Variables

The second major building block of a Python program is called a variable. A variable is pretty much just a box for storing a data value. The variable has a name and we can use that name to access the data stored in the variable or to replace the data with a new value.

The function parameters in the previous examples were variables, as was area:

(name, radius):  

To set the data stored in a variable, we use an assignment statement. An assignment is a statement, so remember this means that it can't be combined with any other statement. It gets a line of source code all for itself and the expressions that are part of it.

An assignment statement consists of the variable's name on the left-hand side of an equal to symbol and the value we want to store in the variable on the right-hand side, as shown in the following code:

outer = "Hello world"  

If the variable didn't already exist, it will be created. Irrespective of whether the variable existed before or not, the value is stored in the variable.

Variables that are created inside a function are only visible inside that function and each time the function runs they're created a new.

The following code provides an example of this in action:

outer = "Hello world"  
def example_function(param): 
    inner = "Hello function: {}".format(param) 
    print(inner, outer) 
example_function("first") 
example_function("second") 
print(inner) 

The last line of the preceding example demonstrates that the variable created inside the function does not exist for code outside the function, as shown in the following output of the code:

This code example also shows what happens when we try to ask Python to do something impossible. It tells us what we did wrong and gives us the information about where the problem occurred and how we got there.