Learn Python Tutorial - Variables and assignment

Python Basics

(Learn Python Programming in 20 Minutes)

3 Python commands

3.5 Variables and assignment

An assignment statement in Python has the form variable = expression. This has the following effect. First the expression on the right hand side is evaluated, then the result is assigned to the variable. After the assignment, the variable becomes a name for the result. The variable retains the same value until another value is assigned, in which case the previous value is lost. Executing the assignment produces no output; its purpose it to make the association between the variable and its value.

>>> x = 2+2 

>>> print x 

4

In the example above, the assignment statement sets x to 4, producing no output. If we want to see the value of x, we must print it. If we execute another assignment to x, then the previous value is lost.

>>> x = 380.5 

>>> print x 

380.5 

>>> y = 2*x 

>>> print y 

761.0

Remember: A single = is used for assignment, the double == is used to test for equality.

In mathematics the equation x = x + 1 is nonsense; it has no solution. In computer science, the statement x = x + 1 is useful. Its purpose is to add 1 to x, and reassign the result to x. In short, x is incremented by 1.

>>> x = 10 

>>> x = x + 1 

>>> print x 

11 

>>> x = x + 1 

>>> print x 

12

Variable names may be any contiguous sequence of letters, numbers, and the underscore (_) character. The first character must not be a number, and you may not use a reserved word as a variable name. Case is important; for instance Sum is a different name than sum. Other examples of legal variable names are: a, v1, v_1, abc, Bucket, monthly_total, __pi__, TotalAssets.