An expression is a combination of values, variables, and operators. A value all by itself is considered an expression, and so is a variable, so the following are all legal expressions (assuming that the variable
x has been assigned a value):
17 x x + 17
If you type an expression in interactive mode, the interpreter evaluates it and displays the result:
>>> 1 + 1 2
But in a script, an expression all by itself doesn't do anything! This is a common source of confusion for beginners.
Exercise 1: Type the following statements in the Python interpreter to see what they do:
5 x = 5 x + 1