Overview
A variable is a convenient placeholder that refers to a computer memory location where you can store program information that may change during the time your script is running. For example, you might create a variable called ClickCount to store the number of times a user performs a certain operation.
When a variable is stored in memory, the interpreter will allocate a certain amount of space for each variable type. Where the variable is stored in computer memory is unimportant. What is important is that you know that a variable has a type, and you refer to a variable by name to see or change its value.
In Python, variables are always one of the five fundamental data types:
For a detailed look at each variable type see Python Variable Types in this guide.
While each variable has its own properties and methods, there are common methods we use to deal with all variables in Python.
Declaration
In Python, variables are created the first time a value is assigned to them. For example:
number = 10
string = "This is a string"
You declare multiple variables by separating each variable name with a comma. For example:
a, b = True, False
This is the same the multiple line declaration of:
a = True
b = False
Naming Restrictions
Variable names follow the standard rules for naming anything in Python. A variable name:
- Must begin with an alphabetic character (A -Z) or an underscore (_).
- Cannot contain a period(.), @, $, or %.
- Must be unique in the scope in which it is declared.
- Python is case sensitive. So “selection” and " Selection" are two different variables.
Best practices for all Python naming can be found in the (Style Guide for Python Naming Conventions)[https://www.python.org/dev/peps/pep-0008/#naming-conventions]
Scope & Lifetime
Scope of a variable defines where that variable can be accessed in your code. For instance a global variable can be accessed from anywhere in your code. A local variable can only be accessed within the function it was declared in. Generally a variable’s scope is determined by where you declare it.
When you declare a variable within a procedure, only code within that procedure can access or change the value of that variable. It has local scope and is a procedure-level variable. If you declare a variable outside a procedure, you make it recognizable to all the procedures in your script. This is a global variable, and it has global scope.
Here are few examples:
global_var = True
def function1():
local_var = False
print (global_var)
print (local_var)
function1() # this runs the function
print (global_var) # this works because global_var is accessible
print (local_var) # this gives an error because we are outside function1
It is important to be careful when declaring variables. It is easy to create duplicate variable names that do not reference the correct values. For instance do not declare a global variable this way:
g_var = 'True'
def function2():
g_var = 'False'
print ('inside the function var is ', g_var)
print ('outside the function var is ', g_var)
The example above will create a Global variable named g_var. When dropping in the function2 function, there will be a second local variable created named g_var with a different value. The proper way to work with a global variable is to be very explicit with the global statement in the local scope:
g_var = "Global"
def function2():
g_var = "Local"
print ('inside the function var is ', g_var)
return;
function2()
print ('outside the function var is ', g_var)
For more scope example see the (Notes on Python Variables)[http://www.saltycrane.com/blog/2008/01/python-variable-scope-notes/]
The lifetime of a variable depends on how long it exists. The lifetime of a global variable extends from the time it is declared until the time the script is finished running. At procedure level, a variable exists only as long as you are in the procedure. When the procedure exits, the variable is destroyed. Local variables are ideal as temporary storage space when a procedure is executing. You can have local variables of the same name in several different procedures because each is recognized only by the procedure in which it is declared.
Assigning Values
Values are assigned to variables creating an expression as follows: the variable is on the left side of the expression and the value you want to assign to the variable is on the right. For example:
B = 200
The same value can be assigned to multiple variables at the same time:
a = b = c = 1
And multiple variables can be assigned different values on a single line:
a, b, c = 1, 2, "john"
This is the same as:
a = 1
b = 2
c = "john"
Scalar Variables & Lists
Much of the time, you only want to assign a single value to a variable you have declared. A variable containing a single value is a scalar variable. Other times, it is convenient to assign more than one related value to a single variable. Then you can create a variable that can contain a series of values. This is called an list variable. List variables and scalar variables are declared in the same way, except that the declaration of an array variable uses brackets [ ] following the variable name.
A = [ ] # This is a blank list variable
B = [1, 23, 45, 67] # this list creates an initial list of 4 numbers.
C = [2, 4, 'john'] # lists can contain different variable types.
For a detailed look at managing lists, take a look at the the Python List Datatype Article
