Python basics - cheatsheet

In

Basics

Variables

  • assign

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    message = "Hello world"
    print(message)

  • concatenantion

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    full_name ="First name: {}\nLastname: {} ".format(
      first_name,
      last_name)

User input

  • Strings

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    # default is string
    name = input("What's your name? ")
    print("Hello, " + name + "!")

  • Numbers

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    age = input("How old are you? ")
    age = int(age)
    pi = input("What's the value of pi? ")
    pi = float(pi)

Lists

  • definition

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    # assign
    bikes = ["mountain bike", "aegean blue"]
    # clone a list
    copy_of_bikes = bikes[:]

  • get item

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    # get first
    first_bike = bikes[0]
    # get last
    last_bike = bikes[-1]

  • add and remove items

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    bikes = []
    bikes.append("mountain bike")
    bikes.append("aegean blue")
    bikes.remove("aegean blue")

  • List comprehensions 

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    double_values = [x*2 for x in range(1, 11)]

  • Slicing a list 

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    finishers = ["Ada", "Bob", "Charles"]
    first_two = finishers[:2]

Tuples

  • Non-editable lists 
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    definition = (1366, 768)

Dictionaries

  • Key-value pairs items

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    alien = {
    'color': 'green',
    'points': 2
    }

    print("The alien's color is " + alien['color'])

  • Adding a new key-value pair

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    alien['x_position'] = 0

  • Looping on dictionaries

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    # loop on pairs
    fav_numbers = {'eric': 17, 'ever': 4}
    for name, number in fav_numbers.items():
    print(name + ' loves ' + str(number)) 

    # loop on keys
    fav_numbers = {'eric': 17, 'ever': 4}
    for name in fav_numbers.keys():
    print(name + ' loves a number') 

    # loop on values
    fav_numbers = {'eric': 17, 'ever': 4}
    for number in fav_numbers.values():
    print(str(number) + ' is a favorite')

Control structures

Conditionals

  • Conditional tests

    Condition Representation
    equals x == 42
    not equals x != 42
    greater than x > 42
    greater or equal to x >= 42
    less than x< 42
    less or equal to x <= 42

  • Conditional test with lists 

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    'mountain bike' in bikes
    'car' not in bikes

  • Conditional blocks

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    if age < 4:
      ticket_price = 0
    elif age < 18:
      ticket_price = 10
    else:
      ticket_price = 15

Loops

  • Conditional block
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    current_value = 1
    while current_value <= 5:
      print(current_value)
      current_value += 1

Files

  • Read

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    file_name = 'demofile.txt'
    with open(file_name) as file_object:
      lines = file_object.readlines() 
    for line in lines:
      print(line)

  • Write

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    # overwrite
    file_name = 'journal.txt'
    with open(file_name, 'w') as file_object:
      file_object.write("I am coding Python.")
    # append
    filename = 'journal.txt'
    with open(filename, 'a') as file_object:
      file_object.write("\nThis is some appended text.")

Functions

  • Simple

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    # defintion
    def greet_user():
      # Display a simple greeting
      print("Hello!") 

    # call
    greet_user()

  • With default value parameters

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    def make_pizza(topping='bacon'):
      """Make a single-topping pizza."""
      print("Have a " + topping + " pizza!") 

    make_pizza()
    make_pizza('pepperoni')

  • Returning a value 

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    def add_numbers(x, y):
      """Add two numbers and return the sum."""     return x + y 
     
    sum = add_numbers(3, 5) print(sum)

Classes

  • Simple class

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    class Dog():
      """Represent a dog.""" 
      def __init__(self, name):
        """Initialize dog object."""
        self.name = name 
      def sit(self):
        """Simulate sitting."""
        print(self.name + " is sitting.") 

    my_dog = Dog('Buddy') 
     
    print(my_dog.name + " is a great dog!")
    my_dog.sit()

  • Inheritance

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    class SARDog(Dog):
      """Represent a search dog."""
      
      def __init__(self, name): 
      """Initialize the sardog."""
        super().__init__(name)
        
      def search(self):
        """Simulate searching."""
        print(self.name + " is searching.")
        
    my_dog = SARDog('WToby') 
    print(my_dog.name + " is a search dog.")
    my_dog.sit()
    my_dog.search()

Exceptions

  • Try/catch/else:

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    prompt = "How many tickets do you need? "
    num_tickets = input(prompt) 

    try:
        num_tickets = int(num_tickets)
    except ValueError:
        print("Please try again.")
    else:
        print("Your tickets are printing.")