Python 101

Part 1: Variables

What is a variable? Techtarget.com says:

In programming, a variable is a value that can change, depending on conditions or on information passed to the program.

In order to utilize variables, you need to first assign it, then use it. To assign a variable, first you need to set the variable name. Then have an equal sign, followed by the value you want the variable to be.

Note: Python automatically formats variables into a type, like integers or strings. You don't need to do an formatting on your part.

Here's an example:

a = "Hello, World!"
var1 = 1
var2 = 10

The first variable was a string, and the last two are integers. The main types of variables are:

• Boolean
  • True or False
• Integer
  • Whole numbers ONLY.
• String
  • Text
• Floating Point
  • A number with decimals, like -0.05 or 12.618
• Array
  • A list, or collection of Integers, Floating Points, or Strings.

To use a variable, simply call it with the same name you assigned it as. Two examples are: Printing variables:

print a

Doing math functions with variables:

var3 = var1 % 4

var4 = var1 * var2 - var3

Part 2: Conditions

What are conditions?

Conditions are a requirement that the program or a variable must pass before executing a specific portion of code. A conditions evaluates to True or False (written as True and False in Python), using a Boolean variable. They also use expressions.

What are expressions?

Expressions include the following:

• Boolean expressions: Evaluate to True or False
• Integer expressions: whole numbers
• Floating-point expressions: Real numbers, including decimals.
• String expressions: Evaluate to character strings.

Expressions in programming is a combination of symbols that represent a value. An expression could be x+6 or the string "Hello, World!".

Operators

These are some operators in Python used for conditions:

Boolean operators: "And" and "Or" operators allow you to build more complex boolean expressions.

if a == 1 and b == 2 or a == 2 and b == 1:
  return True

The "is" operator checks to see if the instances themselves are equal, not the values of the instances.

a = "hi"
b = "hi"

print a == b # True
print a is b # False

And finally, the "not" operator switches the return from True to False, or vise versa.

Branches

In python, the basic type of branch is the if or if-else statement. The if statement is what your program uses to decide whether or not to execute a piece of code. You can use an if statement, elif statement, which means else if, or else: statement. Here's what the syntax would look like:

a = 2

def if_a(a):
  if a == 1:
    return "Yes!"
  elif a == 2:
    return "Maybe..."
  else:
    return "No."

You can also have if statements within if statements, like this:

a = 2
b = 1

def if_a_and_b(a):
  if a == 1:
    if b == 1:
      return "Yes!"
  elif a == 2
    if b == 1:
      return "Yes!"
  else: return "No."

Part 3: Functions

Functions are the bits of code that you use to store lines of code and call them. You can assign variables or arguments to a function by putting them into parenthesis. You can do math, set variables, return values, and print statements with functions. You can also have if-then statements within functions. The syntax looks much like what you saw before, like this:

def function_1(a, b):
  c = a + b
  if a == b:
    return "Nope"
  else: return c

To call a function:

function_1(2, 3)

You can also call a function with different variable names:

x = 2
y = 3

function_1(x, y)

Want to learn more? Go to learnpythonthehardway.org/book