Intro to Python

Prerequisites

There are two versions of Python, Python 2 and Python 3. For this guide we will be using Python 3, which you will need to install on your computer. The easiest way to do this, on a Mac, is with Homebrew, a tool that allows you to install and manage command line programs.

Install Homebrew

Visit Homebrew's website and follow the instructions there, or just copy and paste the following into your terminal:

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"

Install Python 3

Once Homebrew is done, you can use it to install Python and a number of other extremely useful command line tools.

brew install python3

Install a Text Editor

To create and edit Python files you'll need a good text editor, specifically designed to edit code. Here are a few good options:

Python Basics

Python is a command line application, just like cat, grep and sort.

To execute Python code you run the python command with a text file as an argument.

To start, let's make a simple program that prints a message on the terminal. To do this we will use the print command.

Create a new file called hello.py and put this in it:

print("Hello comrade")

Open your terminal and navigate to the directory where the file is saved, and then type:

python hello.py

You should see "Hello comrade" printed on the screen.

Expressions

An "expression" is a set of instructions for the computer to execute. Python will read or evaluate your expressions and return a result. For example you can add numbers:

print(1+1)
print(10/2)
print(100 * 6.2 - 70/3.5)

You can also test to see how different expressions relate to each other.

== tests for equality
< less than
> greater than
<= less than or equal
>= greater than or equal

print(1 == 1)
print(1 == 2)
print(1 < 2)
print(5 * 20 >= 100/13)

All of these expressions will evaluate to either a True or a False

Variables

You can store the value of expressions inside named variables using the = symbol.

x = 2
y = 5
z = x + y
print(x * 100)
print(z)

Types

Values have different "types" or categories. For example, 1 is an integer, 1.5 is a float.

You can see what type a value is is by using the type function:

print(type(1))

Some important types are:

a_number = 1                # an integer
another_number = 5.1        # a float
some_string = "Hello!"      # a string
some_boolean = True         # a boolean (notice the capitalization)
a_list = ["a bunch", "of", "stuff", a_number, some_string]
a_dictionary = {"key1": 10, "key2": "a string"} # a dictionary (key/value pairs)

In Python you do not need to declare variable types, or even that you are declaring a variable, you simply type a name, the equals sign, and then a value or expression.

Strings

Strings are a variable type that stores text. To create a string, surround some text within quotation marks. It doesn't matter if you use single or double quotes as long as you are consistent.

first_name = "Karl"
last_name = 'Marx'

print(first_name)
print(last_name)

If you add two or more strings together, Python will combine a new string for you.

first_name = "Karl"
last_name = 'Marx'

print(first_name + last_name)

print(first_name + " " + last_name)

Each character in a string is indexed numerically, and can access individual characters using [] square brackets.

name = "Karl Marx"
first_letter = name[0]
print(first_letter)

second_letter = name[1]
print(second_letter)

The character index begins with the number 0. If you wish to access the last character, you use -1. The second to last, -2 and so on.

name = "Karl Marx"
last_letter = name[-1]
print(last_letter)

You can also get a range of characters in a string by entering a starting and ending index in your square brackets:

name = "Karl Marx"
first_three_letters = name[0:3]
print(first_three_letters)

To get the total length of a string, use the len() function.

print(len("hello!"))

You can also determine if a string exists within another string with the in keyword.

sentence = "A spectre is haunting Europe"
print("spectre" in sentence)

String methods

Python's string implementation comes with many useful methods that allow you to transform and get information about strings.

For example, to make a string uppercase:

sentence = "hello there!"
uppercase = sentence.upper()
print(uppercase)

Here are a few more examples of things that you can do

sentence = "   HELLO THERE   "

# make it uppercase
lowercase_sentence = sentence.lower()

# make it title case
titlecase_sentence = sentence.title()

# remove white space at the start and end
stripped = sentence.strip()

# replace one set of characters with another
goodby_sentence = sentence.replace("HELLO", "GOODBYE")

Here's a full list: https://docs.python.org/3.7/library/stdtypes.html#string-methods

Lists

A list is a numerically ordered collection of values, also known as an array.

# 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"]

# get the length of a list
len(my_list)

# 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"]

You can iterate through every value in a list with the for keyword:

for item in my_list:
    print(item)

Dictionaries

Methods

Reading files

To open a file in Python, use the open() keyword function. The function takes two arguments. The first is the name of the file to open, and the second is a flag that states if we are opening the file with the intent of reading to it (use "r"), or writing to it (use "w").

Once we have opened a file, we use the read function to grab it's contents and return then as a string.

In this example, we open a file and store its contents in a string. We then uppercase the entire file and print it to the screen.

content = open("communist_manifesto.txt", "r").read()
loud_manifesto = content.upper()
print(loud_manifesto)

You can also store a file as a list of lines using readlines() instead of read()

This example prints the first 5 characters of a text file.

all_lines = open("communist_manifesto.txt", "r").readlines()
for line in all_lines:
    print(line[0:5])