Python

A reference

Variables

strings, ints, doubles, and booleans

String variables are used to store any series of characters. Strings can represent a letter, a word, a sentence, a paragraph, numbers, equations, etc. Integers are used to represent whole numbers, and can be very useful for mathematic situations. Doubles are used to represent numbers with decimal points. Booleans are used to represent true and false and are used in boolean logic.

Data Structures

lists, and dicts

Lists are essentially just a series of variables. Lists, in Python, are similar to arrays in other languages. If you're not familiar with an array, they store a series of variables together. If you want to access a single element in the list you reference its index. Lists are 0-indexed, meaning that the first element has an index of 0, the second has an index of 1, and so on.

Dicts are similar to lists except each entry has a key and a value, similar to a dictionary. Dicts are similar to maps in other languages. If you want to access a single value, you have to use the provided key. All keys have to be unique within a single dict. The key can be thought of as a custom index in the list example. Lets do another interactive example.

Control Flow Tools

if elif else statements

The if block is your program's mitochondria (mitochondria is the powerhouse of the cell!). It allows for multiple paths to be taken through your code. The code inside of the first if will be executed if the boolean check immediately after the if is true. If it is false, then it will check the next elif boolean check to see if it is true. If nothing evaluates to true, the else block will execute. The only required piece is the if block. The following are all correct.

for loops

The for loop is also a crucial part of controlling the flow of your program. If you want to repeat the same block of code multiple times, or iterate through every element in a list, then the for loop is for you!

The range() function is a built-in function in python. It will create a list out of the integer parameter (we'll cover functions and parameters soon). If you call range(4) a list will be created of integers from 0 to 3. This is very useful with for loops in Python. There is an example below that makes use of the range() function, a for loop, and an if/else block.

while loops

while loops are very similar to for loops. They allow you to repeat certain sections of your code for the duration that some check is true. While loops perform a boolean check before every iteration to make sure it should repeat, or break. The boolean check comes immediately after the while keyword. Below is a simple example:

def functions

Sometimes you might want to use the same block of code in multiple places and you'll be tempted to copy and paste. Never ever copy and paste. If there is a piece of code that you would like to reuse, turn it into a function! A function is your own custom set of commands that you can call with a single command instead of copying and pasting multiple lines of code. Don't worry if your output doesn't exactly match on this one; 5 random colors should be printed out.

function arguments

Functions become much more powerful if they can accept variables that you can then manipulate. If we modify the previous function we wrote, we can have it accept incoming arguments, or parameters. But we'll modify it so that the functionality stays the same. Again, don't worry about the output being exactly the same.

Custom Structures AKA Objects

classes

If you would like to create your own object, like a string or int, you have to build it out of the existing primary types. You could create and object called Person, and it could contain a name, a birthday, and an ethnicity. And you could give it custom functions like get_age(). Here's an example of what a Person class might look like.

from datetime import datetime

class Person():
  def __init__(self, name, birth_year, ethnicity=None):
    self.name = name
    self.birth_year = birth_year
    self.ethnicity = ethnicity
    
  def get_age(self):
    age = datetime.now().year - self.birth_year
    return age
    
john = Person("John", 1990)

Scope

visibility

The scope is always something you have to keep in mind. All variables, functions, and data structures maintain some kind of scope. The scope refers to what is visible in certain parts of your code. If you declare a global variable at the top of your code, all of your functions will be able to see it. But if you declare a variable in one function, only that function can see the variable. Here's a quick demo.

x = "Hello, World!" # global variable

def local_one():
  y = "Hello locals."
  print x # global variables are visible here

def local_two():
  z = "Howdy folks."
  print z # printed from the same scope z was declared

print x
print y # error! local_one() scope has ended, along with y