Python basics

Following is a summary of basic Python concepts covered in this class.

Variables

A variable references a value:

x = 5

This means x references the integer 5, or x → 5.

Likewise:

name = 'Emma'

means the variable name references the string 'Emma'. name → 'Emma'.

Types

The following are basic types in Python:

• string — e.g. 'hello', can use single or double quotes
• integer — e.g. 5 — convert a string to an integer with int()
• float — e.g. 5.2 — convert a string to a float with float()
• boolean — must be True or False

Operators

Operators compute new values from a type.

Arithmetic operators

Arithmetic operators take two numbers and produce a result.

Comparison operators

Comparison operators take two numbers and produce a boolean result.

String operators

Some operators can also apply to strings.

If statements

An if runs a block of code once if the condition is true. For example:

if age < 35:
  print("Sorry, you can't run for President.")

You can add else to handle the other case:

if age < 35:
  print("Sorry, you can't run for President.")
else:
  print("Good luck in your candidancy.")

You can use elif to check multiple conditions:

if age < 25:
  print("These are the best years of your life.")
elif age > 65:
  print("These are the golden years of your life.")
else:
  print("Well, just muddle through, I guess.")

You can use multiple conditions and combine them with or and and:

if age < 5 or height < 3:
  print("Sorry, you are not able to ride this roller coaster.")

if not qualified and money > 1000000:
  print("You're not qualified for this opportunity, but if you make a big enough donation we'll consider you anyway.")

While loops

A while loop runs a block of code as long as the condition is true. Some examples:

while bit.can_move_front():
  bit.move()

x = 5
while x > 0:
  print('hello')
  x = x - 1

while True:
  age = int(input("What is your age?"))
  if age >= 18 and age <= 120:
    break

  print("That is not a valid age.")

Functions

Functions let you organize your code into smaller pieces. For example, here is a function that tells you if you have guessed the right number:

def check_guess(guess, answer):
    if guess == answer:
        print("Good job!")
        return True
    if guess > answer:
        print("Lower")
    else:
        print("Higher")

    return False

A function has these parts:

• def — a keyword that means you are starting a function definition
• function name
• parameters — zero or more parameters, separated by a comma (these are also called arguments)
• colon
• indented block of code to run
• optional return statement, which returns one or more values — if no return statement the function returns None

Input

You can get input from the terminal using the input() command:

name = input('Enter your name: ')

If you need to convert to an integer, use int():

age = int(input('Enter your age: '))

Likewise you can use float() to convert to a float.

Output

You can print with the print() statement:

print('Hello')

Formatted strings are particularly useful for output that needs to use a variable:

value = 5
result = f'Your value is: {value}'
print(result)

A formatted string starts with an f and then can use a variable name (and even operators) inside of curly brackets.

value = 5
result = f'One more than your value is: {value + 1}'
print(result)

Breaking from a loop

You can break out of a loop with break:

while True:
  age = int(input("Enter an age for an adult: "))
  if age >= 18:
    break
  print("Try again.")

print(f'The age you entered: {age}')

If a while loop is inside of a function and you want to break out of the loop and return at the same time, you can just use return:

def get_age():
  while True:
    age = int(input("Enter an age for an adult: "))
    if age >= 18:
      return age
    print("Try again.")

Putting this all together

Here is a complete program that is a guessing game for you and a friend to play:

def get_number():
    print("Enter a number between 1 and 100 for your friend to guess.")
    while True:
        number = int(input("Number: "))
        if number >= 1 and number <= 100:
            return number
        print("Try again.")


def print_blank_lines(number):
    while number > 0:
        print()
        number = number - 1


def get_guess():
    return int(input("What is your guess? "))


def check_guess(guess, answer):
    if guess == answer:
        print("Good job!")
        return True
    if guess > answer:
        print("Lower")
    else:
        print("Higher")

    return False


def main():
    answer = get_number()
    print_blank_lines(10)
    while True:
        guess = get_guess()
        found = check_guess(guess, answer)
        if found:
            break


if __name__ == '__main__':
    main()