Python Constants, Variables, and Reserved Words

Constants

• Fixed values such as numbers, letters and strings are called constants because their value does not change
• Numeric constants are as expected
• String constants use either single quotes (') or double quotes (")

Reserved Words

• Reserved words cannot be used as variable names or identifiers
• Examples: False, None, True, and, as, assert, break, class, etc.

Variables

• A variable is a named memory location where a programmer can store data
• The data can be retrieved later by using the variable's name
• Programmers can choose the name of the variables
• The contents of a variable can be changed in a later statement

Python Variable Name Rules

• Variable names must start with a letter or underscore
• Variable names must consist of letters, numbers, and underscores
• Variable names are case-sensitive
• Reserved words cannot be used as variable names

Sentences or Lines

• Sentences or lines can be classified as assignment, assignment with expression or print statements
• x = 2 is an assignment statement
• y = x + 2 is an assignment with expression
• print(y) is a print statement
• The components of a line or sentence are variables, operators, constants and functions

Assignment Statements

• Values are assigned to variables, using the = assignment statement
• An assignment statement consists of an expression on the right-hand side, and a variable to store the result

Expressions

• Lack of mathematical symbols on computer keyboards requires "computer-speak" to express classic math operations

Numeric Expressions

• + is used for addition
• - is used for subtraction
• * is used for multiplication
• / is used for division
• ** is used for power
• % is used for remainder

Order of Evaluation

• Python must know which operation to perform first when stringing operators together
• This is known as "operator precedence"

Operator Precedence Rules

• Highest precedence rule to lowest precedence rule:
  • Parentheses are always respected
  • Exponentiation (raise to a power)
  • Multiplication, Division, and Remainder
  • Addition and Subtraction
  • Left to right

Type

• Variables, literals, and constants have a "type" in Python
• Python recognizes the difference between an integer number and a string
• The plus sign (+) indicates "addition" if something is a number, and "concatenate" if the thing is a string

String Concatenation

• String concatenation is putting strings together

Type Matters

• Python automatically recognizes the variable type
• Certain operations are prohibited
• It isn't possible to add a number to a string
• type() determines the variable type

Types of Numbers

• Numbers can be either int or float
• Integers are whole numbers, and have the type int
• Floating point numbers have decimal parts, and have the type float

Type Conversions

• An integer is implicitly converted to a float, if an expression contains both an integer and a floating point number
• The built-in functions int() and float() can control this conversion

Integer & Float Division

• Integer division result uses the // operator
• Floating point result uses the / operator
• There was a different behavior in Python 2.x

String Conversion

• Use int() and float() to convert between strings and integers
• An error occurs if the string does not contain numeric characters

User Input

• Python pauses and reads user data entered using the input() function
• input() returns a string

Converting User Input

• int() is used to convert user input from a string to a number

Comments in Python

• Use # to comment out lines of code, as Python ignores anything after it
• Comments are used to describe code, document the author, or turn off a line of code

Coding Exercise 1: Band Generator Game

• Create a greeting for the program
• Ask the user for the city that they grew up in
• Ask the user for the name of a pet
• Combine the names of the user's city and pet to show them their band name
• Ensure the input cursor shows on a new line
• Use escape sequences

Coding Exercise 2: Correct Errors

• Correct the errors for display to match these lines
  • Day 1 - String Manipulation
  • String Concatenation is done with the "+" sign.
  • e.g., print("Hello " + "World")
  • New lines can be created with a backslash and n.

Coding Exercise 3: Input Function and len Function

• Google to find a function that calculates the length of a string
• Write a program that prints the number of characters of a user's name

Coding Exercise 4: Swapping Values

• Write a program that switches the values stored in the variables a and b
• e.g. swap variable a = 5 and b = 6, to a = 6 and b = 5