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Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

What are we going to do today?

• Learn a bit about Python in general
• Learn how to execute a Python script
• Write a little Python program and execute it

A future-birthday calculator

• Let's write a silly program today!
• We want to call our program with a birthday and an age:

  python birthday.py 1982-09-08 10

• It should print the weekday for each birthdays up to the given age
• The output should look like this:

  I will turn 1 on Thursday, 1983-09-08
  I will turn 2 on Saturday, 1984-09-08
  I will turn 3 on Sunday, 1985-09-08
  I will turn 4 on Monday, 1986-09-08
  I will turn 5 on Tuesday, 1987-09-08
  I will turn 6 on Thursday, 1988-09-08
  I will turn 7 on Friday, 1989-09-08
  I will turn 8 on Saturday, 1990-09-08
  I will turn 9 on Sunday, 1991-09-08
  I will turn 10 on Tuesday, 1992-09-08
  

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Python In A Nutshell

• Invented by Guido van Rossum in the late 1980s
• Very easy to read syntax
  • Uses plain English for many functions and operators
  • Uses four spaces of indentation for code blocks
  • Has a comprehensive style guide: PEP8
  • No unnecessary special characters, like semicolons at the end of the line

Python In A Nutshell

• Python is not a compiled language, but an interpreted language
• This means, in order to run a Python script, you need to install the Python interpreter
• Pro: You never need to compile your code.
• Con: If you want to install your software on other people's computers, they need to install the interpreter as well.

Python In A Nutshell

• Python is not the fastest language
  • If you want to write a game, better use C
• Python is not the safest language
  • If you want to fly a rocket to Mars, ask Elon Musk what he is using
• Python is a very readable, learnable and maintainable language
  • If you want to build a business, use Python

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Using Cloud9 IDE

• Setting up Python on your local computer can be time consuming
• We have a Python Introduction workshop to help with this
• For this workshop we will save some time and just use Cloud9 IDE
• Create an account at http://c9.io
• Create a new workspace "pyladies-python101"

How to Execute a Python Script

• Open your workspace in Cloud9 IDE
• Rightclick at the workspace name in the left sidebar
• Chose "New file" and name it "test.py"
• Doubleclick the new file to open it
• Enter the following code:

  print("Hello World")

• Save the file
• Press the green "Run" button at the top

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Variables

• Every program needs to store data in memory
• We use Variables to remember values and use them later
• The syntax for defining a variable is this:

  variable_name = 1

  message = "Hello World!"

  is_simple = True

  something = None

Exercise

• Let's create a file called birthday.py

BIRTHDAY = "1982-09-08"
print(BIRTHDAY)

• Always run your script after each exercise!

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Strings

• Strings are the most common data type
  • Some programming languages also have Char as a data type
  • A char is a single character
  • Therefore, a whole word, is a long string of characters
• The syntax for defining a string is this:

  variable_name = "Your string"

Strings

• Like all things in Python, Strings are also Objects
• For this workshop we will not worry about Objects.
• Just remember one thing: Objects usually have useful methods
• Strings have a ton of useful methods
• You can call string methods like this:

  my_string = "Hello World!"
  my_string = my_string.upper()
  print(my_string)
  

• The output should look like this:

  > HELLO WORLD!

Strings

• One especially useful method of the String object is format()
• It allows you to add placeholders into your strings
• Then you can fill in variable values into those placeholders
• It looks like this:

  my_age = 33
  my_string = "I am {} years old".format(my_age)
  print(my_string)
  

• The output should look like this:

  > I am 33 years old

Exercise

• Change birthday.py to look like this:

BIRTHDAY = "1982-09-08"
message = "I am born on {}".format(BIRTHDAY)
print(message)

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Input

• Command Line Programs often take arguments as input
• This way you pass in data into the program
• For example the python command takes a filename as a parameter:

  python birthday.py

• We can even pass in more than one argument:

  python birthday.py 1982-09-08

Imports

• Python has an extensive Standard Library
• We can use things from that library by importing them at the top of our files:

  import datetime
  now = datetime.datetime.now()
  print(now)
  

• The output should be the current date and time:

  > 2015-12-08 02:19:15.529046

Exercise

• With this new knowledge we can extend birthday.py a bit...
• Let's make it so that we pass in the date via the command line
• Make it look like this:

import sys

BIRTHDAY = sys.argv[1]
message = "I am born on {}".format(BIRTHDAY)
print(message)

• Try to press the "Run" button

Exercise

• Whoops! Pressing the "Run" button generates an error:
  • "IndexError: list index out of range"
• This is because our program now expects one command line argument
• From now on, we will execute our script from the Terminal
• Press "ALT+T" to open a new Terminal
• Enter

  python birthday.py 2015-01-01

• Leave that tab open, we will run our program from here from now on

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

The If-Statement

• You need to control the program flow in your programs
• That means, if some condition is true, then certain code should be executed
• If it is not true, either nothing should happen or some other code should be executed
• The If-Statement helps you to do this:

  if 1 > 2:
    print("One is bigger than two")
    # This code is actually never reached
  else:
    print("One is not bigger than two")
  

• The output should be:

  > One is not bigger than two

Exercise

• Let's build some error handling into our birthday.py

import sys

if len(sys.argv) < 2:
    print("ERROR: Please provide a date")
else:
    BIRTHDAY = sys.argv[1]
    message = "I am born on {}".format(BIRTHDAY)
    print(message)

• By the way: "len" is a built-in function to return the length of a list
• ...and "sys.argv" is just a list of command line arguments
• The first argument is always the Python script filename
• The second argument is supposed to be birthdate

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Datetimes

• The built-in datetime module has three important classes: date, datetime and timedelta
• We can use these classes to do powerful things with date calulations
• In order to use the module, you need to import it
• This is how you create a datetime object:

import datetime
my_date = datetime.datetime(2015, 12, 08)
print(my_date)

• The output should be:

  2015-12-08 00:00:00

Datetimes

• The datetime.datetime class has a powerful function called strptime
• It helps you to turn a string into a datetime object:
• You can learn more about the formatting options here

import datetime
my_string = "2015-12-08"
my_date = datetime.datetime.strptime(my_string, "%Y-%m-%d")
print(my_date)

• The output should be:

  2015-12-08 00:00:00

Datetimes

• There is much more you can do with datetimes
• For example, you can get the weekday number for any date:

import datetime
my_date = datetime.datetime(2015, 9, 8)
print(my_date.isoweekday())

• The output should be:

  2

• Because it's Tuesday, the second day of the week

Exercise

• Let's convert the birthdate input string into a datetime object

import sys
from datetime import datetime

if len(sys.argv) < 2:
    print("ERROR: Please provide a date")
else:
    BIRTHDAY = datetime.strptime(sys.argv[1], "%Y-%m-%d")
    message = "I am born on {}".format(BIRTHDAY.date())
    print(message)

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Exception Handling

• Sometimes you can't avoid program crashes
• Especially when your program accepts user input
• You can use try-except blocks to handle exceptions:

  value1 = 1
  value2 = 0
  try:
    result = value1 / value2
  except ZeroDivisionError:
    print("ERROR: You can't divide by zero")
  

• The output should be:

  > ERROR: You can't divide by zero

Exercise

• Let's make sure that the date is formatted correctly:

import sys
from datetime import datetime

if len(sys.argv) < 2:
    print("ERROR: Please provide a date")
else:
    try:
        BIRTHDAY = datetime.strptime(sys.argv[1], "%Y-%m-%d")
    except ValueError:
        print("ERROR: Please enter a correct date")
        sys.exit()
    message = "I am born on {}".format(BIRTHDAY.date())
    print(message)

• By the way: "sys.exit()" stops the program

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Functions

• You can group code into functions
• You can then call your function many times and therefore re-use your code
• Functions accept arguments and keyword arguments
• Functions have return values
• You can define a function like this:

def function_name(arg1, arg2, kwarg1=None, kwarg2=None):
  result = arg1 + arg2
  return result

• You can call a function like this:

result = function_name(1, 2, kwarg1="Hello"):
print(result)

Exercise

• Let's create a function that adds some years to a given date

import sys
from datetime import datetime, date

def get_future_date(base_date, years):
    future_date = date(
        base_date.year + years,
        base_date.month,
        base_date.day
    )
    return future_date

if len(sys.argv) < 3:
    print("ERROR: Please provide a date and an age")
else:
    try:
        birthday = datetime.strptime(sys.argv[1], "%Y-%m-%d")
    except ValueError:
        print("ERROR: Please enter a correct date")
        sys.exit()
    years = int(sys.argv[2])
    future_date = get_future_date(birthday, years)
    message = "I will turn {} on {}".format(years, future_date)
    print(message)

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Lists

• You can define an empy list like this

  my_variable = []

• You can store many values into lists

  my_variable = [1, 2, 3]

• You can add or remove items from lists

  my_variable.append(4)
  my_variable.remove(4)

• You can access an item in the list by it's index:

  my_variable[1]
  # This should return `2`, because lists are zero-indexed
  

• Find out more about lists here

Lists

• Here is a little example of things you can do with lists:

numbers = [1, 2, 3]
more_numbers = [4, 5, 6]
all_numbers = numbers + more_numbers
print(all_numbers)
all_numbers.reverse()
print(all_numbers)
all_numbers.append(7)
print(all_numbers)
all_numbers.sort()
print(all_numbers)

• The output should look like this:

  [1, 2, 3, 4, 5, 6]
  [6, 5, 4, 3, 2, 1]
  [6, 5, 4, 3, 2, 1, 7]
  [1, 2, 3, 4, 5, 6, 7]
  

Tip: The Range Function

• "range()" is a built-in function
• It generates a list of numbers for you:

  my_list = range(100)
  my_other_list = range(1, 100, 1)
  print(my_list)
  print(my_other_list)
  

Loops

• You often need to repeat the same code in a loop
• Loops look like this:

var my_list = [1, 2, 3, 4]

for my_number in my_list:
    if my_number == 1:
        continue
    print (my_number)

• You can use "continue" to skip this iteration of the loop

Exercise

• Let's extend the output of our program so that it shows all future birthdays up to the provided age

import sys
from datetime import datetime, date

def get_future_date(base_date, years):
    future_date = date(
        base_date.year + years,
        base_date.month,
        base_date.day
    )
    return future_date

if len(sys.argv) < 3:
    print("ERROR: Please provide a date and an age")
else:
    try:
        birthday = datetime.strptime(sys.argv[1], "%Y-%m-%d")
    except ValueError:
        print("ERROR: Please enter a correct date")
        sys.exit()
    years = int(sys.argv[2])
    years_list = range(1, years + 1, 1)
    for year in years_list:
        future_date = get_future_date(birthday, year)
        message = "I will turn {} on {}".format(year, future_date)
        print(message)

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

Dictionaries

• Dictionaries are so called "key-value-stores"
• You can define an empty dictionary like this:

  my_dict = {}

• You can also define it with initial values:

my_dict = {
    "Martin": 33,
    "Eva": 24,
}

• You can access an item by it's key:

  martins_age = my_dict["Martin"]

Exercise

• Let's add the final piece to our program
• We will write a function that returns the weekday name for a given weekday number

import sys
from datetime import datetime, date

def get_weekday_name(weekday_number):
    weekdays = {
        1: "Monday",
        2: "Tuesday",
        3: "Wednesday",
        4: "Thursday",
        5: "Friday",
        6: "Saturday",
        7: "Sunday",
    }
    return weekdays[weekday_number]

def get_future_date(base_date, years):
    future_date = date(
        base_date.year + years,
        base_date.month,
        base_date.day
    )
    return future_date

if len(sys.argv) < 3:
    print("ERROR: Please provide a date and an age")
else:
    try:
        birthday = datetime.strptime(sys.argv[1], "%Y-%m-%d")
    except ValueError:
        print("ERROR: Please enter a correct date")
        sys.exit()
    years = int(sys.argv[2])
    years_list = range(1, years + 1, 1)
    for year in years_list:
        future_date = get_future_date(birthday, year)
        weekday = get_weekday_name(future_date.isoweekday())
        message = "I will turn {} on {}, {}".format(year, weekday, future_date)
        print(message)

Table of Content

• Introduction
• Python In A Nutshell
• Cloud9 IDE
• Variables
• Strings
• Input & Imports
• The If-Statement & Type-casting
• Datetimes
• Exception Handling
• Functions
• Lists & Loops
• Dictionaries
• What's Next?

What's Next?

• Work your way through learnpythonthehardway.org
• Come visit our Python 102 workshop :)
• Like our Facebook group: PyLadies SG
• Join our Slack channel