Python Intro

Overview of Python

To get install Python and set up your environment, check out Setting up your Environment.

Running Python Code

You can run Python code by running a .py file or by running the REPL.

To run a Python file, use python <filename> on Windows or python3 <filename> on Linux. Python is an interpreted language, so when a file is run through the python interpreter, it is processed line-by-line until the end.

To open the REPL, just run python or python3 in a terminal. The REPL is also known as the interpretor or “python in Interactive Mode”. It is an interactive line-by-line-interpreting shell that reads an expression from the user into a data structure in internal memory, evaluates the data structure, yielding a result, and prints to standard output the result yielded by eval.

Syntax

message = "Hello World"

if x == 5:
    print("Number:", x)

if ( # single-line comment
    x == 5
    and y == 7
    and z == 10
):
    print(
        "Numbers:",
        x, y, z,
    )

"""
multi-line
comment
"""

Naming Conventions

Basic I/O and REPL Functions

Dynamic Typing

Python Data Model

Basic Python Data Types

Numbers

More on Integers

Sequences

More on Slicing

Immutable Sequences

More on Strings

Mutable Sequences

Set Types

Mappings

If-Else

If-Else statements in Python are formatted as follows:

if condition1:
    <block1>
elif condition2:
    <block2>
else:
    <block3>

The elif condition is only checked if the if condition is False. The else block is only executed if the if and elif are both False. The elif and else are optional.

Loops

There are 2 kinds of loops in Python, for loops and while loops.

For Loops

For loops interate over the elements of an iterator. The range function can be used to iterate over a range of numbers.

for i in range(10):
    print(i) # will print the numbers 0 through 9

You can also iterate over the elements of a set, such as a list.

l = ['a', 'b', 'c']
for character in l:
    print(character) # will print the elements of l

While Loops

While loops will continue while a condition is True.

number = 10
while number > 5:
    print(number) # will print out 10 through 6
    number -= 1

Functions

Functions in Python are defined using the def keyword and can be defined with any number of parameters. However, since Python has dynamic typing, the type of the parameter cannot be enforced without adding additional statements to check. You can, though, use type hints to specify what kind of input a function expects.

def func1():
    print("This function has no parameters!")

def func2(var1):
    print("This function has a parameter with a value of", var1, ".")

def func3(var1: str):
    print("This function has a string parameter with a value of", var1, ".")

# Calling the functions
func1()

func2(5)

func3("my value")
func3(7) # NOTE: even though func3's var1 has a type hint of string,
         # it isn't enforced by the interpretor

Lambda

Lambda functions are a type of function in Python that is anonymous. A lambda function is defined with a number of arguments and can have one expression.

my_func = lambda x : x + 1

print(my_func(1)) # prints out 2

Classes

Users can define types in Python with classes. A class can have any number of data elements and methods associated with it. The __init__ function is called when an instance of that class is created. The self keyword refers to a specific instance of that class.

class my_custom_class:
    def __init__(self, y):
        self.var1 = 5
        self.y = y # self.y refers to the instance, y refers to the parameter
    
    def mult_var1(self, x):
        self.var1 = self.var1 * x
    
    def get_y(self):
        return self.y

y = 7
my_obj = my_custom_class(y)

my_obj.mult_var1(8) # multiply var1 by 8
print(my_obj.var1) # print its new value

print(my_obj.get_y()) # get the value of y