it

class Song:
    def __init__(self, artist, title):
        self.artist = artist
        self.title = title

    def play(self):
        print('PLAYING: ' + self.artist + ' - ' + self.title)


song = Song(artist='Hot Chip', title='The Warning')
song.play()  # => 'PLAYING: Hot Chip - The Warning'

Composing objects

or "has-a"/"has-many" relationships

How would we design a Playlist class?

Playlist

  • Data: name, songs, current song
  • Behavior: play_one(), play_all(), shuffle()

Let's develop it

Add the following to music.py

class Playlist:
    def __init__(self, name):
        self.name = name
        self.current_song_index = 0
        # A list of Song instances
        self.songs = []

    def add_song(self, song):
        self.songs.append(song)

    def play_one(self):
        current_song = self.songs[self.current_song_index]
        current_song.play()
        self.current_song_index += 1

Note: We'll come back to this later

Using our classes

playlist = Playlist('Old Kanye')

song1 = Song(artist='Kanye', title='Stronger')
song2 = Song(artist='Kanye ft. Lupe Fiasco', title='Touch The Sky')

playlist.add_song(song1)
playlist.add_song(song2)
playlist.play_one()  # => PLAYING: Kanye - Stronger

Don't write this code just yet. We'll revisit this later.

"Modularizing"

Breaking big programs into smaller parts

Modularizing

  • Simplicity: focus on on thing at a time
  • Maintainability: changing one piece won't break other pieces
  • Reusability: less copying and pasting!

The building blocks of programs

Your building blocks

Modules

  • Collection of related code
  • May contain variables, functions, and classes

Built-in modules

Built-in modules

Python's standard library contains lots of convenient modules

Importing modules

import math

Let's develop it

Run the python shell and try following

>>> import math
>>> math.sqrt(4)
2.0

Let's develop it

Go to https://docs.python.org/3/library/math.html .

Try out some of the functions in the math module.

Importing individual members

from math import sqrt

Let's develop it

Try the following:

>>> from math import ceil, floor
>>> ceil(12.3)
13
>>> floor(45.6)
45

Let's develop it

Now that you know how to import built-in modules, let's use modules in a program.

Let's develop it

Create a file called coin.py.

Write a function flip that randomly returns either 'H' or 'T'.

Hint: Check out the random module in the standard library.

import random

def flip():
    return random.choice(['H', 'T'])


print(flip())  # => 'H'
print(flip())  # => 'T'
print(flip())  # => 'H'

Also valid

from random import choice

def flip():
    return choice(['H', 'T'])

Before we move on, make sure your coin.py looks like this:

import random

def flip():
    return random.choice(['H', 'T'])

Note: remove any print statements

Creating your own modules

Creating your own modules

Turns out, you've already created modules.

In Python, a module is a file with a .py extension.

Yay, no new syntax!

Let's develop it

Run the python shell and try following

>>> import coin
>>> coin.flip()
'H'
>>> coin.flip()
'T'
>>> coin.flip()
'T'
>>> from music import Song
>>> s = Song('The Beatles', 'Help!')
>>> s.play()
PLAYING: The Beatles - Help!

Let's develop it

In the python shell, import the Song and Playlist classes.

Make a Song instance and a Playlist instance.

Add the song to the playlist.

Then play the song.

>>> from music import Song, Playlist
>>> s = Song('The Beatles', 'Help!')
>>> p = Playlist('faves')
>>> p.add_song(s)
>>> p.play_one()
PLAYING: The Beatles - Help!

Packages

Modularizing further...

Modularizing further...

module: A collection of related variables, functions, and classes

package: A collection of related modules

Packages

If a module is a .py file, what do you think a package is?

Packages

A folder of .py files!

Let's develop it

Create a folder called fun. Move coin.py and music.py into fun.

fun
├── coin.py
└── music.py

Importing from a package

import fun.coin
from fun.music import Song, Playlist

Let's develop it

Try the following in the python shell:

>>> import fun.coin
>>> fun.coin.flip()
'T'
>>> from fun.music import Song
>>> Song
<class 'fun.music.Song'>

Subpackages

Packages can be nested

fun
└── writing
   ├── base.py
   └── markers.py
├── coin.py
├── music.py

Subpackages

# fun/writing/base.py
class WritingImplement:
    def __init__(self, color):
        self.color = color

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

# fun/writing/markers.py
from fun.writing.base import WritingImplement

class DryEraseMarker(WritingImplement):
    # ...

Absolute vs. relative imports

Absolute vs. relative imports

These are equivalent.

Absolute

# fun/writing/markers.py
from fun.writing.base import WritingImplement

Relative

# fun/writing/markers.py
from .base import WritingImplement

Absolute vs. relative imports

These are also equivalent.

Absolute

# fun/writing/markers.py
from fun.coin import flip

Relative

# fun/writing/markers.py
from ..coin import flip

.. refers to the package above the current one.

Let's develop it

How would we...

fun
└── writing
   ├── base.py  # <-- 1. import Song?
   └── markers.py
├── coin.py
├── music.py  # <-- 2. import DryEraseMarker?

Let's develop it

# fun/writing/base.py
from fun.songs import Song
# OR
from ..songs import Song
# fun/music.py
from fun.writing.markers import DryEraseMarker
# OR
from .writing.markers import DryEraseMarker

Installing 3rd-party packages

with pip

pip

A way to install packages from the Python Package Index (PyPI)

Let's develop it

pip install crayons

Let's develop it

Run python and use the package you installed.

Using crayons

Let's develop it

Now uninstall crayons:

pip uninstall crayons

Installing multiple requirements

pip -r requirements.txt

Let's develop it

Create a file called requirements.txt with the following 2 package names to install:

crayons
hodor

Let's develop it

Install the packages.

pip install -r requirements.txt

Let's develop it

List your installed packages.

pip freeze

Let's develop it

Use your packages.

hodor and crayons both installed

Virtual environments

virtualenvs

Note: To get an understanding of what virtualenvs are and why they're useful, let's look at Netflix

Virtualenvs are like Netflix profiles

Note: Virtual environments are useful for the same reason netflix profiles are useful...avoiding conflict...

The problem

Different projects on the same computer may depend on conflicting 3rd-party packages.

They may even depend on different Python versions.

The solution

Each project gets its own isolated environment with

  • its own copy of python
  • its own copy of pip
  • its own folder of 3rd-party packages

Using virtual environments

with pew

"Python Env Wrapper"

Install pew

pip install pew

Before we get into a virtualenv...

Let's see which python we're using now.

which python
# => /usr/local/bin/python
which pip
# => /usr/local/bin/pip

Right now, we're using the system python.

Create a new Python 3 virtualenv

pew new --python=python3 gdi

pew will automatically activate the new virtualenv

Check yourself

which python
# => /Users/sloria/.local/share/virtualenvs/gdi/bin/python
which pip
# => /Users/sloria/.local/share/virtualenvs/gdi/bin/pip

We're no longer using the system python.

We're using our virtualenv's python.

Exiting the virtualenv

Hit ctrl+D to exit.

which python
# => /usr/local/bin/python

Back to system python.

Activating the virtualenv

pew workon gdi

which python
# => /Users/sloria/.local/share/virtualenvs/gdi/bin/python

Let's develop it

  • While your gdi virtualenv is activated, install the django package
  • Enter the python shell and import django
  • Now exit your virtualenv
  • Enter the python shell and import django again. You should see an error.

Questions?

Looking ahead

How do we go from...

Python code

to

App

Extra practice

  • Add a play_all() method to Playlist that plays all the songs in the playlist.
  • Modify your WritingImplement.write method to actually print out a colored string using the crayons package installed from PyPI.
  • Write a script that creates two Playlist instances and add songs to each of them. Then use your coin.flip() function to choose which playlist to play. Play the chosen playlist.