Python

The Snake-Charmer Guide

A bit about you

  • Deutsch / English?
  • Ersti?
  • Why do you want (have 😉) to take this course?
    • ​Interest
    • RoboLab

Resources

git.io/JeuPA

Where is Python? 🐍

Introduction

C

C++

Rust

Go

Javascript

PHP

Perl

Java

Python

Compiled

Interpreted

Bytecode

Introduction

def hello()
  print("Hello, World!")
0 LOAD_GLOBAL              0 (print)
2 LOAD_CONST               1 ('Hello, World!')
4 CALL_FUNCTION            1

Where is Python? 🐍

Introduction

PHP

Slow

Fast

Easy

Hard

Python

Rust

 

C

Java

Javascript

C++

 

Go

But it is so slow! 🐢

  • developer time is a lot more expensive than computing time
  • many applications are not bound by processing time
  • many algorithms implemented in C

Introduction

State of Python 📈

Introduction

most popular languages (stackoverflow 2022)

most loved languages (stackoverflow 2022)

Why Python? 🤔

  • Easy & Intuitive
  • Huge community
  • Robolab
  • Mature Packages for almost everything
    (202.022 @ 10.2019 ... 403.290 @ 07.2022)

Introduction

Installation

macOS
windows
linux

Normal:

https://www.python.org/downloads/

 

For brew users

brew install python3

Ubuntu / Debian:

apt install python3

 

Arch / Manjaro:

pacman -S python3

Normal:

https://www.python.org/downloads/

 

For chocolatey users:

choco install python

Editor / IDE

Installation

VS Code
Atom
PyCharm
Vim oder Emacs are also fine of course 😉
with official
python extension
with python extension

Open Python

macOS
windows
linux

Applications/Utilities/Terminal.app

 

python3 + enter

Open the terminal

 

python3 + enter

Windows Key and type "cmd", hit enter

 

python + enter

Installation

Variables

# I'm a comment, everyone ignores me

a = "Hello 👋"	# string
b = 5		# integer
c = True	# boolean

Basics

Types

# Strings
'Python'
"Another String"
"""I
am
incredibly
long
"""


# Boolean
True
False

Basics

# Integer
1
42
69
-420
123e3 # 123000


# Float
0.123
0.5e2 # 50.0
# List
[1, 2, 3]
["foo", "bar"]
[True, 2, "three"]


# Tuples
("Hello", 2)


# Dictionaries
{
  'name': 'Yoda',
  'age': 900
}

Built-ins

# Conversion

int(5.3)  # 5
int(5.7)  # 5

float(5)  # 5.0

int('5')  # 5
float('5.5')  # 5.5


# Length
len('Hello')  # 5
len([1,2,3])  # 3

Basics

Control Flow

# if & else
if something is True:
  do_this()
else:
  do_that()


# and
if a and b:
  do_this()

# or
if (a and b) or c:
  do_this()

Basics

# numbers
if 42 < 69:
  magic()

# numbers
if 42 == 42:
  magic()

# string
if input == 'awesome':
  yaaas()
else:
  nope()

Reserved Words

Basics

and exec not
assert finally or
break for pass
class from print
continue global raise
def if return
del import try
elif in while
else is with
except lambda yield

Terminal for Dummies

Command macOS / Linux Windows
navigate cd cd
list files & directories ls dir
create folder mkdir mkdir
create files touch test.txt type nul > test.txt
remove files rm del
remove folder rm -r myfolder rmdir \s myfolder
run python python3 hello.py python hello.py

Loops

Overview

for x in range(0, 3):
  print("Running 3 times 🏃🏼")


while True:
  print('To infinity and beyond!')

Loops

For

# range(start, end) generates numbers from `start` to `end`
# `start` is included, but `end` is not
# => range(0, 3) will generate 0,1,2
for x in range(0, 3):
  print(x)  # 0,1,2

for x in range(3):
  print(x)  # 0,1,2

for name in ['Lisa', 'Günther', 'Sigfriede']:
  print(name)  # Lisa, Günther, Sigfriede

Loops

While

n = 5
while n > 0:
  print(n)
  n -= 1
  # 5,4,3,2,1


while True:
  # forever


l = ['A', 'B', 'C']
while l:
  element = l.pop() # Change l "in-place" until it's empty (=false)
  print(element)
  # A, B, C

Loops

Controls 

l = [1, 2, 3, 4, 5]

for x in l:
  if l == 3:
    break
  print(x)
  # 1,2


for x in l:
  if l == 3:
    continue
  print(x)
  # 1,2,4,5

Loop

Functions

  • Reuse logic & code
  • Readability
  • Split up complex tasks
function
x1
y
x2
add
1
3
2

Declaration

def greet():
  print("Hello 👋")

Functions

greet() # "Hello 👋"

Parameters

def greet(name):
  print(f"Hello {name} 👋")

Functions

greet('Python') # "Hello Python 👋"

greet('Batman') # "Hello Batman 👋"

Example

def add(a, b):
  addition = a + b
  return addition

Functions

three = add(1, 2)
five = add(2, 3)

Default Values

def add(a, b=1):
  addition = a + b
  return addition

Functions

three = add(1, 2)
five = add(4)
def add(a, b):
  addition = a + b
  return addition
five = add(4)
# ❌ ERROR ❌

Named Parameters

def eat(pizza, slices):
  print(f"Eating a pizza {pizza} with {slices} slices")

Functions

eat('Margherita', 8) # ✔

eat(pizza='Margherita', slices=8) # ✔

eat(slices=8, pizza='Margherita') # ✔

eat('Margherita', slices=8) # ✔

eat(slices=8, 'Margherita') # ❌

Scope

name = 'Milka'

def hi():
  print(f'Hi {name}')


hi()

Functions

Scope

name = 'Milka'

def hi():
  name = 'Paula'

hi()

print(f'Hi {name}')

Functions

Scope

x = 0

def do_stuff():
  x = 1

do_stuff()

print(x)

Functions

Scope

x = 0

def do_stuff():
  x = x + 1

do_stuff()

print(x)

Functions

Scope

x = 0

def do_stuff():
  global x
  x = x + 1

do_stuff()

print(x)

Functions

Overview

# Simple
def hello():
  print('Hello')


# Parameter
def greet(name):
  print('Hello ' + name)

greet('Horst')

Functions

# Return
def add(a, b):
  return a + b

c = add(1, 2)


# Named Parameters
def gradient(x, y):
  return y / x

m = gradient(y=5, x=10)

Lists

  • Reuse logic & code
  • Readability
  • Split up complex tasks

List

list = [1, 2, 3]

colors = ['blue', 'green', 'yellow']

mixed = [8, 'some string', True]

Basics

List

list = [1, 2, 3]

first = list[0]
last = list[-1]

first_two = list[0:2]
first_two = list[:2]

last_two = list[-2:]

everything_except_the_first = list[1:]
everything_except_the_last = list[:-1]

Basics

List

names = ["Peter", "Thomas", "Lisa", "Claire"]

for i, name in enumerate(names):
  print(f"Person {i} is called {name}")

Basics

List

Basics

All methods are documents here:

 

https://docs.python.org/3/tutorial/datastructures.html#more-on-lists

Dicts

  • A.k.a Maps
  • Key-Value pairs
  • Group data together
  • Less variables

What?

Why?

Syntax

{
  key1: value1,
  key2: value2,
  ...
}

Dict

Assignment

person = {
    'age': 900,
    'name': 'Yoda'
}

Dicts

Reading

person = {
    'age': 900,
    'name': 'Yoda'
}

Dicts

person['age']  # 900
person['name']  # Yoda

Writing

person = {
    'name': 'Yoda'
}

person['name']  # Yoda

person['name'] = 'Luke'
person['name']  # Luke

Dicts

Iterating

person = {
    'name': 'Elon Musk',
    'age': 48,
    'height': 184,
    'worth': '23,7 Billion',
    'companies': ['Paypal', 'SpaceX', 'Tesla']
}

for key in person.keys():
    print(key)  # name, age, height, worth, companies

for value in person.values():
    print(value)  # Elon Musk, 48, 184, ...

for key, value in person.items():
    print(f'{key} has the value: {value}')
    
    # name has the value Elon Musk
    # age has the value 48
    # ...

Dicts

Nesting

person = {
    'name': 'Elon Musk',
    'age': 48,
    'height': 184,
    'worth': '23,7 Billion',
    'companies': {
        'Paypal': {
            'founded': 2000,
            'revenue': '15.451 billion',
            'employees': 21800
        },
        'SpaceX': {
            'founded': 2002,
            'revenue': '2 billion',
            'employees': 7000
        },
        'Tesla': {
            'founded': 2003,
            'revenue': '21.461 billion',
            'employees': 45000
        }
    }
}

Dicts

List Comprehensions

Is it a map? Is it a filter? No, it's both!!

# Option 1: For loop
squares = []
for x in range(10):
  squares.append(x*x)

print(squares) # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# Option 2: Map()
def square(x):
  return x*x

squares = list(map(square, range(10)))

# Option 3: List comprehension
squares = [x*x for x in range(10)]
#          │       │    └ Existing list
#          │       └ Iteration variable (similar to for loop)
#          └ The transformation for each element

Classes

  • Group functions and variables together
  • Basis of Object Oriented Programming (OOP)
  • Group logic that belongs together
  • Domain specific code
  • Extendable

What?

Why?

class (1)
instance (0..n)

Person

Alice, Bob, ...

Car

Fiat, Tesla, ...

Mate

Club, Kolle, ...

class
attributes
methods
(variables)
(functions)

Person

Classes

Class

Instance

  1. Alice, 20
  2. Bob, 7
  3. Charlie, 43
  4. Diana, 32
  • Name
  • Age
  • Static: Homo sapiens

Basics

class Car:
    pass

Classes

Constructor

class Car:

    def __init__(self):
		pass

Classes

Attributes

class Car:

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

Classes

my_first_car = Car('red')

my_second_car = Car('blue')

Methods

class Car:

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

    def clean(self):
        print(f'I am {self.color} again!')

Classes

my_first_car = Car('red')
my_first_car.clean()

my_second_car = Car('blue')
my_second_car.clean()

Static Attributes

class Car:
    wheels = 4

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

    def clean(self):
        print(f'I am {self.color} again!')

Classes

my_first_car = Car('red')
print(my_first_car.wheels)

my_second_car = Car('blue')
print(my_second_car.wheels)

Example

class Car:
    wheels = 4
    capacity = 5

    def __init__(self, color):
        self.color = color
        self.passengers = 0

    def clean(self):
        print(f'I am {self.color} again!')

    def get_in(self, how_many):
        self.passengers += how_many

    def available_seats(self):
        available = self.capacity - self.passengers
        print(available)

Classes

my_first_car = Car('blue')
my_second_car = Car('red')

my_first_car.available_seats()  # 5

my_first_car.get_in(1)
my_first_car.available_seats()  # 4

my_first_car.get_in(3)
my_first_car.available_seats()  # 1

my_second_car.available_seats()  # 5

__str__

class Car:
    def __init__(self, model):
        self.model = model

    def __str__(self):
        return f"This is a {self.model}."

Classes

fiat_1 = Car('Fiat 500')
fiat_2 = Car('Fiat 500')
vw = Car('VW Golf')

print(fiat_1) 
print(fiat_2)
print(vw)

# This is a Fiat 500.
# This is a Fiat 500.
# This is a VW Golf.

__eq__

class Car:
    def __init__(self, model):
        self.model = model

    def __str__(self):
        return f"This is a {self.model}."

    def __eq__(self, other: object) -> bool:
        return self.model == other.model

Classes

fiat_1 = Car('Fiat 500')
fiat_2 = Car('Fiat 500')
vw = Car('VW Golf')

print(fiat_1 == vw)
print(fiat_1 == fiat_2)

# False
# True
print(fiat_1 == fiat_2)
print(fiat_1 is fiat_2)

# True
# False

__gt__

class Car:
    def __init__(self, model, length):
        self.model = model
        self.length = length

    def __str__(self):
        return f"This is a {self.model}."

    def __eq__(self, other: object) -> bool:
        return self.model == other.model

    def __gt__(self, other: object) -> bool:
        return self.length > other.length

Classes

fiat = Car('Fiat 500', 3.6)
vw = Car('VW Golf', 4.2)

print(fiat > vw)
print(vw > fiat)

# False
# True

Class Overwrites

  • Basic: init, repr, str, bool
  • Comparison: lt, le, gt, ge, eq, ...
  • Math: add, sub, mul, div, ...
  • Utility: len, iter, contains, ...
  • Even further: call

Classes

https://docs.python.org/3/reference/datamodel.html

Inheritance

class Car:
    def __init__(self, seats):
        self.seats = seats
        self.odometer = 0

    def drive(self, km):
        self.odometer += km

    def __str__(self):
        return f'I have driven {self.odometer}km.'


class Truck(Car):
    def __init__(self, seats, capacity):
        super().__init__(seats)
        self.capacity = capacity

    def load(self, kg):
        self.capacity -= kg

    def __str__(self):
        return f'I have driven {self.odometer}km and have {self.capacity}kg of space left.'

Classes

car = Car(4)
car.drive(100)
print(car)

# I have driven 100km.
truck = Truck(2, 100)
truck.load(25)
truck.drive(50)
print(truck)

# I have driven 50km and have 75kg of space left.

Inheritance

class Car:
    pass


class Truck(Car):
    pass


car = Car()
truck = Truck()

Classes

print(isinstance(car, Car))  # True
print(isinstance(truck, Car))  # True

print(isinstance(car, Truck))  # False
print(isinstance(truck, Truck))  # True

Naming Conventions

  • How do i call x?
  • variables, functions, parameters, etc.
  • It's a convention, not a requirement
  • Makes it easier to read code
  • Especially if it's not yours

What?

Why?

Cases

Naming Conventions

SNAKE_CASE

CamelCase

lower_case_with_underscores

Example

myVariable = 'foo'  # 👎
MyVariable = 'foo'  # 👎

my_variable = 'foo'  # 👍

MY_CONSTANT = 'foo'  # 👍 , but not only for constants


def some_function(some_parameters):  # 👍
    pass


# Classes
class SomeClass:
    pass

Naming

Error handling

What happens when our programm runs into an error?

Exceptions

an Exception is raised

Common Exceptions

a = 2/0
# ZeroDivisionError


number = int(input("Enter a number: "))
# ValueError


song = {'name': "Sandstorm"}
darude = song['artist']
# KeyError

Error Handling

Raise an Exception

raise Exception()

raise Exception("I'm tired, don't want to work anymore")


def foo():
  raise Exception("I decided to crash now")

Error Handling

Handle it

def try_not_to_procrastinate():
    raise Exception("This 30 minute Youtube video about Ant colonies looks really nice!")


try:
    try_not_to_procrastinate()
except Exception:
    print("Well...")


# # Well...

Error Handling

Handle it

def try_not_to_procrastinate():
    raise Exception("This 30 minute Youtube video about Ant colonies looks really nice!")


try:
    try_not_to_procrastinate()
except Exception as e:
    print(e)


# This 30 minute Youtube video about Ant colonies looks really nice!

Error Handling

Custom Errors

class MyError(Exception):
    pass

Error Handling

Custom Errors

class MyError(Exception):
    pass

try:
    raise MyError()
except MyError:
    print('Something happened')

Error Handling

Modules & Imports

  • Split code into multiple files
  • Use code that people have already written
  • Don't reinvent the wheel
  • Much easier to maintain
  • Keeps everything organised

What?

Why?

Python

PIP

a.k.a the interwebz

Local

Python

  • Batteries Included
  • 250+ Build-In Packages

Python Standard Library

math, datetime, random

PIP

  • 208,439 projects
  • 1,576,568 releases
  • 2,362,966 files
  • 390,584 users

pypi.org

Basics

import datetime  # Standard Library

timestamp = datetime.datetime.now()
print(timestamp)

Modules

Import

import math

Modules

math.sqrt(4)  # 2

math.ceil(2.1)  # 3
math.floor(2.1)  # 2

math.pi  # 3.14159...
math.cos(math.pi)  # -1.0

from math import sqrt, pi
sqrt(4)  # 2

math.ceil(2.1)  # ❌ Error

pi  # 3.14159...

Aliasing

import something

something.do()

Modules

from something import do

do()


import something as s

something.do() # ❌
s.do() # ✅


from something import do as foo

foo()

Import own code

# main.py

import ascii

ascii.order('Mate')

Modules

.
├── ascii.py
└── main.py
# ascii.py

def order(drink):
  return f'Here is your {drink}'

Asynchronous Programming

Synchronous

print('Guess my number!') # This one is always executed first
rounds = 1
found = False
start = time.time()

while True:
    try:
      	# Blocks the current thread until the user has 
      	# entered something.
      	# => We can't do anything in the meantime.
        guess = input('What is your guess?: ')
        
        guess = validate_guess(guess) # May throw a ValueError
        is_correct = play_round(guess) # May throw a InvalidGuess
        
        if is_correct:
            found = True
            break
        rounds += 1
    except ValueError as e:
        print('You are too dumb for this game, it says NUMBER!')
        break
    except InvalidGuess as e:
        print('INVALID: ', e)
        print('Please try again. 🙏')
        continue

EMAIL EXAMPLE!!!

Task:

- We have to do 10 conversations

- For each conversation:

  - We send a question (little time)

  - Recipient answer (a lot more time)

  - We send a confirmation (little time)

Me:    Question 0
They:  Info 0
Me:    Ok 0

Synchronous (Naive)

Process & Thread

Process 1

CPU 1

CPU 2

Thread

Thread

Time

Process 2

Thread

Thread

  • Processes run on a single CPU
  • Process request resources (CPU, memory, etc.)​
  • Thread is an instance of a process that has all resources (hence, it can run)
  • Threads are stopped by OS when:
    • ​time slice is exhausted (20ms)
    • thread blocks because it waits for something
  • ​Threads are started by OS when:
    • ​they are the next in the queue
    • some event for which the thread has waited happened

Why is it slow?

compute-bound

memory-bound

IO-bound

Your computer does not have enough processing power to handle the task.

 

e.g. Your processor can do 100 iterations of something an hour, but the task is to do 10,000,000 a day.

Your computer does not have enough memory to store all values.

 

e.g. You have 16Gb of RAM but need 12 TB to store all the values.

Your processor is doing almost nothing and is waiting for input and/or output to happen.

 

e.g. Writing large data to disk, fetch information from the internet.

Solution:

- parallel computing

- distributed computing

Solution:

- more money

- declare it as a feature

- back to the drawing board

Solution:

- async programming

  (parallel waiting)

- multi-threading

Computation approaches

Easier

Complex

Synchronous

Multithreading

Asynchronous

Parallel

Distributed

One after another

Parallel waiting

Parallel doing

Parallel doing

in multiple places

HTML

  • Similar syntax to XML
  • Born in 1993

Hello world

<html>
<body>

  <h1>Hello, I'm a title</h1>
  <p>This is a paragraph.</p>

</body>
</html>

HTML

Hierarchy

<html>
  <body>
    <h1>Some title</h1>
    
    <div>
      <p>
        Some text
        <b>
          I am thick
        </b>
      </p>
    </div>
    
  </body>
</html>

HTML

Attributes

<a href="https://tu-dresden.de">Go to uni 🔗</a>

HTML

Attributes

<input type="text" placeholder="your name" />

HTML

Selectors

<div> ... </div>

HTML

<div id="my-id" > ... </div>
<div class="my-class" > ... </div>
div
#my-id
.my-class

Selectors

<div>
  <div id="some" > ... </div>
</div>

HTML

<div id="my-id" >
  <p>
    <span class="text" >Some text</span>
  </p>
</div>
div #some
#some
div p span
#mi-id p span
div span
#my-id span
#my-id .text
.text
Made with Slides.com