6: Math and Variables

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6.1: Arithmetic Operators
6.2: Order of Operations
6.3: Constants and Variables
6.4: Identifier Names
6.5: Variable names and Keywords
Programmers generally choose names for their variables that are meaningful and document what the variable is used for. Variable names can be arbitrarily long. They can contain both letters and numbers, but they cannot start with a number. It is legal to use uppercase letters, but it is a good idea to begin variable names with a lowercase letter.
6.6: Choosing Mnemonic Variable Names
As long as you follow the simple rules of variable naming, and avoid reserved words, you have a lot of choice when you name your variables. In the beginning, this choice can be confusing both when you read a program and when you write your own programs. For example, the following three programs are identical in terms of what they accomplish, but very different when you read them and try to understand them.
6.7: Statements
A statement is a unit of code that the Python interpreter can execute. We have seen two kinds of statements: print being an expression statement and assignment.
6.8: Assignment
6.9: Variables
One of the most powerful features of a programming language is the ability to manipulate variables. A variable is a name that refers to a value. An assignment statement creates new variables and gives them values.
6.10: Updating Variables
A common pattern in assignment statements is an assignment statement that updates a variable, where the new value of the variable depends on the old.
6.11: Python Examples
6.12: Arrays and Lists
6.13: Index Notation
6.14: A list is a sequence
Like a string, a list is a sequence of values. In a string, the values are characters; in a list, they can be any type. The values in list are called elements or sometimes items.
6.15: Lists are mutable
The syntax for accessing the elements of a list is the same as for accessing the characters of a string: the bracket operator. The expression inside the brackets specifies the index. Remember that the indices start at 0:
6.16: Debugging
At this point, the syntax error you are most likely to make is an illegal variable name, like class and yield, which are keywords, or odd~job and US$, which contain illegal characters.

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