Technically, Python does not support named constants, meaning that it is possible but never good practice to change the value of a constant later. There are workarounds for creating constants in Python, but they are beyond the scope of a first-semester textbook. Named constants must be assigned a value when they are defined. Variables do not have to be assigned initial values. Variables once defined may be assigned a value within the instructions of the program.
Skip to content Overview A constant is a value that cannot be altered by the program during normal execution, i. Constants are used in two ways. You use the Const statement to declare a constant and set its value. By declaring a constant, you assign a meaningful name to a value.
Once a constant is declared, it cannot be modified or assigned a new value. You declare a constant within a procedure or in the declarations section of a module, class, or structure.
Class or structure-level constants are Private by default, but may also be declared as Public , Friend , Protected , or Protected Friend for the appropriate level of code access. The constant must have a valid symbolic name the rules are the same as those for creating variable names and an expression composed of numeric or string constants and operators but no function calls.
Your computer might show different names or locations for some of the Visual Studio user interface elements in the following instructions. The Visual Studio edition that you have and the settings that you use determine these elements. For more information, see Personalizing the IDE. Write a declaration that includes an access specifier, the Const keyword, and an expression, as in the following examples:.
The compiler determines the type of the constant from the type of the expression. For more information, see Constant and Literal Data Types. Write a declaration that includes the As keyword and an explicit data type, as in the following examples:.
A program variable is a data item held in computer memory that holds the value currently assigned to it. Each variable has a name, an address in memory, and a size usually measured in bytes that determines the amount of memory it occupies. The variable's size is dependent on its data type. Before you can use a variable, you must declare it. An initial value is often assigned to a variable when it is declared, but this value may change many times during program execution.
To declare a variable, you use the Dim statement you can also use a variable without first having declared it, although this is discouraged. The Dim statement tells the system what data type the variable will be, and reserves space in memory for the variable accordingly. Variables can be declared at any point in a program as long as this occurs before the variable is actually used , although it is customary to declare them at the beginning of a procedure or code module.
The following statements declare a string variable called "lastName", and an unsigned short integer variable called "age":. A string variable contains alpha-numeric and other printable characters. String variables are commonly used to hold text information.
Once you have declared a variable, you can assign a value to it using the assignment operator.
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