Introduction to the Python Language

Agenda

• Introduction: Why, What and Where?
• REPL and the Debugger
• Syntax
  • Structure
  • Variables
  • Types
  • Operators
  • Flow Control
  • Functions

Learning objectives

• Get an idea of what Python is
• Get an idea of when to use Python

Resources and Material

• docs.python.org
• realpython.com
• learnpythontherightway.com
• automatetheboringstuff.com

Why use Python?

• Simple
  • Lean Syntax
  • Readability (PEP8 )
• Flexibility
  • procedural, object-oriented, functional
  • Interpreter, Just-in-time Compiler, Cython, Jython
• Spread
  • Free and Open Source
  • For all OSs
  • Extensive libraries and modules
  • Big Community
• Relevance: Data Science, Machine Learning

But really: Why use Python?

• Extensive libraries and modules: pypi.org
• Big Community

There is nearly every problem already solved and every question already asked!

• xkcd: antigravity
• xkcd: environment

What is Python?

• General purpose programming language
• Dynamically typed (like JavaScript)
• Interpreted (like JavaScript)
• Not one of that "curly-braces" languages (unlike JavaScript)

def main():
    print('Hello, world!')

main()

Where is Python used?

• Data Science and Machine Learning
• Webservers (Backend)
• Shell scripts and tools
• etc.

Read-eval-print loop (REPR) and the Debugger

Replit

Showcases

Replit

Syntax

Structure

• One line per instruction

  sum = a + b + c
  sum = (
      a
      + b
      + c
  )

• Blocks by indenting with 4 spaces

  def abs_diff(a, b):
      if a > b:
          return  a - b
      return b - a

Comments

• Comments start with a #

  c = a + b  # I'm a comment

• No block comments

  # There are
  # no block
  # comments
  

Variables

• Variables are defined using a =, no keyword needed.
• Names can contain _, numbers and UTF-8 letters, but no number at the start!
• Upper and lower case matters.
• Convention: snake_case.

  my_variable = 1  # valid
  π = 3.14         # valid
  2_chars = 'ab'   # not valid
  

• Unpacking

  a, b = 1, 2

Types

Types

Types

More on Strings

• Strings can be defined with ' or with "

  a = 'Hello'
  b = "World"

• Both versions can be used within each other

  a = "'Hello'"
  b = '"World"'

• Multi-line strings can be defined with three quotation marks (typically used for documentation)

  a = """I am a string
  that spans over
  multiple lines
  """
  

More on Lists and Tuples

• They can contain arbitrary items

  [1, 'a', None]

• They can be nested

  (1, [2, 3])

• They can be converted

  list((1, 2))
  tuple([1, 2])
  

More on Sets and Dictionary

• Both use {} ← this is an empty dict!

  set()  # this an empty set

• Dicts can be nested, sets not

  {1: {1, 2}}

• Dict keys and set items can be anything ...

  {1: 1, '2': '2', 3.0: 3.0}

• ... but need to be hashable

  hash(dict())

Indexing Lists and Tuples

• Access a subset of the items with []
• Access one item with one index

  'Hello'[0]

• Access a slice with a range from:to (to is not included)

  'Hello'[1:3]

• Ranges can be empty (everything until) or negative (from the right)

  'Hello'[2:]
  'Hello'[:2]
  'Hello'[-2:]
  'Hello'[:-2]

Indexing Dictionaries and Sets

• Access a dictionary item with []

  a = {1: 'a'}
  a[1]  # 'a'
  

• Sets are not ordered and can not be indexed.
• You could convert them to a tuple or list

  a = {1, 2}
  tuple(a)[0]
  

Indexing

Replit

Operators

Logical Operators

What evaluates to not? (5')

Replit

Mathematical Operators

Comparison Operators

Python as a Calculator (5')

Replit

• 10 ÷ ( 3.44 x ( 2.34 - 2.2 ÷ 4 ) ) = 1.624
• 2⁷ - 1 = 127
• (12300.88 modulo 8003.35) ² = 18468764.1
• √ 2 = 1.41

Flow Control

if

• if: conditional block
• elif: additional conditional block(s)
• else: if no condition met
• No brackets around the condition needed

  if x > 0:
      do_somethig()
  elif x < -10:
      do_something_else()
  elif x < 0:
      do_yet_something_else()
  else:
      fall_back()
  

while

• while: run while the condition is met
• break: stop the while loop
• continue: skip the rest of block
• else: in case no break occured

  while x > a:
      x = x - b
      if x < a:
          break
      if x < c:
          continue
      x = x - d
  else:
      print('no break!')
  

for

• for: iterate over all items
• break: stop the while loop
• continue: skip rest of block
• else: in case no break occured

  for x in (1, 2, 3, 4):
      if x > 2:
          continue
      if x < 3:
          break
  else:
      print('no break!')
  

• "for-able": sets, dict keys, lists, tuples, strings, ...

Functions

Functions

• Define functions using the def keyword
• Functions can return (something). If not, None is returned
• Arguments can have a default value (=)
• Arguments can be positional or by keyword

  def sum(a, b, c=0, d=0):
      return a + b + c + d
  
  sum(1, 2)
  sum(1, 2, 3)
  sum(a=1, b=2, c=3)
  sum(1, 2, d=4)
  

Built-in Functions

• List of built-in functions: docs.python.org/3/library/functions.html
• Replit

Countdowns (15')

How can you print a countdown (10 to 0)?

Replit

Importing

• Functions, variables, classes etc. can be imported with from ... import ...

  from datetime import date, datetime

• Built-in modules: docs.python.org/3/library/index.html
• Replit

String formatting

• How to embed variables in strings?
• f-strings can contain expressions within {}

  name = 'World'
  f'Hello, {name}'  # 'Hello, World'
  

• Expressions can have formatting options

  result = 1.23345
  f'The result is {result:.2f}'  # 'The result is 1.23'
  

• Expressions can be expressions

  a = 5
  b = 6
  f'{a} * {b} is {a*b}'  # '5 * 6 is 30'
  

Exercise 1

Create a random password generator. The password has to have a length of at least 8 characters, at least 1 digit and 1 special char (_ or -).

Exercise 2

Validate a telephone number, as if written on an input form. Telephone numbers can be written as ten digits, or with dashes, spaces, or dots between the three segments, or with the area code parenthesized; both the area code and any white space between segments are optional.

Exercise 3

Create a program that prompts for a quote and an author. Display the quotation and author as shown here:
[Author] says, "[Quote]" (Replace [Author] and [Quote] with the actual values.

You might want to use quotes.rest and requests.

Exercise 4

Create a program that prompts for ingredients. Display possible food recipes.