python-basics.md

Python Basics

Here's a very quick intro to Python, mostly aimed at covering things I show the in examples folder. If you're interested in learning more, I recommend Zed Shaw's Learn Python the Hard Way.

Hello World

To run a python script, just type python scriptname.py from the command line.

Here's a basic example.

Make a new file called hello.py and stick this in there:

print "hello"

From the terminal, run:

python hello.py

You should see "Hello" on the screen.

Variables

You don't need to declare variable types in python, or even that something is a variable.

a_number = 1 # an integer
another_number = 5.1 # a float
some_string = "Hello!" # a string
some_boolean = True #notice the capitalization on booleans
a_list = ["a bunch", "of", "stuff", a_number, some_string]

Lists

A list is an ordered collection of variables.

# make an empty list
my_list = []

# add something to our list with the "append" method
my_list.append("hi") # the list will now look like this: ["hi"]

# add some more stuff
my_list.append(45)
my_list.append(100.2)
my_list.append("whatever")

# now our list will look like this:
# ["hi", 45, 100.2, "whatever"]

# add one list to another
my_list = my_list + [4, 5, 6]

# now, like this: ["hi", 45, 100.2, "whatever", 4, 5, 6]

# you can access individual items in the list by referrring to their index value
print my_list[0] # prints "hi"
print my_list[2] # prints 100.2

# use negative numbers to start at the back
print my_list[-1] # prints "6" - the last item

# you can access part of a list with a ":"
my_list[1:3] # will be [45, 100.2, "whatever"]

# iterate throught the list:
for item in my_list:
	print item
	
	
# you can also have a list of lists, like so
cool_list = [[1,2,3], ["hello", "friend"]]

print cool_list[0][2] # prints 2
print cool_list[1][0] # prints "hello"

Dictionaries

Dictionaries are key-value pairs. They are like lists, but they are not ordered, and instead of accessing items with a number, you use a key, which could be a string, number or other python object.

# create an empty dictionary
my_dict = {}

# create a dictionary with values
my_dict = {"name": "Sam", "height": "short"}

# pull values out of a dictionary
print my_dict["name"] # prints "Sam"

# override or insert new items
my_dict["name"] = "Todd" # changes "name" to "Todd"
my_dict["last_name"] = "Anderson" # adds a new key "last_name" with the value "Anderson"

Functions

You can define a function with the def command.

def say_hi(name):
    print "Hello " + name
    
say_hi("Sam") # prints "Hello Sam"

def add_two_numbers(a, b):
    return a + b

my_number = add_two_numbers(1, 3) # my_number will now equal 4

Flow control

name = "Sam"

if name == "Sam":
	print "Hi Sam!!!!!"
else:
	print "Hi " + name

Modules

Python has tons of useful libraries that you can put in your scripts to add extra functionality. These are called "modules" in python. Some are included with your installtion of Python, like the sys module, or the random module. There are also tons of external modules that you can install via the pip command (like moviepy).

Use the import command to import a module.

# import the "random" library
import random

# to use the library, first type the module's name, and then a "." and then the method you want to use
# in this case, there is a method in "random" called "randint" which gives us a random integer
random_number = random.randint(1, 100) # gets a random integer between 1 and 100)

Command Line Arguments

When you run a python script you can add additional arguments that get passed in to the script. These are accessible via sys.argv which takes the arguments and sticks them into a list for you.

import sys

# get the first argument and store it in the "name" variable
name = sys.argv[1]
print "Hi " + name

You would run this script by typing python hello.py Sam